October 2, 2006

I couldn't leave it alone The last two pages at http://kirbgood.googlepages.com/ are new.

September 2, 2006

For those still interested, the results of the energy analysis from making glass/clay bricks are now posted at


The last three pages are newly posted. The last two pages, in particular, summarize our results.

This sub-project will be written as a technical article and published for the benefit of ceramics manufacturers in California. If you know manufacturers who might be interested, please do not hesitate to have them contact me.

Otherwise, this may be the final time we communicate in this manner. I'm still exploring options, and have learned to never say never, but if this is, indeed, the end for this project, I'd like to express my thanks to all who are still reading. I know how unusual it is for people to care enough to read others' rantings. You are most kind.

August 12, 2006

If you are interested in the brick testing progress, we finally have strength test results.

Please go to


The last two pages are new.

July 19, 2006

subject: Brick Testing Update

If you're interested, there's more info on the brick project. Go to the Table of Contents at: http://kirbgood.googlepages.com/

The last two pages, "Finding equivalent absorptions," are new.

July 3, 2006

Subject: more on brick making with glass

For those interested in this, we have now posted a Table of Contents at http://kirbgood.googlepages.com/

The pages from "Why do this" on are new.

The new information includes more than you wanted to know about glass recycling and a great effort to solve a small problem.

June 28, 2006

Subject: the brick tests

There was enough interest in the last announcement I sent out on this list that we've decided to keep updating the web site on the brick testing project and let you know when we add things.

A report will need to be made of the testing anyway, and this discipline will help. Also, since most of you have more experience with clay than I, we would greatly appreciate your input if you see me making correctable mistakes.

For example, since the last announcement, procedures for brickmaking and initial absorption test results can be seen at http://kirbgood.googlepages.com/brickmaking

June 26, 2006

Today's topic: Zoom Zoom

It's been along time between Recycled Glass List messages, hasn't it? Having finished the series of workshops and the Recycled Glass Kiln Arts Competition, this part of the project is basically complete. We are pursuing other means to continue, but for now this will be the last gasp for this email list.

Early this year I included some information on the special kiln we were building to do some research on the amount of energy used by making bricks with recycled glass vs. regular ceramic raw materials. I have had several inquiries on how that kiln worked out.

Well, the kiln is done and we are in the midst of the research. For those who are interested, there's some information and pix posted at http://kirbgood.googlepages.com/quartzbulbkiln

So far so good. In the web blurb you will see that the kiln goes from 1100 to 1800 degrees F in three minutes. It's remarkable the difference this makes in how fused glass looks. I hope to be able to show you some time.

Thanks again to all of you who participated in the competition. I hope you had a chance to see the display in Sacramento.

Much of the information we presented in the workshops was so far removed from conventional wisdom that it's been hard to get traction, but as for me, I believe that it's all part of learning to make things smarter, and we will eventually have an effect. How can we not try?

If you're interested in knowing how the brick testing goes, I could post periodic info and pix at the web site above. Just let me know you're interested.

February 4, 2006

Today's topic: Competition entry glass percentage

One question has come up often enough that I thought I'd shoot out a clarification. It pertains to the question on the entry about the percent of glass.

I think the best way to describe how to think about is to define what your entry is.

Say you entered a set of 100 percent recycled glass tiles mounted on a board and grouted. The entry is the tiles. The board, grout, and mortar are incidental to the display. So you say 100 percent.

Tiles made with 5 percent CMC, where the CMC burns out during firing, are 100 percent, since the entry contains no CMC.

Of course if you enter a 500 pound piece of carved granite with a recycled marble sitting on top, I would consider it unreasonable to call the entry 100 percent recycled glass.

I generally feel that the term "reasonable" answers a lot of questions. Let's say the predominant number of scientists in the world say that we are going to wreak havoc on the environment and cause untold suffering to man and beast if we continue to ride around with one 120 pound person per 5000 pound steel vehicle. A reasonable person might say a major focus of all nations should be how to prevent that from happening. An idiot would say, as I heard an idiot say at gathering after a funeral once, "stuff the bugger and keep the party going."

Also, I wish I had the ability, in this document, to put the word "weasel words" in a red circle with a slash through it. Please remember, this is about recycling. Once again, if you're using Bullseye and calling it recycled, go enter someone else's competition.

February 2, 2006

Today's topic: The Competition, Opportunity knocks

Less than two weeks until the closing of the Art Competition. For info/entry materials go to www.ceedweb.org/glass

Whether you think your pieces are ready for prime time or not, I encourage you to make an entry. I don't sound nervous, do I?

Opportunity knocks

Owning your own home is often referred to as "the American Dream." I take small issue with that. I think there are two major American Dreams: owning your own home AND owning your own business. Which of us haven't suffered fools ungladly at work, dreaming about setting off on our own. So many American icons have been entrepreneurs.

But exigencies of daily life and lack of exciting enough ideas and lack of capital make most of us relatively risk-averse. Personally, I've owned or been part owner of four businesses in my life. Of course the only one that was truly lucrative was also the most onerous and killing to the spirit. I mention this just to show that I know a little about what I'm saying.

Anyway, I saw a quote from a publication the other day: Waste News, Jan 16, p.22: "San Francisco has set a goal to divert 75 percent of its solid waste by 2010 and have zero waste going to the landfill by 2020." This sounds like Opportunity knocking to me. Amidst the rest of the garbage the City is going to try to divert is one heck of a lot of glass. And if the city actually hopes to achieve this goal, it will need to help people get their hands on waste materials and encourage people to buy products made from it.

Some of the techniques you saw at the workshops were expressly developed to use the crummiest glass around. And in addition it's apparent that even President Bush has come to realize that reducing energy inputs to products is a national defense issue. Which isn't to say he'll walk the walk, but at least he's talking the talk.

I make a promise to you. Ten years from now, there will be products marketed using glass as a raw material in combination with clay or high temperature cement, and for sure there will be more fused glass.

It's damn hard developing a product and a business. I've periodically had the twinge over the last ten years, but Seattle rents scare the heck out of me. I really like where I live, right next to downtown Seattle, but the nearest light industrial space I could afford is farther than I'd want to commute. So for now I do developmental work I can do in a garage with 100 amps of 240 volt power.

Many of you are in a similar situations in the Bay Area, where I assume rents are even more outrageous than here. But some of you are not. You either live far enough out of town or have "alternative" living and working situations with more flexible expenses.

It's unclear whether this little List will live on after the competition ends, but I want to leave you with an encouraging word. Try stuff. Do experiments. Learn to work with the materials. It would be a big payoff for me for someone to have taken my half-baked information and really done something with it.

January 24, 2006

Today's topics: The Competition, Quartz Halogen Heating Elements

The due date for entries for the Art Competition is less than a month away! Please let me emphasize again that we intend to have a wide range of types of entries on display. So even if you don't feel that you could compete in, say, a conventional fused glass competition, this may be the opportunity to have one of your pieces shown in a high-profile show because your work is innovative from a process point of view.

If you send in the pictures via email, please remember to still send in the signed liability statement on the entry via snail mail.

Quartz Tubes -- more than you wanted to know In the previous Tip I asked for leads for good kiln builders. I got several, thanks very much. I also got a couple of questions about what the deal is with quartz bulb heating elements.

I first saw a quartz bulb heated furnace a number of years ago in a materials science lab at San Jose State. The unit was a fugitive from high-tech ceramics manufacturing, where they sometimes need to fire very thin ceramic layers very quickly.

When the lab tech turned the furnace on, it came up from room temp to 1800 degrees in just a few minutes. I couldn't believe it. And I was more amazed to find out that the heating elements were basically just quartz halogen bulbs.

Quartz halogen bulbs are the really white, very hot kind you see in many desk lamps and trouble lights. In trouble lights they are usually thin tubes about 4 inches long, which plug into each end of a little fixture. Installation instructions generally recommend that you use cotton gloves when handling them, because oil, even from your hands, can cause them to fail when lit.

The element in quartz bulbs is tungsten, just like in incandescent bulbs. But the atmosphere inside the lamp is halogen. Google it if you need more info on halogen. The bulb itself is what's quartz. Rather than a thin conventional glass like with incandescent lamps, the glass part is basically pure SiO2. That's for two reasons: First, SiO2 glass is very thermal shock resistant, and Second, SiO2 glass passes light in the desired range better.

The halogen atmosphere in the bulb must make the tungsten element burn hotter than in the semi-vacuum of regular bulbs.

The elements in most of your kilns are metal alloys, either kanthal or nichrome. They are just big resistors that generate heat when electricity is passed through them. When the kiln first starts up, the metal element is quickly losing heat to the air by conduction and convection to the air in contact with the element.

Then as the kiln starts to heat up, the element can't get rid of its heat as fast and gets hotter and hotter until it glows red, then toward white. At high heat, most of the heat transfer in the kiln is by radiation rather than conduction, because it's a more efficient mode of heat transfer.

Think about the heat of the sun. Those photons left the sun and traveled at the speed of light, for about 8 minutes as I recall, through an almost perfect vacuum to get here and heat the surface of the earth. Remarkable.

Since the element in the quartz bulb is enclosed in a sealed tube and is very fine, it heats up and starts radiating immediately. No heat-up time. And because of the halogen atmosphere, it heats up to what thermo geeks call a "black body equivalent" temperature of close to 3000F. Even at high heat, the elements in your regular kiln don't get over 2400F.

One cool semi-technical thing about radiation. If you're conducting heat, say through a wall, the amount of heat passing is proportional to the temperature difference on each side of the wall. So a temp diff of 20 degrees passes twice as much heat as a diff of 10 degrees.

But for radiation, the amount of heat transfer is proportional to the difference of the temps to the fourth power! To see what this means, let's say the temp on one side of a wall is 1000 degrees, and the other side is 0 degrees. Now increase the temp to 2000 on the hot side. The conduction through the wall doubles.

Now let's say you have a wall that's clear to radiation. With radiation, you need to work with absolute temperatures, so 1000F is the same as 1460 Rankine, and 0F is 460 Rankine. Now double the temp on the hot side to 2460 Rankine. The heat transfer by radiation increases by

(24604 - 4604)/(14604 - 4604) = 8.1

Instead of doubling, it increases by a factor of 8!!!

Back to quartz bulbs. The quartz bulbs used in kilns have special end pieces to take the heat. One manufacturer is Ushio. Again, Googling will find them.

Once I bought some of those cheap trouble lamp quartz light bulbs from Home Depot. I took two 4x8 molds for fusing glass and put fiber insulation in the bottoms and made a kind of clam-shell mold. Then I drilled holes in the sides of the top mold to hold the bulbs inside the mold.

Then I put a 2x2 tile mold with glass in it into the bottom mold, attached alligator clips to the bulbs from the outside and plugged into the wall. Inside that mold got unbelievably hot. I didn't have control, so I did it by time, and found I could make a glass tile in ten minutes!!

Then once I forgot it was on when I left the room and destroyed the whole thing. Don't do this at home.

So now that I want to build a very fast kiln, I will use quartz tubes (real ones intended for kilns).

January 11, 2006

Today's topic: Know a Good Kiln builder?

Today I'm asking for your help.

I need to build a small kiln that will be a little more sophisticated than the Crucible Kiln genre.

The guy I usually use for this sort of thing is not available. If you know of someone who can build the following, please reply to this List.

The kiln will be small, about 12-inches square, 8 inches high inside dimensions.

Constructed of expanded metal exterior lined with 2-inch lightweight rigid fiber.

Here's the gotcha. The heating elements will be quartz halogen bulbs, which will enable the kiln to go from ambient to 1800F in five minutes. Quartz bulbs require phase-angle Silicon Controlled Rectifier (SCR) power. I'm assuming the SCR's will be controlled by 4-20 milliamp controllers, probably two (top and bottom).

The construction will require someone with good fabrication skills for bending metal and knowledgeable about installing lightweight fiber, and the ability to design and implement a somewhat creative wiring harness for the bulbs, controllers, and power supply.

If you know someone who fits the bill, please pass along contact information.

Thanks. If I find someone and this works out well, I'll pass along the basic design and info for this rocket kiln.

January 4, 2006

More mold info.

Something I forgot to mention in the Tip yesterday. Many of you probably already know this, but one type of mold you can have fun with, no stress, no strain, is cast iron.

Go to a kitchen place and get one of those heart, or little corncob, or just little circle cookie cast iron molds. Pre-fire it to at least 1000 to burn the oil out, coat it with release, put granular glass into the holes, and you're good to go.

I've made 50,000 glass hearts for a client, and my first couple of molds were just those cast iron things. They last surprisingly well. Be sure to always re-coat between firings. Again, if you miss a spot and it sticks, that's it for that particular hole.

You need to find cast iron molds with gently sloped side, like the heart molds. Anything with severe sides, like cake pans, will grab the glass upon cooling.

One of the first times I was asked to make some stuff for display I used cast iron loaf pans. I put about an inch of crushed glass in the bottom. I quickly found out that the pan grabbed the glass as it cooled. But I also found out that if I could get the "loaf" out when the the glass was 800 degrees F, it would come out because the pan hadn't grabbed it yet.

So I set my controller to cool down and hold at 800. Then I'd take a channel lock in each hand, whip open the top of the kiln, take each pan and dump the glass pieces onto the floor of the kiln before slamming the kiln shut.

I had no hair on my forearms for years.

In my experience, steel oxidizes like crazy, flaking off in sheets, but cast iron works pretty well. I don't understand this, because I thought cast iron had more carbon in it, which I would have thought would aggravate oxidation.

But the cast iron works pretty well, and lasts pretty well. Eventually it will warp and crack.

January 3, 2006

Today's topic: Permanent mold materials and the Competition

Happy New Year. I hope many are working on developing entries for the competition. I would like to re-emphasize that the intent is not necessarily to show the most glassy artwork we can get. The intent rather is to demonstrate the potential breadth and depth of working with recycled glass, preferably using some of the techniques demonstrated in the workshops.

If all we have to show are objects that can be made better using Bullseye, then why do it? That would only reinforce the old adage for marketing recycled content products: "Sure it's ugly, inferior and expensive, but it's recycled!"

Rather, we want to show things that CANNOT be made using Bullseye. For example, go ahead and try mixing 50 percent Bullseye frit with clay and firing to 1800 degrees. Then spend the next day cleaning up the puddle of glass in your kiln.

You can download info and app's at www.ceedweb.org/glass

It's free.

Mold Materials During the workshops, I assert that the ideal permanent mold for glass fusing would have the following characteristics:

  1. Lasts through many firings without thermal shocking or cracking
  2. Has a lower coefficient of thermal expansion than glass
  3. Is lightweight and thin-walled
  4. Doesn't stick to hot soft glass,
    Takes mold release well
  5. Is inexpensive
  6. Has no undercuts
  7. Transfers bas relief or texture cleanly to soft glass
  8. Is easily made into any shape

Of the above, I believe everything is possible to have in one mold except number 4a and its uncompromised corollary, number 7. Although some castables will not stick to glass during the first, or even second, firing, all that I have ever seen (except graphite -- see below) start sticking and require mold release. I believe this is because the surface characteristics of any static material evolve with firing. And since you have to apply mold release, you always lose a little relief or texture.

Graphite Ten years ago when I was first experimenting with glass, I visited the glass container manufacturing plant in Seattle. They had a black tarry emulsion they were applying to container molds. It was a graphite composition. An engineer in the plant gave me some and suggested that graphite would give me the clean release I was seeking. I tried the emulsion on one of my molds. I was dumbfounded the next day when I took the mold out of the kiln to find the glass stuck to it. And what's more, the mold was white, rather than black!

I hadn't known that graphite (or carbon) sublimates and goes into the air as CO2 (or maybe CO) in the presence of excess oxygen, elevated temperature, and given time. Later, someone sent me a solid machined graphite mold. It weighed about ten pounds. When I tried it for warm glass, I couldn't believe the results. The fused glass that came out of it was so smooth it was slippery. But when I weighed the mold and used it repeatedly, it lost about 5 grams every firing. So first, the mold would have been very expensive, and second, the machined hole would grow larger and larger over time. But it was still amazing.

Stainless Steel Over the years I have met several people trying to start fused glass production businesses using metal molds. Good stainless steel will take repeated firings. I have some nice 6x6 bent metal stainless steel molds. There are at least five problems with them:

First, since the coefficient of thermal expansion of glass is lower than virtually any metal alloy I have tested (there may be some titanium alloys -- but I've never had the pleasure), they have a tendency to shrink and grab the glass as they cool. This problem can be dealt with by building enough "draft" into the mold. But it's a challenge.

Second, you need a good coat of mold release. Stainless will stick to glass really tenaciously. And once it happens, kiss that mold good-bye. And the fact that stainless is non-porous makes it hard to get the release to uniformly coat.

Third, in a fast heat-up situation like a belt kiln, stainless will warp like crazy.

Fourth, you can have stainless flat molds fabricated by a sheet metal shop, but you don't have the flexibility to make whatever shape you want any time.

Fifth, although some have tried, I've never seen an effective way to "gang" stainless steel molds. For example, I recently got around to making myself a master to make 1x1 tiles. I made a 4-inch square mold that makes 9-1x1's at a time. Those would be itsy bitsy stainless molds.

I have tried using some of industrial castables, but have found them to be hard to work with, especially when one wants thin walls and light weight.

Porcelain My concept of the ideal mold material is porcelain. I would call the slumping molds Bullseye sells porcelain. It's a high-alumina, fine-grained, thin-walled, low-expansion, shock-resistant ceramic. The problem is, all of the Bullseye molds I've seen are obviously slip-cast, and all of the slip-casting molds I've ever seen are convex, so it's not possible to get a sharp inside corner.

Fiber I have seen people use stiffened fiber for molds. Personally, although I work with powdered and granular glass, I believe I know enough and take enough precautions to feel safe. What I know about fine fibrous materials scares the heck out of me. I may be wrong. Also, I have found stiffened fiber molds to get whimpy pretty quickly. I need more than three or four cycles from a mold.

Do-it-yourself Castables All of this is why I've gone to compositions using calcium aluminate cement and aggregates, as shown in the workshops. Once you get the materials and methods down pat, you will find that your initial nightmare of working with them was just part of the learning curve.

December 20, 2005


A quick clarification to the previous Tip.

After going slowly to 1300, I then go fast up to the max temp, 1600, 1700, or whatever. To minimize devit, it's generally best to minimize the time between 1300 and max temp.

December 20, 2005

Today's topic: An offer of glass and firing profiles

First, please excuse typos in my mailings. I compose these things off-line as Word docs, re-read them several times, then copy and paste on-line to the list server. Sometimes I decide to add a sentence or two after I've pasted and read the memo through. That's inevitably when the typos sneak in, like writing "stiff" rather than "stuff" in the last paragraph of the previous tip.

I have plenty of coarse crushed green container glass. At this point we need to ask you to pay the UPS charge, but the glass is free. For about 30 pounds, please send a check for $20 (UPS sure has gotten expensive -- at least for us little people) to:

Dogged Enterprise
Robert Kirby
113 17th Ave East
Seattle, WA 98112

Make certain to include a UPS-able mailing address.

Lately I've become more and more an advocate of soaking for a while at a temperature above organic burn-off but below full fusing. If you've fired any number of recycled glass fused pieces, I'm certain you've experienced the "mega-bubble."

You can be certain you soaked at 1000 for plenty of time to burn off organics, but you still get a whopper bubble. I believe that's because besides the obvious paper and plastic, there is a wide range of contamination in recycled container glass.

There are little pieces of aluminum and other metals, and any kind of dust you could possibly imagine. In one instance when I was running a manual crusher, just as I was dropping a wine bottle into the machine, I saw a dead mouse in the bottom of the bottle. The bottle was already in the air, so in it went. So there were bone fragments in that batch. And hemoglobin, I suppose.

Inorganic materials can volatize at any temperature. Here's how I minimize the problem.

I take the kiln up to 1200 degrees, then give it 30 minutes or an hour, depending on the thickness and weight of the piece, to get up to 1300. I believe that at 1200-1300 degrees soda lime glass fuses a little, but doesn't seal over, so the burn-off and volatiles can escape. It just makes sense to me to soak at the highest temp at which you know the bad air can escape.

If you soak too long at much higher temps, you make devit worse and may seal in the volatiles when they need to escape.

If you've used calcium oxide release on bigger pieces, you may have seem big bubbles that obviously originated from the bottom of the mold. No calcium oxide is perfect. All contains both chemical water and carbon dioxide. So the calcium oxide "calcines" at higher temps. In 4x4's I've found this to not be a problem. But on large pieces it can be.

I've done two things that were generally successful to offset calcining. I've drilled or cast small offgassing holes in the bottom of the mold, and I've dry-screened rather than adding the calcium oxide wet. Both help. Usually.

Any suggestions are always welcome.

December 18, 2005

Today's topic: Atomized metals as colorants

When I claim in the workshops that in my experience if you use granular glass you don't have to worry about most of the incompatibility issues that plague warm glass artists, I get a lot of sideways glances, indicating that some people are pretty sure I don't know what I'm talking about.

I have fused together all manner of crushed Bullseye and Spectrum glass with container and plate glass. I've only had a problem when I tried to do it with pieces rather than grains, or tried to layer different glasses, creating areas of a single type of glass.

And now an extreme example. I purchased from Douglas and Sturgess some samples of what they call Atomized Metals. These are very finely ground metals. I don't know exactly how fine, but my guess would be about 200 mesh. Fine, but not superfine. Definitely not truly atomized.

I bought bronze, brass, copper, aluminum, and stainless steel. I think these fine metals may be used to colorize some of those resin-type sculpting compounds. First I tried mixing one percent by weight with a granular glass without fines, about 20x30 mesh. I put 50 grams of glass/metal in each compartment of a 2x2 mold and fired to 1650 in 90 minutes, holding for 20 minutes.

The Aluminum really interfered with the glass fusing. I think because the amount of surface area of the fine aluminum severely oxidizes in an atmosphere with excess oxygen at this temperature.

The Brass is kind of a murky blue/green, from the high copper content, I suppose.

The copper is a nicer blue/green, but nothing significantly different from using copper carbonate.

The bronze is a more metallic gray with a touch of blue/green.

I found the stainless steel to be most interesting. It has the slick grey look of shale. And the mold side picked up the brush strokes from the application of calcium oxide mold release, making it look even more stony.

I tried just the stainless steel in varying amounts from 1 to 5 percent with the coarse glass we gave out the last time at the Crucible, including all of the fines. At 4 percent by weight, 4x4 tiles are gunmetal grey and stony looking. I'm anxious to try making some larger paver pieces.

After adding the 4 percent stainless steel and mixing it with the glass, I added a small amount (one part in 400 by weight) of water and stirred it up to keep the large and fine pieces mixed up before putting it into the mold.

Of course there are many grades of stainless steel. I once worked with a wise guy materials engineer who was fond of saying that "stainless steel is neither," meaning that it's neither stainless nor steel.

Stainless steel is generally recognized as being a family of alloys of nickel and chromium. Many people check to see that they're getting stainless rather than steel by checking with a magnet. Higher grades of stainless steel are not magnetic. Good stainless steel (like 300 series) should be pretty stable at 1650F.

The Stainless from Douglas & Sturgess is a little magnetic, indicating that it's not the best quality, but definitely not really just steel.

Now the main point. Imagine putting a stainless steel bolt in the middle of a fused glass tile. Well, you couldn't make a fused glass tile that way. But mixing finely ground stainless into glass is just fine.

Try it. Mix some wierd fine stiff into your granular glass. You have to keep the percentage low enough that it doesn't actually interfere with the glass fusing around it.

November 27, 2005

I have had two unexpected email incidents through this list in the past week, so I'm writing to make sure I haven't stiffed anyone.

When you reply to this message, it is supposed to forward to my personal email address.

Last week a friend emailed me through the list and I didn't receive the message for about eight hours. I've also heard from a person who said they replied to this list and I never wrote back.

If you replied to this list and I never wrote back, then I never received the email. I feel very strongly about this. I always reply, even if it's to say I can't help you.

It irritates me no end that in this age of email communication many people seem to think it's okay simply to not reply if they don't think they can profit from doing so.

Through the web site www.recycledglasskilnarts.com I get inquiries from around the world from people wanting info on glass recycling. I always reply.

I have a low setting on my spam filter, but it saves spam to a mailbox so I can scan it before deleting. So I'm sure that's not the problem, if there is one.

For more assured delivery, write to my personal email address above.

November 26, 2005

Today's topic: Competition priorities

I'd like to pass along some information on the goals and preferences of this project and the competition.

Once again, you can download rules and applications for the competition at http://ceedweb.org/glass/awards.html

The operative word for the competition is "Recycled Glass." I posted a notice of the competition on a glass art web site discussion group this week, and a discussion that ensued made me realize that we should carefully define what we mean by recycled glass.

There are people in the world who make the argument that by the laws of conservation of material and energy, all materials are recycled, because there are no new atoms in the universe, except those created and destroyed in nuclear processes. In the context of this project, that's sophistry.

There is a lot more to recycling than just convenient self-justification. It's also about energy conservation, pollution reduction, and minimization of the exploitation of natural resources.

As a practical matter, this competition is paid for by funds generated from the state's bottle bill, so the sponsors get to define the priorities. And the sponsors' priority is improving markets for recycled containers.

We have excluded blown glass from the competition, and have a strong preference against hot cast glass, because those are energy-intensive processes that we feel are not conducive to the spirit of this project.

In the display, we want to illustrate many possibilities for recycled glass. I'd like to show some things that make people exclaim "THAT'S made of glass?"

There are two judging committees, one focused on aesthetics and one focused on technical accomplishment. The priorities of the project will be communicated to both sets of judges.

That said, no recycled glass is excluded, EXCEPT your old Bullseye scraps. Recycled containers, plate glass, mirrors, used lab glass, etc., are all fair game, within the context of the priorities mentioned above.

And again, we want to show a wide range of pieces in the exhibit. I can imagine all of the following:

The contest is definitely NOT limited to processes we show in the workshops. Please teach us something new.

My own preferences in glass and ceramic art have evolved a lot during my work in this area over the past ten years. There's a certain "gee whiz" factor to seeing big blown pieces, but for my tastes, they don't stand up over time. I highly respect Dale Chihuly's marketing skills, but if I never see another blown glass shell or bulbous cherub, I don't think my life will be the poorer for it. On the other hand, the appreciation of a rough-hewn Raku piece is something I wouldn't want to lose.

A few years back a glass artist in Seattle kiln cast a five-foot high free-standing sculpture from granular container glass. It integrates copper washers and studs and has a matte granular verdant green jungle kind of color. It looks like it could have been a set piece from Apocalypse Now. Every time I see it I notice new nuances and appreciate the texture.

So whether you're a professional artist, a hobbyist, or a hack like me, please don't be afraid of try some stuff and enter. It doesn't cost anything except time, energy, and emotion.

Please feel free to contact me about your needs and concerns.

November 20, 2005

Today's topic: The Right Kiln for the Job

We held the last workshop in this series on November 11 and 13 at the Crucible in Oakland. For that workshop Strategic Materials, the biggest glass processor in the country, with a facility in San Leandro, donated a ton of glass. It was the same glass they sell to fiberglass manufacturers to make into new fiberglass. The glass was a mix of the three colors of containers, and was graded to about 14 mesh and finer (about 1/16-inch and smaller).

On November 11, two days before the presentation workshop, we held a hands-on workshop for people who had attended earlier workshops, so they had already been exposed to the information and, hopefully, already had tried working with recycled glass. Ericka Clark Shaw, a ceramic artist from Marin County, was there to give us her experience on working with the glass in ceramics processes.

After Ericka gave her input, everyone did their own thing with the glass. Some mixed glass with clay, others worked with glass mixed with corn starch or flour.

On Saturday I came back to fire the pieces people had made on Friday. Two kilns were available, so I dedicated one to the clay/glass pieces and programmed it to go to 1800F in two hours. The other I filled with the glass and organic binder pieces, and programmed it to go to 1600F in 90 minutes.

After baby-sitting the kilns for about two hours, one thing was clear. Both kilns are under-powered, for my preferences. At that point I decided to override the controllers. I put both kilns into "full on" mode, telling them to get up to temperature as fast as possible.

I won't get into the differences between programming various electronic controllers, but with the Digitry controllers they have at the Crucible it is easy to override. I often do it on my own kilns. It is sometimes more difficult with other makes of controller. Digitry is owned by warm glass people. I have found them to be good listeners and advisers when you have problems. Not the cheapest, but good equipment. www.digitry.com

When I left after five hours, the kiln set for 1600 was done, but the one set for 1800 was only up to 1300F! I was worried about that kiln making it to 1800F at all, so I reset the target temp to 1750. After confirming that the Crucible didn't mind leaving kilns on at night, I left.

The next morning both kilns were off and cooling. I cracked both and raised the lids when they got below 400F. The clay/glass pieces, which had been fired to 1750, looked okay and had a "ring" to them, but didn't seem as dense as I would have expected.

The organic binder and glass pieces were ugly. How ugly? They were so ugly their parents denied knowing them. The glass hadn't completely fused, and was dull and craggy.

I was worried that I had provided non-functional glass for the workshop. I took some of the exact same glass home and tried it in my own kiln. At home I made one tile where I put 250 grams loose into a 4x4 mold. I made another where I combined 250 grams of glass with 10 grams of corn starch, added 40 grams of water, formed it into a 4x4 tile, and dried it out on a warming plate.

I fired both tiles as follows:

Both tiles are completely fused, smooth, and have uniform matte surfaces, just like the 4x4 tiles I showed people at the workshop. The surfaces of both are quite similar.

So I convinced myself that:

  1. the glass fused fine, and
  2. the organic binder didn't cause the crummy surfaces we saw at the Crucible.

So that leaves the kiln as the cause of the ugly pieces.

I have said before that I believe that taking too long to get up to temp sucks the life out of the glass. This experience reinforces that notion.

In the workshops I say I believe much of what artists call "devit" is actually loss of sodium from the surface of the glass. This experience also reinforces that belief. The sodium "boils" off the surface of the glass. Since sodium is the primary flux in the glass, the surface stiffens, becoming resistant to flowing or fusing to other particles.

Some artists have tried to convince me that surface fogging happens primarily during cooling. That may be true for Bullseye glass, but I don't believe it is true for container glass. The fact that the glass at the Crucible never flowed enough to get a smooth surface indicates that the stiffening happened during heating, not during cooling.

If the funky fused glass had been caused by crystal growth, it would make sense that it happens during cooling. The mechanism for crystal growth (also called devitrification) has two phases: nucleation and growth. Nucleation is like germination of the crystal. It happens at higher temperatures (above 1500F). Then crystal growth takes place at lower temperatures (between 1000 and 1200F). That is one reason many people think the glass fogs only as it cools. If you want to learn more about this, Google glass-ceramics, or go to any materials science library. There are many books on it.

What to look for in a kiln Buying kilns is the one area of life where power doesn't corrupt, and actually saves energy. The more amps you build into your kiln, the faster it will heat and the more reactive it will be. Since it's faster, it will use less energy than an underpowered kiln. And when you're looking at kilns, lightweight insulation is better because it doesn't suck up the heat and slow down the kiln.

So if you have an old kiln, consider at least getting that heavy brick lid replaced with one made from lightweight fiber.

So if you took some of the glass from the Crucible, don't despair. It works great with the right kiln and firing profile.

I welcome your thoughts.

October 26, 2005

Today's topic: Final Hands-on Recycled Glass Workshop

We will be holding two sessions on November 11 & 13 at the Crucible in Oakland. November 13 will be the presentation you have probably already seen. Several people have told me in the past that sitting in on a couple of these, even at $10 each, was really helpful. I am always learning new things myself, and there is a lot of info to absorb at one sitting.

More importantly, there will be a hands-on demo on November 11. We set this up JUST FOR YOU. I know many people have gone away from earlier workshops and tried, with varying degrees of success, to work with the materials I talk about. I apologize for any difficulties you've had. I suspect it's partly that I've worked so much with these things that they've become second nature to the extent that I have a hard time communicating the basics.

With time running on this project, and getting toward the art competition, we set up this Friday session EXPRESSLY for working with you who have found it challenging. Ericka Clark Shaw and I have gone to a lot of trouble to set this up, and we only have fifteen seats for this. We still have a number of empty seats.

THIS IS YOUR CHANCE TO ASK QUESTIONS AND GET MORE EXPERIENCE BEFORE THE COMPETITION. If we don't get enough sign-ups there's a chance we will have to cancel the Friday session.

YOU SHOULDN'T MISS THIS OPPORTUNITY. Bring the pieces that didn't work for you and hit me over the head with them!

If you want to bring a friend who didn't attend an earlier presentation, please be my guest.

The reason I got involved with this project in the first place is that I've been experimenting with these processes for a number of years. I thought it was time to get the info out to creative people. I have an abiding belief that ten years from now glass will be a standard ceramic raw material. The potential energy savings and process efficiency will make this inevitable.

Pretend that it's 1980 and a young guy with a funny haircut named Steve Jobs has approached you to show you a little computer he's developing that he says will be make computing democratic rather than just for the cognoscenti. He respects your creativity and is offering you a ground floor opportunity, literally on the ground floor of his garage, to work on his project. Just do it. And the next time you're in Seattle, visit my garage and see how one fits six kilns and two tons of glass into 250 square feet.

October 6, 2005

Today's topic: Final Recycled Glass Workshop and Art Competition

We will be holding the last workshops in this series on recycled glass kiln arts on November 11 and 13 at the Crucible in Oakland.

Sunday, November 13

On Sunday, November 13, from noon to 3pm, we will be doing basically the same presentation we did at previous workshops. All of the information and binders with workshop instructions will be distributed. Some people have told me that there was so much info that attending twice really helped them to better absorb.

As before, the presentation will be wide-ranging with many samples. You can read more about it at www.ceedweb.org/glass

There will be a $10 charge for this workshop.

Each attendee will receive a 25 pound bag of processed glass for free. This glass is being donated by Strategic Materials Corp., the glass processor in San Leandro. It is coarser than the glass given out at the last Crucible workshop and works much better for some things.

Please tell anyone who might be interested about this workshop. If you have ideas about lists of potentially interested people we could email, please feel free to contact me.

This is the LAST workshop of this series. It is unclear either if or when this information will be presented again. If you know anyone who would like to see this information, please encourage them to come to this one. If you attended a previous workshop, you are also welcome to come and ask your most challenging and confrontational questions. We bend but we do not break.

To register call 510-444-0919

Friday, November 11

On Friday, November 11, from 10am to 3pm we will have a hands-on session focusing on using glass with clay. Ericka Clark Shaw, a ceramic artist from Marin County, and I will start the day with a one to two-hour presentation. Ericka has been testing glazes with glass/clay combinations and experimenting with workability and firing issues when granular glass is mixed with clay.

THERE IS SPACE FOR ONLY 15 PEOPLE IN THE NOVEMBER 11 SESSION, because we need table work space for each person. This session is especially for people who already attended one of the previous workshops and want to both expand their knowledge and ask questions about what they've already seen.

The general background information on using recycled glass will NOT be presented during this session. The workshop binders will NOT be given out. Again, this is primarily intended for those who already attended a previous workshop and want to expand their knowledge or have questions.

If you sign up for this November 11 session, please bring a lunch so we can keep working without taking a break. After the initial presentation, attendees will have the option of working with materials we have there or bringing their own projects to work on.

There is no charge for the November 11 session. Strategic Materials Corp, the glass processor in San Leandro, will deliver a ton of crushed glass in a big bag. If you bring your own bucket or bag, you can take up to 50 pounds for free.

Ericka and I will be there through the afternoon, so if you just have specific questions, you can also stop in from noon to 3pm to ask them informally.

To register call 510-444-0919


At long last, the promised Recycled Glass Art Competition is taking place. Information and registration forms will be available shortly at www.ceedweb.org/glass

The competition will focus on the processes we talk about in the workshop:

but is open to any variation on "warm glass" or ceramics. Blown glass is excluded from this competition.

Deadline for entry is February 15, 2006, so you have plenty of time to experiment, but you need to start planning now.

There will be at least four cash prizes in both artistic and technical categories.

At the end of the competition at least twenty pieces will be displayed at one of the State headquarters buildings in Sacramento. It should be a great opportunity for both artists and technologists to strut their stuff.

Watch the CEED web site for information packets. More info soon through this List.

September 1, 2005

Today's topic: Cleaning up Coarse crushed glass

Shipping out bags of coarse crushed glass to those who requested it since the Crucible workshop has reminded me that people interested in using this as a raw material for fusing tiles may want some pointers on dealing with contamination and grading. Those using the glass in clay blends or for mold making don't have the problem because the contaminants are not much of an issue in those processes.

You receive the crushed container glass with most of the same contaminants the recycling center receives. We have tried to avoid including caps, but the labels and residual sugars, etc. from the original contents are there. The whole containers were just run through a crusher.

In many parts of the country you can go to a site where the community or a recycling company owns a crusher. Often you can shovel all you can carry for free. On Orcas Island, Washington, all of the recycled glass from San Juan County, a county of islands, is run through a crushing system and given away to whomever wants it. At least that has been the case for the past ten years and until three years ago, the last time I was there.

I know similar crushing systems are used in Santa Fe, Albuquerque, and Taos, New Mexico. Many other communities at long distances from container manufacturing plants are doing similar things. Often it's the cheapest way to deal with recycled glass. Whether it's the right thing to do is a source for endless debate, but in many places crushing and giving away the glass as an aggregate is the only way to maintain collection because transport is so expensive. Which makes our development of new products from this minimally processed glass all the more important. Please keep that in mind as you suffer through the learning curve.

As in so many things, California is different in this regard. For at least two reasons, California has very robust glass markets that make coarsely crushed glass less available. The first reason is the existence of multiple glass container and fiberglass manufacturing plants that use vast amounts of recycled glass. The second reason is legislation that requires the plants to use certain percentages of recycled, and also helps the system move glass with a variety of incentives.

I only mention this issue because the question of easy availability of crushed glass comes up often and is not always easy to explain.

Regardless, if you have the crushed container glass, it can look pretty ugly at first. The labels, even though they comprise less than one percent by weight, stand out like a sore thumb. And after firing they remain as pieces of white or grey ash.

There are at least three basic ways to get rid of the contaminants: screening, pre-heating, and washing.


Although as you know, I'm an energy freak, the one reliable way to get rid of organics is to burn them out. Remember the 1960's film Fahrenheit 451? It was about book-burning. The name came from paper's ignition temperature of 451F. Rent if you can. You'll see why Michael Moore riffed on the name.

More importantly, the maximum temperature to heat soda lime glass without it starting to fuse is 1200F. So put the glass into something durable, like an unglazed pot or a stainless steel pan, heat it to 1200 and hold it long enough for the heat to penetrate the load. For a few pounds in a stainless steel bowl I hold for an hour.

After cooling, the labels will be seen as pieces of ash that break up when touched. If whole labels are still there, you didn't hold it long enough at 1200.

I've wondered about, but never tried, a little alcohol, a match, and a Hibachi to accomplish the same thing. Let your conscience be your guide.


Now that the labels are burned to ash, you need to get them out. Screening is the way to go. I have a 50 mesh screen I bought at a pottery supply store. I find that the 50 mesh screen gets the ash and finest glass out without sacrificing too much of the good glass.

You can use a coarser screen, but will lose more glass. Kitchen supply stores carry kitchen screens that are finer than the usual 16 mesh, are pretty cheap and have a bowl shape that's easy to use.

Use a dry brush to brush the glass around in the screen as you hold the screen over a garbage bag. If you shake the screen a little the fines will automatically fall to the bottom.


You might want to try washing (or rinsing), if only to know why you don't want to bother with it. In my experience, the marginal effects are not great. And drying is a pain.

I haven't found the perfect washing strategy. I've tumbled glass in rock tumblers with soap, alkalis, and acids, but I've never found anything that works discernibly better than putting the glass into the screen mentioned above and swirling it around in a bucket of water. Small particles that had been adhered to the larger ones soon cloud the water.

Then you've got to dry it. You can put it in a sock or towel and hang it outside in the sun. Or spread it out on a plastic sheet or towel outside. Microwaving can also be a pretty efficient drying strategy. Only use microwave-safe containers. Don't use plastic, the hot glass will probably melt it.

Once it's dry, play around with it. Screen to different grades, fire at different temps. Keep a record of your results. I have found green and amber glass to fuse at somewhat lower temps than clear.


One tip for firing. It's very hard to get all of the fines out. And any finer pieces that remain have a tendency to fall to the bottom of a mold and make tiles look uneven. So I add just a bit of water (1 gram of water in a pound of glass) and mix it up just to get any fine particles left to adhere to larger particles and stay mixed up.

Try it with and without a little water and see if you notice the difference.

Contact me with any questions.

August 25, 2005

Today's topic: Making Countertops

I have been asked a couple of times about the feasibility of making countertops using molds and granular glass.

Please recall that we presented four categories of glass processes in the workshops:

  1. Combining glass with clay
  2. Combining glass with calcium aluminate cement
  3. Combining glass with organic binders like CMC
  4. Fusing glass in reusable molds

Calcium aluminate cement and glass

Many companies make concrete countertops now. I assume that they have large molds or frames that are adjustable depending upon the size of the countertop being made. I also assume that they pour the concrete into the mold, scree the top and trowel it to give it a nice finish, then grind the edges after it sets up. So the "up" side becomes the finished countertop.

If that is the way they're made, why not use a glass/calcium aluminate cement mixture instead of Portland cement and aggregate and pour the countertop right onto the bed of a top hat kiln? You could scree and trowel it the same way to get a nice finish, but after it sets up, you could glaze it and fire it in the top hat! You end up with a monolithic glazed countertop. Sounds very nice to me.

The top hat would need to be capable to higher temps than most, probably a cone 06 equivalent, or at least 1850∞F.

Clay and glass

Adding glass as the grog in clay accelerates the drying and lowers the firing temperature. Glass reduces drying shrinkage in two ways:

  1. The clay/glass mixture shrinks less in an absolute sense because the glass pieces are incompressible, non water-absorbent particles, and
  2. It appears to me that if you have larger glass particles (like 8 mesh) in the mix, it also puts most of the shrinkage into the vertical dimension. That means that rather than shrinking in from the sides until cracks appear, as in pure clay mixtures, the shrinkage is mostly in height, so you don't get shrinkage cracks.

I have never made anything larger than 12 inches square, so this is theorizing on my part. It's on my "when I get around to it" list.

So if you can roll out a big monolithic clay piece, and dry it without cracking, you can glaze and fire it.

Organic binders and glass.

I can't picture how one would use the CMC/glass process to make a large monolithic piece with clean edges. So I'll skip over that process.

Glass fused in calcium aluminate molds

This is the specific topic I wanted to expand upon in this Tip. I have been asked whether one could make a mold the size of a countertop and fill it with glass, making a solid glass piece.

I think this is probably not feasible with recycled glass and in most people's facilities.

First, the size of the mold is a problem. My 12-inch square molds weigh about 7 pounds. To make a 3/4-inch thick glass piece takes about 13 pounds of glass. So the glass with the mold weighs 20 pounds per square foot. A 24-inch square piece will weigh 80 pounds. Quite a load to lift in and out of a kiln.

And my mold making formulas were optimized to make 4-inch square tiles. I like the process because it's cheap, fast, and fires at low temperatures. However, it simply is not good enough to make huge pieces. The largest mold I've made is 15 inches square, and it developed cracks after firing five pieces. Too much effort for that amount of return.

I have also thought about fusing the glass on a flat kiln shelf with a dam around it. I have actually tried this, using my mold mix to cast dams to hold in the glass. This might work okay, except in my experience the glass always wants to slip under the dam, so you end up with all of these little pointies that have to be ground down and re-finished. Once again, big effort for moderate gain.

And on your 100 percent glass piece, which side do you envision to be the "good" side? In the workshop I explained that there are certain contaminants inherent to recycled glass. That's just the way it is. So if you envisioned a perfect glassy surface on the "up" side after firing, forget it. Expect pocks and flecks and devit all over the place. Definitely not countertop material.

If you use the "down" side for the good side, then you can come closer to getting a uniform surface, because all of the nasty things tend to rise and blow out the top side. But the best surface you can get on the down side is the brush strokes from the shelf release. I don't think most people would find it to be an acceptable countertop. And as I said above, the edges are a finishing nightmare.

I'm the last one to say anything is impossible. I'm just saying that if you want to kiln cast a 100 percent glass countertop using recycled glass, you have a whole lot of work to do between here and there, and there are some specific issues that I can't see resolving easily.

If I were going to try monolithic countertops, I think I'd try clay first, fondu cement second. Maybe I will.

I welcome your thoughts.

August 15, 2005

Today's topic: Fines vs. Coarse in fusing molds

I have had a couple of comments on the finer glass distributed in Oakland and Marin developing cracks on the surface when it's fused in molds. That's both because of all of the air contained among the glass particles and because of the size of the particles vs. gravity.

By my scale, an 8 ounce yogurt container, with 263 cubic centimeters (cc) of volume, filled with the 100F fines weighs 282 grams. The same container filled with 1/8 inch and finer weighs 364 grams. A piece of solid glass the same size would weigh 657 grams. The specific gravity of glass is just about 2.5 gm/cc, so there are 150 cc of air in the fines cup, and 117 cc of air in the coarse cup.

So there is 25 percent more air to be squeezed out of the cup of fines. We all know what happens when there's too much air or water in a clay mix: it cracks as it dries or fires.

In addition, the larger particles tend to push the whole mass down as it fuses. One way this is manifest is that the corners of tiles made from all fines are generally much more rounded than the corners of those made from coarse. The coarse seems to push down harder and fill the bottom of the mold.

If you have both coarse and fine, try this. Fill the bottom of a mold with fines, then put a layer of coarse behind this and press it down. I'll bet the back doesn't crack when it fires.

There are other ways to reduce or eliminate cracking. When I fill a mold with loose glass, I have a small piece of wood that I press on top of the loose glass to make certain all of the glass is "seated." That helps reduce air, as well.

I also add a small percentage (no more than 1/200 by weight) of water to the loose glass and mix it up before putting it into the mold. This does two things. First, I believe the water helps to clump the finer particles around the coarser ones to get a more uniform mixture. Second, I believe the water adds to the "lubricity" of the glass, helping the particles slip across each other as you press them down.

Don't add too much water. Too much water will create havoc with the mold release.

One last way to reduce or eliminate cracking is to use CMC binder. To each 100 grams of fine glass, add two grams of CMC and mix it up. Then add 35 grams of water (or more or less to make a sort of dough). And slap, pat, and other wise work the glass dough into the shape you want. Dry it out in a toaster oven & fire. I find that the glass/CMC mixtures seldom crack. I suppose that's because of the close proximity of the particles to each other as they dry with the binder. I imagine quite big fine glass/CMC mixtures would crack just because of the amount air in there.

Please let me know how it goes.

August 12, 2005

Today's topic: Processed Fines vs. Coarser Glass

For the first three workshops in this series we found ways, either through shipping or by on-site processing, to supply crushed glass to attendees. For the last two workshops, in Oakland and at the College of Marin, we had finer, more highly processed post-industrial recycled glass shipped in.

The reasons for the change had to do with the complications of shipping and easy access to inexpensive processed glass. Many of you will remember that in the workshops I emphasize the need to work with minimally processed glass, as it is available through recycling programs. That is the need, that is the purpose, and that is one of the principles around which this project was designed.

We saw using the more highly processed glass as only a partial compromise for convenience. The processed glass we supplied, while finer than the coarser glass given out in the first workshops, is still a lot coarser than traditional frits or the very fine glass that has been the subject of earlier glass-in-ceramics studies.

Prior to the Crucible workshop, I had done enough testing to satisfy myself that the finer glass worked okay. Still, it was not totally consistent with the "Principle of Reactive Aggregate" that I talk about in the workshop.

Since the Marin workshop I have heard from a couple of participants and have done more testing on my own. I now believe that the finer glass supplied in Oakland and Marin does not work as well as the coarser in some of the applications.

I have found that the fine glass works well in ceramics mixes. I have blended the fines 50/50 with OM-4 ball clay and fired dense bodies at Cone 03, which is way below OM-4's normally densifying temperature.

However, the fine glass does not work well in fusing and CMC binder processes. It fuses to kind of an unattractive gray that is hard to control. For nice fusing and binder processes, I believe that a mix of coarse and fine glass works best.

In future workshops we may send a combination.

WE ARE THEREFORE MAKING ANOTHER OFFER. We have arranged to make available a limited number of 25 pound cartons of crushed green container glass. If you attended either the Crucible or the College of Marin workshop, we will send you, for free via UPS, 25 pounds of coarsely crushed recycled container glass. The crushed glass includes everything from pieces of about 1/8 inch to dust.

If you want to take advantage of this offer, please copy the following into an email reply to this listserve: _____________________________________________________________

The Crucible__
College of Marin__



If you did NOT attend at either Marin or Oakland, we will still ship you the glass, but we will need to ask you to pay the UPS charge. If you are interested, please send a reply email to the list and I will check on UPS charges. After we see what kind of response we get from the Crucible and Marin attendees, we'll ship the remaining glass on a first come/first serve.

It would have been easy to hide from this problem or act like it hadn't happened. But for us, the ultimate purpose of this project is to get creative people like you experimenting with this material and these processes.

And remember, there will soon be an announcement for the art competition. So now's the time to practice.

August 3, 2005

Today's topic: 4x4 mold offer

A number of people have expressed frustration with the mold making process. Either it has been frustrating finding and learning to work with the raw materials, or the number of steps required to start making reproducible molds is too daunting right now.

So here's our offer. We can't afford to supply everyone with free permanent molds for testing. But we do have fixtures set up to make a variety of mold sizes. So if you want to try permanent molds, here's the deal.

For your $25 check, we will send you via UPS 4 reusable 4x4 molds

These molds are all pre-fired, ready to be filled with glass, coated with mold release, and repeatedly fired as high as 1800F. With proper handling, the molds should be good for hundreds of firings.

The molds are 4 inches square, one inch deep, with a 5 degree draft so tiles easily drop out. They have 3/16 inch thick walls and weight less than 300 grams each.

We will include a pound of calcium oxide mold release in each carton, along with a one-page instruction sheet and a pound (or more -- depends on shipping cost) of crushed glass for your initial tests.

This will give you a chance to just throw some glass into a mold, fire it, and see what it looks like. Even warm art-glass people using products like Bullseye Glass like these molds, because you get the same 4x4 size each time you fire. No worrying about the glass slumping and forming too large of a foot print.

If you would like a set, please send a check for $25 (no cash or credit cards, please) to:

Dogged Enterprises
CA Mold Offer
113 17th Ave East
Seattle, WA 98112

One other thing. If you plan to do this, it would be helpful if you respond to this email by telling us you plan to send a check. Having a few days to anticipate orders would be good. We have no idea how many people might want this.

After we receive your check, we'll email you a shipping date, so you'll know when to expect them.

If you like, you can print the following and use it. Please print clearly.


Please send me 4 -- 4x4 reusable molds. I have enclosed a check for $25.

Name _____________________________

Address ___________________________

Address ___________________________

Address ___________________________

Phone _____________________________

Email ______________________________


We have masters for a number of other sizes:
If you want to try other sizes in a production shop, contact us.

The 2x2's are four tiles in one mold. The only substitution we can make with this offer is to substitute one 2x2 mold for 2 of the 4x4 molds.

Let me be clear that this is not a money-making offer. The cost pays for the materials and a partial offset for labor. We want people to be able to try firing recycled glass with the least amount of hassle.

August 1, 2005

Today's topic: Silica, Safety, and CMC/glass mixtures

For the hands-on workshop at the Crucible, we had 200 mesh silica delivered as well as the glass and CMC. I have found that adding some silica (about 10 percent by dry weight) to the glass/CMC mixture helps to maintain the shape you've formed through the firing. The formed piece doesn't slump so much.

Some people at the workshop did not want to work with dry powdered silica for safety reasons, so instead we worked with just glass and CMC. It was my mistake for asking anyone to do something they weren't comfortable with.

That incident inspired me to get together some safety information on both glass and silica. I am not an industrial hygienist. I have no qualifications in the area. All of my information is by reference and anecdote. Please keep that in mind as you read this.

More than ten years ago I managed a little project at the CWC to develop a fact sheet on glass dust. You can find some of the information from that sheet at http://www.cwc.org/gl_bp/gbp2-0503.htm. Some of the values in that sheet have since been revised by the Occupational Safety and Health Administration (OSHA), but the narrative still applies.

Silica Dust Crystalline silica has been strongly associated with the disease silicosis. I've read studies of hard-rock gold miners where the mortality rate from silicosis was as high as 50 percent. It's an awful way to go. For what appears to me to be a pretty good discussion, please see http://www.workershealth.com.au/facts060.html

For a Materials Safety Data Sheet (MSDS) that includes silica, please go to http://www.lagunaclay.com/msds/3rawmat/adry/MSDS3a.htm and download msil.pdf. This is the MSDS for 200 mesh silica.

Note that the NIOSH TLV (National Institute for Occupational Safety and Health Threshold Limit Value) is 0.05 mg/m3. That's probably less than you can see suspended in the air. So the standards for silica dust are very stringent.

Glass Dust Glass does not contain crystalline silica. It contains non-crystalline silica as part of a glassy or amorphous matrix. For an MSDS on glass, please go to http://www.trivitro.com/tech_msds.html and download the MSDS under Vitro Grit blasting abrasive. You will see that the TLV is 5 mg/m3. So the TLV for glass is 100 times as high as that of crystalline silica.

Personally, I always wear a respirator when working with either silica or glass dust in my shop. But I also know that glass dust appears to lack the level of hazard that is associated with silica dust.

All Dusts Like it or not, virtually all natural dusts contain crystalline silica. When you are walking beside the street, the dust kicked up by the cars, visible or not, probably contains crystalline silica. In trace amounts, our bodies can handle this, but we must be careful not to overwhelm our natural defenses.

You who work only with moist clays avoid this issue in ceramics. But some of us want to explore mixing our own materials, recognizing that most dry clays contain some crystalline silica.

Additions to CMC/glass mixtures Spurred by the issues above, I looked for ways to stiffen the glass/CMC mixtures that would not expose you to crystalline silica. I found that there aren't many ceramic raw materials completely lacking crystalline silica, as either a component or as a contaminant.

Frit. That's the answer. Ferro and other ceramic frits are formulated to add fluxes and other normally water soluble materials to ceramics in the form of very finely ground glass.

For an MSDS on Ferro Frit, go to http://www.lagunaclay.com/msds/3rawmat/cfrit/MSDS3c.htm and download ff3134.pdf That's the MSDS on Ferro Frit 3134. Note that the TLV is the same as glass.

To stiffen the CMC/glass mixtures, I have used Ferro Frit 3134, which has a fusion point of 1450F. In its dry form it is a glass, so it qualifies under the higher TLV. But it is a glass with very high viscosity, so it fuses to the glass but helps it to retain the form.

Try it. Mix dry
90 grams glass
2 grams CMC powder
10 grams Ferro Frit 3134
Add enough water to make a dough. I find that 35 grams of water works well with 100 mesh glass. Form a ball.
Dry it.

Do the same with 100 grams of glass, leaving out the Ferro Frit. Make another ball.

If you have the energy, do it a third time with 20 grams Frit and 80 grams of glass. Make a third ball.

Put all three balls in the kiln. Fire as follows:
1 hour to 1200 (slower is okay)
hold 30 minutes at 1200
30 minutes to 1600 (slower is okay)
hold 15 minutes at 1600
Note how some balls kept their shape better than others. Let this inform your glass/CMC work.

Please be safe.

July 27, 2005

Greetings -

I'm working with the Center for Environmental Economic Development and Bob Kirby on the Recycled Glass Kiln Arts Project. As part of our grant with the California Department of Conservation, Division of Recycling, we may be able to provide assistance to individuals or companies interested in starting up or expanding a business involving the production of recycled content glass art or other products. Our assistance may involve business planning, product testing, product process design or improvement, identification of equipment, facility siting and/or help with other issues.

The purpose of this email is to identify the level of interest in these services. Please contact me if you are considering a business venture involving recycled glass and would like more information on how we may assist you.

Thank you!

Edward Boisson

July 8, 2005

Today's topic: Test your glass

I speak in workshops about the importance of having a consistent source of recycled glass. By consistent I mean, ideally, the same type of glass collected through the same program, processed by the same equipment, and separated to the same gradation.

If any of these things change, it is very important to carefully test the new glass to see how it will work, even in processes you think you know. Don't do a big new piece you have a lot of time invested in the first time you use a new source of glass.

I have just had a humbling experience along these lines.

The glass left at the first three workshops was post-consumer bottle glass, crushed in a mobile crusher and not screened, with pieces from as large as 1/8 inch down to as fine as dust. After the Crucible workshop we distributed bags of glass that were processed by TriVitro Corp of Kent, WA. The glass originated as scrap from a window manufacturing plant, was processed in TriVitro's industrial-scale equipment, and separated to the consistency of fine sand.

The reason we changed sources was that we would like to be able to provide a consistent source of raw material for everyone's explorations. TriVitro processes glass as a business, and we know the glass they provide will be consistent and available.

My hope is to have a retail outlet or two for the glass at very good prices in the Bay Area. So far we have approached two retail outlets in the East Bay area. The store owners and managers have told us that they will consider stocking the glass, but my hopes are not that high. I guess I'm not a very good salesperson. If you have ideas for retail outlets, once again, please let me know.

Back to my little disaster. I was running some tests with thick "slip-cast" tiles, trying to get the glass to bead on the surface. I made two pieces, each using the following formula:
300 grams glass
150 grams OM-4 Ball Clay
100 grams water

The glass in one piece was 16 mesh and finer container glass

The glass in the other was the glass sand from the Crucible.

I poured the slip-like glass/clay mixture into plaster molds to dry. After drying the thick tiles I put them into a kiln and fired a little hotter than usual:
2 hours to 1950
30 minutes at 1950

The container glass piece is very nice. It has a tight surface with medium-sized glass beading on it. The piece with the glass from the Crucible is a molten lump in the bottom of the kiln. It's such a mess that I may need to replace kiln bricks. On the positive side, it looks just like milk-glass. It'd be cool to be able to make thick ivory-looking pieces like this kiln-casting with cheap materials. I'll try again with better containment.

Test. Test. Test. And protect yourself. Even if you have been generally able to form pieces and fire them free-standing in a kiln, if you change sources of glass, be certain to put an old tile or waste clay dish under the first trials of the new glass.

And those of you with the glass from the Crucible, keep in mind that this is very nice glass, a consequence of which is that it may fuse and slump at lower temperatures than you expect.

Please contact me with any questions or new info.

July 4, 2005

Today's topic: The reaction of Iron in clay and making glass bead on the surface

Another soda-lime glass paradox.

In the course of testing oxides as colorants for clay/glass mixtures, red iron oxide was put into a 50/50 mix of OM-4 Ball Clay and recycled glass crushed to 16 mesh and finer. At the same temperature at which the OM-4/glass mixture by itself fuses to below 8 percent absorption, the mixture including 5 percent iron oxide developed glass beads on the surface of the glass.

It was unexpected that iron oxide should flux glass, so additional tests were run. In particular, for one test, four samples were made. All had the following base materials:

The four samples contained

  1. 0 percent red iron oxide (0 grams in the tile)
  2. 1 percent red iron oxide (2 grams in the tile)
  3. 2.5 percent red iron oxide (5 grams in the tile)
  4. 5 percent red iron oxide (10 grams in the tile)

4-inch square tiles were pressed and fired together to cone 03.

  1. the sample with 0 percent iron oxide had a smooth surface
  2. the sample with 1 percent iron oxide had small beads of glass on the surface
  3. the sample with 2.5 percent iron oxide had medium-sized beads of glass on the surface
  4. the sample with 5 percent iron oxide had very large beads of glass on the surface. The glass almost appeared to have foamed.

Some people have expressed interest in being able to duplicate this glass beading phenomenon. Up until now, the samples that beaded were considered to be mistakes, so no great pains were taken to analyze the process.

The function of the iron in the clay/glass mixture is not yet understood, but that shouldn't stop people interested in this from duplicating the result.

The Iron oxide effect

Iron oxide is not generally a flux for granular glass. A variety of tests have been run coating granular glass particles with iron oxide suspensions. In no case was iron oxide seen to flux the glass.

Other tests were run testing whether iron oxide is a flux for the clay. At the temperatures used (up to 1950F), no fluxing action was seen for OM-4 tiles with and without iron oxide.

The current theory is that iron oxide somehow lowers the viscosity of the glass, causing it to more easily be squeezed out of the pores of the clay body. This seems to be different from a pure fluxing function.

Getting glass to bead on the surface

For anyone wanting to try this glass beading phenomenon, here are some tips:

1) Use glass that has pieces as large as 16 mesh. Even up to 1/8 inch. It's probably best to also include the glass fines in the mixture, otherwise the glass/clay mixes have a tendency to crack when drying. Tests run with only fine glass have not beaded.

2) Fire to 1950F or 2000F and hold for 30 minutes, or in a come kiln fire up to cone 03. At temperatures of 1900 and below, the beads that form tend to be very poorly attached and fall off with handling. At higher temperatures, the glass comes closer to a full melt and adheres to the clay with more tenacity.

3) 50 percent glass by dry weight seems to work well. Too much glass seems to suppress beading and too little glass makes the effect less interesting.

4) Try the effect with any dry clay. If it doesn't work, try adding 2.5 percent red iron oxide (by total dry weight) and try again. Red iron oxide really seems to promote the effect.

5) When firing, place the piece on a piece of waste clay or an old tile, because glass will also bead on the down surface, sticking to shelves very badly. Don't ruin your shelf.

More tests will be run to better determine the optimal mesh sizes to create this effect. This would make interesting thrown objects, but the mesh size probably needs to be finer for easy throwing.

Please let me know how it goes.

June 27, 2005

Today's topic: Availability of Calcium Aluminate Cement and Kiln Element Design

Chai Ng has been kind enough to tell us that calcium aluminate cement (fondu) can be purchased at:
Clay Planet
1240 North 13th St
. San Jose, CA 95112.
Tel: 408 295 3352

We are looking for a source in the East Bay.

Element Placement

I was recently asked about where the elements should be for recycled glass processes. We seem to fall into the crack between warm glass and ceramics.

Traditional small electric ceramic kilns usually have elements around the sides and are often top loading and quite deep. The ubiquitous 28" round is about three feet deep. Maximum temperatures range up to cone 8 to 10 (over 2300 degrees F).

Glass is more sensitive to temperature variations than traditional ceramics, so most warm glass kilns are shallow and often have elements in the top, or top and sides. Warm glass kilns often are only guaranteed to heat to 1700 degrees.

For the recycled glass processes, ideally we need a hybrid. These processes, whether it's mixing glass with clay, using methyl cellulose as a burn-out binder, or fusing pure soda lime glass, are more sensitive than pure clay to time and temperature. So I've found that shallow kilns work better. If you use a deep kiln, you may get different results between the top and bottom shelves in the kiln because of temperature differences.

We are also often operating at temperatures somewhat higher than 1700, so getting a glass kiln that only reaches 1700 seems to be working too close to the edges.

I also believe, but can only support anecdotally, that soda lime glass fusing works better with side elements. If you're using Bullseye or some other glass specifically formulated for fusing, having top elements pounds a lot of heat quickly into the glass. However, with soda lime glass's tendency to devit and lose sodium, I believe indirect heat is better.

I own a 3-foot square shallow kiln that was built for me many years ago. I knew even less then than I do now, and I let the kiln builder make it underpowered, with top elements only. As a result, it struggles to get to 1700 degrees. And it devits the heck out of recycled glass.

As it works to get up to temperature, those top elements are on for a long time. And they push a lot of radiant heat into the glass. Warmglass.com says that devit happens during cooling. This underpowered kiln is my best evidence that this is not true for generic soda lime glass. This underpowered kiln also has not enough insulation, so it drops like a rock once it's turned off. The devit that happens to soda lime glass in this kiln definitely happens as the kiln struggles to reach max temp.

To summarize:

  1. I believe the ideal small kiln for recycled glass processes has side elements. For a larger flat bed kiln, in order to avoid large temperature differences, you will need to have both side and top elements. I don't have any experience with bottom element kilns, but I'd love one to play with.
  2. I think the kiln should be capable of 2000 degrees F. That way if you want to try a cone 04 firing, or pre-fire a permanent mold, you can do it.
  3. I would have an electronic controller. My ideal is an electronic controller with external relay and kiln-sitter back-up, for both safety and the ability to do a straight cone firing now and then.
  4. Lots of power. 240 volts and around 1500 watts per square foot. You can't have too much power. Paradoxically, power actually saves energy, because with plenty of power you can get up to temperature faster.
  5. All of my kilns are top loading. I don't know about front loading, but the door better be tight for even heat.
  6. Maximum 12 inch depth.

One tip on kiln elements. All electric ceramics kilns I've seen have kanthal elements. My understanding is that Kanthal elements are alloys of iron, chromium, and aluminum. Kanthal elements make me crazy because once you've used them for a week they get brittle. If they fall out of the channel during firing and you try to put them back, as often as not you'll break them. I know, you can use a torch to make them flexible, but it's still a pain.

Some years back I went to a place in Seattle that winds elements for industry, and after telling the guy I seldom go over 1800 degrees, he suggested Nichrome elements. They are an alloy of nickel and chromium (a type of stainless steel), and they stay flexible indefinitely! I have nichrome elements that are six years old. I can still open the kiln and twist them back into shape at room temp, no problemo.

The surface temp of the nichrome elements is not as hot as kanthal, but when you're dealing in the realm of radiant heat transfer at these temps, wattage density seems to be more important, and the nichrome elements have been plenty hot enough for me.

Please send your questions.

May 31, 2005

Today's topic: Throwing pots containing Recycled Glass

Until recently I had no personal experience with clay pot throwing. But I'm taking a class in throwing right now and took some clay/glass mixture into class to try.

I used 70 mesh and finer glass because the 16 mesh and finer that I use for other purposes seemed too coarse on the hands. I made a mix of 50/50 by weight Redart Fireclay and 70 mesh glass. I used a solution of ten percent sodium bicarbonate for the water because I wanted to try self-glazing the pot.

I also filled my water pail with the sodium bicarb solution, so the exterior water would be alkaline.

In trying it on the wheel, I couldn't get the clay to stick. It kept jumping off the wheel. I assume this is because since half the mix was glass, it didn't have the adherence of 100 percent clay.

I asked the instructor, Sam Scott, a pretty well-known potter in the Pacific Northwest, to give it a try. Sam was able to throw a pretty nice pot. You can see it at http://www.recycledglasskilnarts.com/WkShopInfo.html. Click on "Using Recycled Glass with Clay Mixtures" and scroll to the bottom.

After throwing, I left the pot in the sun to dry. It dried with a fine film of salt on it. I fired it to 1750. It appears that the places the sun was more direct on the pot, the pot glazed more. It didn't glaze inside because there was no air movement inside the pot to draw the salt to the surface.

I picked it up by the bottom, but my clumsy finger touched one spot high on the pot. That spot didn't glaze because I rubbed the salt off. I could see someone masking off spots to not glaze intentionally.

The glass grade we will be giving out at the Crucible workshop June 17 will be useable for throwing because it is quite fine.

By the way, we have a couple of good leads for retailers willing to sell crushed glass in the Bay Area. I hope to announce a pick-up site for glass in the next couple of weeks.

May 27, 2005

Today's topic: Workshop Announcement and Supplies

We will be holding Recycled Glass Kiln Arts workshops at The Crucible in Oakland on June 17 and 18. The 17th will be a presentation session. On the 18th we will hold two "hands-on" workshops.

For information go to http://www.thecrucible.org/ and click on the Glass link under Classes.

We are trying adding the hands-on sessions because some people have said that they have a hard time working with some of the materials after they get back to their studios.

Attending the presentation session is a pre-requisite for attending the hands-on session, but if you want to attend just the hands-on and tell them you already saw the presentation part somewhere else, presumably they will let you in. I have no control over this.

As before, we will be providing a bag of crushed glass to each attendee. When this project was originally conceived, we had a small glass crusher mounted on a pallet that we planned to move to each workshop site, crushing glass to provide to participants. The logistics and expense of moving the crusher proved to be crushing, so we fell back on crushing the glass in Seattle and shipping it to the venue. Now that, too, has become more complicated and expensive because we have lost our excellent connection for shipping materials out of Seattle. It's always something.

So for this workshop we will be shipping recycled glass processed by TriVitro, a glass processor in Kent, WA, who crushes glass and sells it mainly as a blasting abrasive. The biggest differences between the glass provided at previous workshops and the glass we will provide at The Crucible are that this glass will be somewhat finer and definitely cleaner, because TriVitro is set up to do this.

To provide ongoing glass supplies to the Bay Area, we are looking for a ceramic supply retailer to work with on stocking supplies of crushed glass. Please reply if you either know, or are associated with a retailer who is conveniently located and might be interested.


May 4, 2005

Today's topic: Strength tests

I have been using absorption as the test to determine whether glass/clay combinations are stronger than clay/grog combinations because absorption is easy to do: all you need is water and a scale.

In order to get some real data on the actual strength of the glass/clay combos, we sent samples to the Tile Council of America (TCA) for testing.

Two sets of samples were made:

All samples were fired to Cone 06 (1830F) at 270 degrees F per hour. Each tile weighed 200 grams and was 3.5 inches square. All tiles were made in a manual press. No glaze was used.

The TCA tested against ASTM C-648, "Standard Test Method for Breaking Strength of Ceramic Tile." Ten of each type of tile were tested.

Average Breaking Strength for the clay/grog tiles was 257 pounds

Average Breaking Strength for the clay/glass tiles was 539 pounds.

ANSI A137.1 "Specification for Paver Tile (Unglazed)" requires a minimum average Breaking Strength of 250 pounds.

So the clay/grog tiles barely passed, while the clay/glass combos were more than twice the required strength for outdoor paver tile.

This is good evidence that the absorption test is a decent surrogate for strength tests.

We may have tests run using different base clays. The Redart seems to gain strength at a fairly low temperature.

I also think it would be a good idea to run tests with the self-glazed tile, to make certain the sodium in the body doesn't degrade the strength.

Any suggestions are welcome.

April 29, 2005

Today's topic: Self-Glazing clay

Whether you're interested in using glass in clay or not, please try this. I find it fascinating.

Dissolve 10 grams of sodium bicarbonate (baking soda) in 100 grams of water.

Combine 100 grams of dry clay with 100 grams of crushed glass, which can be as coarse as 8 mesh or very fine, but must include some fine particles.

Add enough of the water with soda in it to make a workable clay. Depending on the clay and grade of glass, it may take from 30 to 50 grams of water.

Fashion something with the clay/glass combo.

Dry out the clay/glass combo. Setting it on a heating plate usually works fine. You want the top to dry faster than the bottom, because while drying, the salt in the water is leaching to the surface. If it leaches out the bottom, the bottom will glaze and probably stick to the kiln shelf.

Put the thoroughly dried piece in a kiln, heat it to 1850 in three hours, and hold for 30 minutes. Cool.

The piece should be self-glazed.

I took a garden pot and some of the glass/clay mixture and hand-built a pot by pressing the clay into the inside of the old pot. After pressing it in (as evenly as possible), I let the pot sit in the sun until the clay dried and pulled away from the sides of the old pot. Then I removed the hand-built pot and fired it.

The hand-built pot fired to a high gloss around the top and to a softer gloss inside the pot. I assume that's because the top lip dries the fastest in the open air. I'm testing it for water-proofness right now. Presumably the inside could be made glossier by blowing air into it while drying. The outside of the pot is totally dull because it was covered during drying.

I have used white clay and added body stains in the form of Mason Stains and copper carbonate at about one percent of the dry weight of the clay and glass. The colorants leach to the surface and can be either mottled or uniform.

I'm not certain what's happening here. Obviously the salt is leaching to the surface. But I'm not sure whether the salt then reacts only with the glass on the surface to make the glaze, or whether fine glass also leaches to the surface with the salt, or whether the salt reacts with both the glass and the silica on the surface of the pot. This last may be the most likely.

If you make a thick or heavy piece, be certain to dry it really thoroughly, maybe even hold it in a cracked kiln at 300 degrees for an hour, otherwise salt can make the piece bloat. Please let me know how it goes.

April 5, 2005

Today's topic: Firing Glass/Clay mixtures using Cones

It was pointed out to me a few weeks ago that many ceramic artists use kilns equipped with cones, or manual pyrometers, rather than electronic controllers. Giving all of my tips in terms of times and temps was not helping them.

Of course the biggest difference between cones and electronic controllers is the inability to soak at a pre-defined temperature for a set period of time with cones. I have always soaked at the max temp to get temperature uniformity, and thus maturity, throughout the piece. I wasn't sure how the lack of a soak would effect the glass/clay combinations. These results will vary depending upon the thickness of the piece. My tiles are 1/2-inch thick.

I undertook to run a series of tests of glass/clay combinations using cone controls. I mixed one red clay base, one white clay base, and one grog-containing base.

I pressed 3-1/2 inch square tiles for testing.

One of the standards for cone firing ramp-up is 270 degrees Fahrenheit per hour, so I set the following as my firing standard:

I know the convention is to get up to 1000 degrees much slower, but I'm impatient and find that it doesn't matter in glass/clay combinations.

I tested at cone 05, 06, 07, and 08. Here are the results. I based tile efficacy on absorption, with the general assumption that absorption above ten percent is unacceptable:

It looks to me like Cone 06 is the best firing temp. At 06 the mixtures containing glass achieved a low absorption, indicating good strength, and very nice color. All of the straight clay/grog mixtures were too weak at these temperatures.

Again, using the Redart fire clay, the richness of the red achieved at low firing temps is really impressive. I'd like to try other non-white clay mixtures. I'd appreciate your recommendations.

I am sending samples of the Cone 06 firings to the Tile Council of America for strength tests. We will send out the results when we receive them.

A peripheral issue in this discussion is how to get compatible and attractive glazes that work with these tiles. This is an important question. We are having some commercial glazes tested by a tile manufacturer, and will get back to you on that.

This is a development process. I think we have demonstrated how to make strong and beautiful base tiles using 50 percent glass, while saving time and energy. However, there is already an infrastructure of support that has been created over hundreds of years in the development of equipment and glazes that work with conventional clay mixtures.

We need people willing and interested in starting to develop the same thing for glass/clay mixtures.

March 18, 2005

Today's Topic: Controllers vs. Cones

I received feedback from two ceramics artists after the latest TIP regarding firing times. In particular, it seems that most artists use cones or manual pyrometers rather than electronic controllers, making soak times difficult or impossible.

Point taken.

I have one kiln with an electronic controller with a cone-controlled kiln sitter as a safety. So I will run a series of tests using moderate ramp rates and cone shut-off to fire glass with clay. A future TIP sheet will give the results.

Any feedback you can give with regard to practical ramp rates is appreciated. I'll start with the following:
300 degrees/hour to 1500 degrees
100 degrees/hour to cone shut-off

I'll try a series of mini-cones, probably cone 07 to cone 04 for starters.

March 6, 2005

Today's topic: Getting a Nice Mold Surface

"Thus Conscience doth make cowards of us all. And thus the native hue of resolution is sicklied o'er with the pale cast of thought, and enterprises of great pitch and moment with this respect their currents turn awry, and lose the name of action."

I'm always amazed at how Hamlet, a fictional character created 400 years ago, nails the human condition today. In this case what Hamlet is saying to me is "Don't make assumptions. Run the tests."

I've given you bad information on making molds using clay masters. Most of my experience with making permanent molds is for production shops. The positives from which the production molds are cast are made of pourable rubber, which is both resilient and non-porous. Vegetable oil makes a fine mold release on rubber molds, simply providing a thin non-stick coating.

I've made few molds from clay masters, and I didn't have enough knowledge before I claimed in the workshops that you could get good permanent mold surfaces using vegetable oil on moist clay masters. Some people have tried using vegetable oil on moist clay, gotten grainy mold surfaces, and quit, "and enterprises of great pitch and moment with this respect their currents turn(ed) awry, and los(t) the name of action."

After the SJSU workshop two weeks ago, I had the opportunity to spend some time with Board Members of the Clay and Glass Arts Foundation, practicing some of the things I'd been preaching. Artists' running their own tests will naturally make their first masters from moist clay, because it's familiar, available, and cheap. I was unhappy with the coarse surfaces on the molds the Board members were making, so I decided that you needed better information. My particular apologies to Nina, Penelope, and April.

First I confirmed what I'd seen. I pressed the following four 300 gram 4"x4" clay tile masters:
Moist clay with no coating
Moist clay with a vegetable oil coating
Moist clay dried for 48 hours to a hard green strength with no coating
Moist clay dried for 48 hours to a hard green strength with a vegetable oil coating

I used the following standard mold making formula four times:
180 grams 16 mesh and finer recycled glass
60 grams calcium aluminate (fondu) cement
60 grams nepheline syenite powder (in the mix as an inert fine)

I added 48 grams of water from a bottle containing:
400 grams water
1 gram sodium silicate
2.5 grams sodium bicarbonate (baking soda)

First I mixed by hand, then I was able to pick up the mixture and make a ball of it. Squeezing and patting the ball, I was able to make a nice "thyxotropic" pancake. I put each of the four masters above in turn on an oiled piece of aluminum foil. Then by patting and shaping I got the mold mixture to take a shell around the 4"x4" clay tile.

I left the molds to sit for 24 hours at room temp, until the backs were dry and a fingernail would no longer dent the surface. Then I turned them all tile side up on a warming plate for three hours to let the clay dry out a little. Then, leaving the clay master in the molds, I put all of them into a kiln and fired to 1850 in four hours, holding at 1850 for 30 minutes.

Before putting them into the kiln, I noticed that both of the molds cast over the dry clay masters had cracked during drying. This is because the dry clay was too hard for the mold materials, which shrunk and cracked while curing.

After firing, the two tests using moist clay both had grainy, unacceptable surfaces. The moist clay seems to have pulled the fine cement particles off of the surface as it dried, leaving a rough texture.

The two molds cast over dry clay had much nicer surfaces, but both had cracked during curing and firing.

So how are we going to use clay masters to make molds? One possibility is to use a thicker sealing mold release on the moist clay. Maybe Vaseline dissolved in mineral spirits? But that's kind of nasty to work with and can cause a build-up, losing detail on the mold. The Vaseline may also have a tendency to hold cement particles, once again causing a rough surface.

Either Penelope or April reminded me that some ceramics shell mold processes coat the master with a finer material before adding the coarse blanket coat. Great idea.

I ran two more experiments.

For the first I used modeling clay. I'm not sure what modeling clay is, but I know it's clay with a non-drying oil base of some sort. Maybe that surface will release better. I used the standard mold mix above and cast a mold over the modeling clay.

For the second test I made a moist clay master with a vegetable oil coating. Then I made the following mixture:
50 grams fondu cement
25 grams water

This is what cement guys call a "neat" mix. All cement, no aggregate. If you leave a neat mix to cure on its own, it will shrink and crack.

I took an old one-inch paintbrush and brushed the neat cement mix on the surface of the moist clay tile. I waited about ten minutes, until the excess in the yogurt container I'd mixed it in began setting up. Then I backed the fine cement coating with the standard mold mix above. You want to let the fine mixture set up a little so it stays on the surface, but not dry out, because then it may not adhere to the blanket mold.

I let both molds sit overnight. Then I turned them over and let them sit for a few hours on a warming plate. The heat softened the modeling clay so much it dug out easily with a spoon. I left about half of it in the mold to see how it would look after firing. I did the same with the moist (now dried) clay mold.

I fired both molds to 1850 in four hours and held for 30 minutes. The surfaces look great! The mold with the cement "liner" in it is so nice that I may start making all of my production molds this way. I'll run tests using these molds to make sure they last.

My wife thinks I should write less, and just cut to the chase. However, I figure that some people will want to know the logic and testing that went into developing these processes and mixtures. It may even stimulate some other new ideas. I wish I'd had this info ten years ago.

Here's the Bottom Line:

To make permanent glass fusing molds with fine-grained surfaces:

1) Make the master out of modeling clay and cast the mold over it using the standard formula above. After about 24 hours, when the mold cement is dry, place the mold upside down on a warming tray. This will soften the modeling clay. Use a spoon to dig out the modeling clay, leaving a thin protective coating of modeling clay next to the mold surface. Save the clay dug out for re-use. Fire the mold to 1850. Hold at 1850 for thirty minutes. Cool naturally. Go in with a spatula and a toothbrush and clean out the rest of the dried-out modeling clay.


2) Make the master out of moist clay and apply a thin coat of vegetable oil. Make a mixture of "neat" fondu: 50 grams fondu 25 grams water Brush this mixture on the oiled clay master surface. Let it set up for about ten minutes, until the surface loses its sheen and it looks like it's thickening. Use the standard mold formula above to cast a mold over the neat cement. So the mold ends up being a pure cement surface with a mold mix blanket. After about 24 hours, dig out some of the clay, leaving a protective layer of clay next to the mold surface. Fire the mold to 1850. Hold at 1850 for thirty minutes. Cool naturally. Go in with a spatula and a toothbrush and clean out the rest of the clay.

Have fun.

March 17, 2005

Today's Topic: Productivity

At some point during workshops I generally assert "Give me dry clay first thing in the morning and I'll hand you a brick by nightfall." I have always been completely confident in this assertion because of what I've seen, but I thought it was time to do a timed demo.

The only tile size I have the capacity to press is 4" x 4". So I decided t make a 4x4 2-inches thick as the standard for half-brick equivalence. This is all about using glass as the binder to hold clay together, so I wanted to use a high temperature clay that wouldn't contribute to the strength or density on its own.

I used Dove Porcelain, a cone 6 dry slip formulated by Seattle Pottery Supply. I made 800 gram 4x4 half-bricks, which are almost exactly 2 inches thick.

I used the following formula:
400 grams 16 mesh and finer glass
400 grams Dove Slip
110 grams water

I pressed the 4x4x2 half-brick in a converted extruder. Here's my day:
7:30 am -- blend and press the half-brick

8:00 am -- put the brick into a toaster oven at 250 degrees

9:30 am -- turn the toaster oven up to 300 degrees

10:30 am -- put the green (hot) brick into a small (.75 cubic foot) electric kiln and fire to 1850 in three hours

1:30 pm -- hold at 1850 for two hours

3:30 pm -- turn off kiln, cool naturally

6:00 pm -- crack kiln, now at 770 degrees, about one inch

7:00 pm -- open kiln, now at 400 degrees, completely

7:30 pm -- hold the warm (about 200 degree) half-brick in my hand. Ta-daa.

The brick seemed very solid and weighed 757 grams. After being submerged in water over night it weighed 822 grams. That's 8.6 percent absorption. The absorption was greater than I wanted. The ASTM standard for freeze resistance in outdoor bricks is 8 percent.

So I ran two other formulations on EXACTLY the same schedule:

400 grams Dove Slip
400 grams 30 mesh and finer glass
120 grams water
This one after firing had an absorption of 7.5 percent. Within Spec!

320 grams Dove slip
480 grams 16 mesh and finer glass
100 grams water
This one after firing had an absorption of 7.8 percent. Within Spec!

So it's apparent that you can increase the density either by using somewhat finer glass or by increasing the percentage of glass.

And look what's happening here. The cone 6 clay isn't reacting at all at 1850 degrees F. So all of the strength is coming from the glass. And incompatibility isn't an issue because all of the materials are fine enough!

It was the issues of productivity and energy efficiency that got me interested in this in the first place. I assumed that greatly increasing both drying and firing speeds would fire the interest and imagination of both ceramic artists and ceramic production companies. Lower energy bills. Less work-in-process. Good for the environment.

Boy, was I wrong.

When I'm discussing things like energy and recycling with groups of artists, I look out to see more glazed looks than I saw during my short career as a math teacher. One young man at the SJSU workshop had such an exciting dream during my talk that I thought he was going to swallow his tongue.

We all naturally work within the limits of our comfort zones. Artists, especially those with a commercial gig that's working for them, naturally don't want to start messing with the formulas.

I'm just surprised that the productivity potentials of this haven't evoked more interest.

February 20, 2005

Today's topic: Water Reducers in Permanent Molds

In a past tip sheet I extolled the benefits of using water reducers when making molds using calcium aluminate cement. Minimizing the water in a mold mix does two things:
Strengthens the mold
Reduces the chance of drying cracks

I recommended using WRDA-19, formerly made by W. R. Grace. This was a water reducer used in the concrete industry. It is no longer made. The problem with using products intended for the concrete industry is that they tend to only be available in gallon volumes, which is enough for me to last ten years or more. And I make a lot of molds.

P* asked whether some of the deflocculants more common to the ceramics industry might work as well. In the interest of finding a new formula with ingredients that would be more accessible to everyone, I decided to run a series of tests.

The book Sculpting with Cement by Lynn Olson (Steelstone Press, 1988) describes water reducing admixtures like this: "water reducers, also known as superplasticizers, give each cement particle a negative electrical charge that makes the particles repel each other. This reduces friction between the particles, promotes better contact with water, improves (sic) workability and hydration."

In the book Slipcasting (A & C Black, 1997), Sasha Wardell, says "Deflocculants . . . alter(s) the charges on the molecules, causing them to repel each other which in turn renders the slip fluid." Sounds like the same function! So let's try some ceramic deflocculants.

I used the following dry mix as the standard:
150 grams 16 mesh and finer glass
50 grams calcium aluminate (fondu) cement
50 grams of custer feldspar
The feldspar is there only as a filler. I described the fines issue in an earlier tip sheet.

I tried the following:

Water with 2 percent WRDA-19.
Water Requirement: 40 grams
Comments: This is the control batch.
Result: Made a mold that dried and could be removed from the form in 12 hours at room temperature.
Judgement: Nice mold. Good surface.

Water with nothing in it.
Water Requirement: 44 grams
Comments: I wanted to see how pure water would do. It was fine. But this 4x4 mold wasn't big enough to induce much cracking.
Result: Made a mold that dried and could be removed from the form in 12 hours at room temperature.
Judgement: Nice mold. Some surface bubbles.

Water with 2 grams liquid Darvan.
Water Requirement: 40 grams
Comments: Darvan is an organic deflocculant that is said to be more forgiving than sodium silicate. Recommended dose is 1 to 2 percent of weight of dry material.
Result: Mold didn't dry in 12 hours. Darvan seemed to interfere with the curing of the cement.
Judgement: Failure.

Water with 1/2 gram sodium silicate.
Water Requirement: 40 grams
Comments: Sodium silicate is the most common ceramic deflocculant. Recommended dose is .1 to .5 percent of weight of dry material.
Result: Mold didn't dry in 12 hours. Sodium silicate seemed to interfere with the curing of the cement.
Judgement: Failure.

Water with 2 percent green soap in it.
Water Requirement: 40 grams
Comments: The literature for WRDA-19 describes it as a "dispersing mixture." I thought, well, soap is a dispersant.
Result: Mold mix was very stiff. Maybe even worse than water without additives
Judgement: Failure.

Water with .1 gram sodium silicate AND .1 gram sodium carbonate.
Water Requirement: 40 grams
Comments: The most common deflocculant in industrial ceramics is a combination of sodium silicate and sodium carbonate. The two chemicals are said to reinforce the deflocculating effects of each other. My logic was this: if sodium silicate interferes with cement curing, then maybe reducing the amount but maintaining the deflocculating characteristics will do the trick.
Result: Mold mix flowed very nicely. Made a mold that dried and could be removed from the form in 12 hours at room temperature.
Judgment: Nice Mold.

THIS IS IT!!!!!!!!!!

Reducing the amount of sodium silicate and adding a little sodium carbonate got the deflocculating effect without interfering with the curing of the cement.

As always, your results may vary.

It is very difficult for most of us to weigh out materials accurately to one-tenth of a gram. Here's my recommendation:

Keep this mixture for making molds. Shake before using.

These ratios are critical. When I had too much sodium silicate in the mix, the mold wouldn't cure. So if you don't have a scale that will accurately weigh one gram, find someone who does.

Personally, I do this in a two-step process. I keep a container in which I have combined 50 grams of sodium silicate with 150 grams of water. I label it "1:3 sodium silicate." So I use 4 grams from that container to equal one gram of sodium silicate. Sometimes when trying to measure small quantities, it helps to give yourself a little leverage.

Something I'd love to hear about from a technically oriented ceramics person. Deflocculants impart a negative charge to the particles in the mix, causing them to repel each other. Glass itself is alkaline, and imparts a negative charge. The runoff from a pile of glass is measurably alkaline because the sodium on the surface of glass particles is available for leaching. So shouldn't the glass in the mix cause a sort of self-deflocculation? If so, I haven't seen it.

February 14, 2005

Today's topic: Control Freak

This is a description of the current situation, a rant, and a plea.

Electronic controllers are ridiculously expensive. Over the years for various projects I have accumulated six, of various types:

I offer this description to demonstrate that I have tried a number of control schemes. When I look at catalogs of supplies for kilns I see nothing with good functionality less expensive than $500. That's crazy.

An immediate practical tip to the cheap, adventuresome, and moderately technically savvy: Love Controls (http://www.love-controls.com/index.html) makes relatively inexpensive proportional and on/off electronic controllers. They work well but you have to read the manual.

Aside from controllers, I have a number of kiln monitors called "Hoboes," made by Onset Computer Corporation (http://www.onsetcomp.com/). They are about the size of an Altoids box. You plug one into your computer and tell it to give you the temperature every minute for the next 12 hours, or whatever. Then you plug the Hobo into a thermocouple on your kiln. Twelve hours later you plug the hobo into your computer again, and there's a chart and table of the temperature in the kiln every minute for the past twelve hours! They cost about $80 each! By the way, I'm sending a copy of this tip to Onset.

I am amazed at the number of accomplished ceramic artists I meet that use kiln sitters or non-powered pyrometers and get great results. However, one thing about working with glass is that you need profile control. Working with cones generally doesn't hack it for two reasons:

First, glass often requires a soak at the max temperature.
Second, glass changes more than clay with relatively minor time and temperature changes. So if you want to duplicate what you just did, without good control and records you could be S.O.L.

Now the rant. There is no technical reason controls are so expensive. An electronic controller is simply a thermocouple with some electronics and some logic. You car is loaded with thermocouples. They monitor engine temperature, outside temperature, and maybe even seat temperature. The auto industry buys them for pennies each.

Here's what I want:

A controller the size of the Hobo described above

And it has two relay accessories:

You know that dimmer switch on the wall? The one that hums when the lights are on low? I suspect that's a phase angle SCR with a manual adjustment. They don't have to be expensive.

The final accessory:

So here you have a small, fast, controllable test kiln for $300.

I know some of you live in Silicon Valley. If you know electronics designers, show them the specs. A little P-ROM programming and a little logic and we'll blow them away at the next Glass Art Society conference.

Seriously, the current situation is an example of specialty equipment costing much more than it should. I have seen how hard it is for artists to run lots of experiments because:

  1. their equipment is large, slow, and tied up with projects
  2. they can't afford to buy good test equipment.

If you can't afford to run a lot of experiments and make a lot of mistakes, you can't develop anything new. We need as many people as possible running as many experiments as possible. Somebody please fix this.

February 8, 2005

Today's topic: fines vs. clay in mold making

Imperious Caesar, dead and turned to clay, might stop a hole to keep the wind away.

Well, Hamlet might have thought that Caesar would make a good plug, but the Imperious One would have made a lousy glass fusing mold. I guess we all have our talents. In an earlier posting, I described how I've come to feel that some fines are necessary in the mix to make a good permanent mold.

In particular, I suggested the following 200 gram mold mix:
120 grams 16 mesh and finer glass,
40 grams calcium aluminate cement, and
40 grams talc.

I used water containing 2 percent waster reducing agent WRDA-19, by W. R. Grace. Considering that W R Grace has since declared bankruptcy, the product may no longer be available. But something just as good is. I pre-fired the mold at 1850 for 30 minutes. That 4x4 tile mold has gone through 16 firings with no cracks, and still rings like a bell.

By the way, I misspoke in that previous posting on the mold thickness. The sides of that mold are 3/16-inch thick, not 3/8-inch. I have found that it's best to make the molds as thin and consistent as possible. In my experience most molds fail from thermal shock rather than strength. So thinner is better. And it uses less energy and is more responsive.

I have undertaken to determine what kinds of materials work as fines in this mix. So far, I've been successful with:
Very fine ceramic iron oxide
Granite dust (from a cemetery monument maker)
Quicksand dust from the flood plain in front of Anchorage Alaska (there's a story there, but I won't get into it) Custer feldspar

These are all very finely graded materials that become inert fillers after the initial firing. I find the feldspar to be interesting in particular. There is, of course, a whole range of sodium and potassium feldspars used in ceramics.

Things I've tried that have NOT worked are:
Ball clay
PV (plastic vitrox) clay
Dry slips
Calcium carbonate

The problem with the clays is that they compete with the calcium aluminate cement for water, driving up the water requirement, so the molds tend to crack during curing. The calcium carbonate doesn't work because it converts to calcium oxide, which interferes with glass fusing.

I've tried 200 mesh and 325 silica but didn't find them to work well. I think the problem may be that fine materials like talc are superfine, much finer than 325 mesh silica. The point of adding the fine material to the mix is to gain particle packing and plasticity, which the silica didn't seem to accomplish. I've also tried fumed silica, but it drives up the water requirement, seemingly because there's so much air in it. I really don't understand that.

So is custer feldspar a clay or not? Not being a ceramicist, I frankly didn't know. So I conducted a little test. I put 20 grams of feldspar into a paper cup, and added just enough water to get it level out. Then I put the cup into a toaster oven at 250 degrees until the feldspar dried out. I dropped the "puck" out of the paper cup and broke it. It had little if any strength.

If you conduct a similar test with kaolin or ball clay, you will see that the puck has considerable green strength. So I'd call feldspar an industrial mineral rather than a clay. Anything that attains significant strength after wetting, forming, and drying, will compete with the cement for water and should NOT be used.

Try making a puck with talc. Talc makes the whimpiest pucks.

So what does feldspar do during firing? My reading of the characteristics of feldspars is that they don't start reacting to heat below 1900 degrees. When pre-fired to 1850, feldspars seem to be just another filler.

This means that the old bag of nepheline syenite you have on the shelf will work as a filler for mold making!

February 2, 2005

Today's Subject: Vibration in mold making and the joys of Value Village

One of the lessons in the workshop packet describes how to build an inexpensive but effective shaker table. I realize that undertaking the construction of such a thing is daunting to many. However, I also think that if you are planning on trying to make permanent molds, you should find a way to experience vibratory compaction, just to see how great it works. If you plan to make production molds, you need to know about vibration.

I have heard comments on the glass/talc/fondu mold mix that the surface next to the master comes out too grainy and rough. Using vibratory compaction improves that and a number of other things. With vibration:

You need a mold box if you use vibration, otherwise the mold material will flow off the piece. But quick and dirty mold boxes can be made using cardboard boxes or plastic garden edging.

Shortcut to Vibration There is a Value Village near where I live. I believe Value Village is a national chain, but if you don't have one of those near by, you probably have a St. Vincent DePaul or somewhere else that sells used household items. Originally I was a little creeped out about going to a second hand store. But heck, it's not like I'm buying a mattress.

I stop by Value Village every couple of weeks. I've found it to be a great source for plastic mixing bowls, tableware for mixing cements, etc., and small handy appliances that I keep in my shop and don't worry about destroying.

I've purchased a blender, a big mixmaster, a couple of warming tables, a dehydrator, toaster ovens, a coffee mill for blending fine materials, and the latest best find: a hand vibrator.

I found a vibrator/massager (I guess) that has straps on the back of it for holding in the palm of the hand. It has low and high vibratory settings. It has a stainless steel surface, so I don't have to think too much about where it's been. And when I hold the vibrator up to the side of mold box, it does a terrific job of making the cement flow! And it cost $4. It reminds me of outdoor concrete guys using a thing they call a Stinger, which appears to be a vibrator on the end of a stick that they jam right into the mass of concrete to vibrate it for filling tight spaces. Sometimes local vibration is all you need.

January 28, 2005

Today's Subject: How the Percentage of Glass in Clay effect the final product

More is less.

You know that bread pudding you make, the one with the crisp crust? Then after years of making it, one time you tried to make it for a special occasion and it just didn't turn out right? So you tried again and the same thing happened? And you never really figured out what had changed?

I had that experience recently with glass in clay. I've been making samples of glass mixed with dry clay and fired for years. I had seen now & then instances where the glass had beaded on the surface of the clay body, but I always thought I knew why.

A few weeks ago every time I fired a sample, glass beads appeared on the surface. They were interesting at first. In fact using quite coarse glass you can get pretty big drops of glass on the surface. But all of a sudden everything I fired was beading. I began to question the practicality of everything I was doing.

So I went back and began looking at and comparing earlier samples. What I was doing when the beading was taking place was testing glass with a dry Redart fire clay. I don't know the specs, but it seems to be a pretty low-fire clay. I was firing at 50 percent glass rather than 67 percent glass because I was trying to confirm some observations from a brick manufacturer we're trying to convince to use glass. They are not interested in taking the chance to put more than 50 percent glass in their clay raw materials.

I noticed that most of the firings I was doing a year ago were with 67 percent glass, and mostly with higher firing clays. Is it possible that 50 percent glass beads on the surface and 67 percent does not? Seems contrary to logic. You'd expect the compositions with more glass to come out on the surface.

So I ran some tests. I fired samples at both 50 percent and 67 percent (by weight) glass in the same kiln at the same time using the same glass and clay. The 50 percent glass beaded, the 67 percent did not.

I speculate that when half or more of the mix is clay, and it's a low-fire clay, the clay attains some amount of strength at 1850 degrees, the same time the glass is softest. The clay body then squeezes the glass out of its pores. When there is more glass than clay, it is impossible for the clay to develop a matrix to squeeze out the glass. Amazing.

The effect doesn't happen with high-fire clays like kaolin. That also supports the theory above.

Try it. I have a small extruder I've converted to a tile press and have been pressing 4x4 tile weighing 200 grams. For you interested in making glass bead on the surface, as soon as I can get some good pictures, I'll make them available either through this list or on a web site.

I know glass fusing is hip these days, but of all of the glass processes I've worked with, I believe that the idea of using granular glass in clay blends has the best chance to be a transformational technology. The methods, materials, and equipment are already in place around the world to make ceramic products. Knowing how glass can improve both productivity and energy consumption is important to give impetus to this transformation.

Please share your experiences.

January 23, 2005

Today's Subject: The Importance of Fines in Permanent Molds

This is the first message going out to everyone on the Recycled Glass Kiln Arts List.

Sorry for it's having taken so long but as they say, Stuff Happens.

Anyway, this List is intended to be for answering questions and giving tips and updates on working with recycled glass in kiln processes. I'm the GateKeeper. When you send a question, it will go only to me initially. I will do one of three things:

  1. forward the question to the List,
  2. answer the question and post the question and answer to the List, or
  3. answer you personally.

Sometimes, like this time, I will post updates to the information from the workshop.

If you have questions or issues you do not want posted to the List, please just say so. I will do my best to answer you in private.

I thought about writing these Tips as separate Word documents and attaching to the email, but attachments create headaches for many people, so I'll just write a long email.

On this first Tip, I'd like to update the information I gave out in the first two workshops on making permanent glass molds. When writing an instruction sheet on something you've invested quite a bit of time in, you have to work to make the information as simple as possible, yet usable.

In the Permanent Molds chapter, I now believe that I simplified too much. I have found myself that just combining glass with Calcium Aluminate cement produces much too grainy of a mixture. I failed to convey it properly in the chapter. As a result, the formulas I gave there don't work very well. I am re-writing that chapter for future workshops.

You need some fine material to fill in the gaps between the cement and the glass and to create plasticity. However, in my experience dry clay does not work well as the fine material because it competes for water with the cement.

Ideally, the fine material added should be superfine powder, but it should not absorb water like clay, and should be stable and inert during pre-firing to 1850. Calcium Carbonate is no good because it calcines to calcium oxide, and effloresces.

Lately I've had good luck with talc. I've made molds using the following recipe to make a 250 gram batch:

150 grams 16 mesh and finer (window screen size) recycled glass
50 grams calcium aluminate (or fondu) cement
50 grams talc.

Mix the dry ingredients together.

Now another thing I didn't emphasize enough in the workshop. You want to use the minimum amount of water necessary to get the mixture just fluid enough to either work by hand or have it flow (under vibration) to nicely fill a mold.

I have found a water reducing agent to be necessary to get a nicely flowing mixture. You who work with cement know about water reducing agents. I like W. R. Grace WRDA-19.

I will be so strong as to say that if you are interested in making a mixture that you spread by hand over a master to make a blanket mold, you don't need a water reducing agent. But if you are serious enough about this that you want to make production molds, you need it. YOU NEED IT. YOU NEED IT. You can get it at any decent cement supply place. You'll have to buy much more than you'll need, so go in with others.

I put 2 percent by weight W R Grace WRDA-19 (it's a brown liquid -- may be lignin) in the water I use to make molds. I just keep it around pre-mixed in a bottle. It's permanently stable, as far as I can tell.

When using the water with the WRDA, here are the final batch amounts:

150 grams 16 mesh and finer (window screen size) recycled glass
50 grams calcium aluminate (or fondu) cement
50 grams talc.
40 grams water w/WRDA

Without the WRDA, I find that even with 50 grams of water, the mixture is workable by hand to make a blanket mold, but not vibratable. If you are working by hand creating a blanket mold rather than pouring into a mold, wear rubber gloves and work the mixture with just enough water so it balls up and flows only if you shake it (call thyxotropic). Then when you lay the clump of mold making cement mixture over the master, it will flow out when you pat it, but won't have excess water. Takes practice.

The mixture above, after pouring, will set up overnight at room (60 degree +) temperature.

It can be demolded the next day and prefired. For a 250 gram mold to make a 4"x4" tile, I fired as follows:

4 hours to 1850
30 minutes at 1850.
Thicker molds need longer soaks.

Including the talc in the mix greatly improves the surface texture and flow of the mold. I have one of these I've been keeping track of. It's a 4"x4"x7/8" deep tile mold with 3/8" thick walls. So far I've fired it 15 times and counting to 1650F with NO CRACKS.

Just today I poured a mold trying Dolomite as the fines. It poured and vibrated very nicely. I know there's calcium in there, so I'll let you know how it acts after firing.

Long email, huh? Well, whether I like it or not, making re-usable molds for glass fusing seems to be the topic that excites most people. I think it's worth the time learning a good and inexpensive way. Remember, you right-brainers, weigh everything. Weight is NOT the same thing as volume!

For those of you who have taken the trouble to read all of the way down to this point, please give me feedback on the accessibility of my instructions. It's how I learn.