Crystalline Glazing

 

In the spring of 2011, Blake and I traveled to India. We were sitting in a cafe in Dharamsala when I got an e-mail notifying me that I had been selected to serve as junior staff at Not Back to School Camp later that year. While I had been asked to staff at the first session, Blake had been invited to the second session as well. We planned on traveling to the first session together, and we had commitments after the second session ended, so I suddenly had a two week period during which I would need to occupy myself on the West Coast.

I immediately knew that I wanted to spend the two weeks doing something related to pottery. I started poking around online and found the website of the fabulous Dale Donovan in Corvallis, Oregon. He made crystalline glazed pots on a Christmas tree farm slightly outside of Corvallis- a town where I had lots of friends! I had been fascinated by crystalline glazing since I had first seen the technique the previous spring while attending Craft Boston, and I was determined to learn how to do it.

I immediately drafted Dale an e-mail, asking if I could come work in his studio and learn how to make crystalline glazes from him. I didn’t expect to hear anything back: I had needed to do quite a bit of fishing when searching for my first apprenticeship the previous spring, and I didn’t think I would luck out on my first try- it was just too perfect.

Much to my surprise, Dale e-mailed me back within a couple of days, telling me that he had worked with many apprentices over the years and that he would love to have me in his studio. When was I going to be in the area?

It was amazing how quickly things fell into place after that. Some family friends in Corvallis graciously opened their home to me, and they even had a bicycle I could use to get to my apprenticeship each day.

Once the apprenticeship began, I was very busy. I slaked down hundreds of pounds of clay in five gallon buckets and mixed the resultant glop with dry trimmings in a pug mill. It’s difficult to do this without creating results that are too dry or too wet, and my initial results were so hard that they clogged the pugmill’s mesh screen. As a result of that I disassembled the entire pug mill, cleaned it, greased it, and assembled it again. In addition, Dale asked me to organize his gallery- a project I adopted with relish.

In return, he showed me how to produce successful crystalline glazes. He threw a series of large and small vases and bowls on the wheel, and I did the same. We bisque fired them, and then I mixed a series of 22 different glazes in 500 gram batches. We used those glazes to run glaze tests on small test shapes and recorded the results. Then we selected a few glazes that we liked the best and I mixed them in larger quantities. Finally, we applied those glazes to our bisqued pots and fired them on pedestals. When they came out of the kiln we separated them from the pedestals and used a bench grinder and then a lapidary wheel to smooth and finish the bottoms.

By the time the two weeks ended, I was totally confident that I could go home and develop my own line of crystalline glazes, and that’s exactly what I did. I had never even mixed a glaze before, but I ordered a precision kitchen scale and a crystalline glazing book and I began buying things like Titianium Dioxide and Alumina Hydrate in bulk at the local pottery supply store.

My first few batches of crystalline pots were inconsistent, and when I got the glazes working properly I went on to have difficulties separating the pedestals from the bases of the pots. Less than a year since I started making crystalline work on my own, however, I can say that I have a line of crystalline glazes that give me consistently good results.

If you were just interested in hearing about my apprenticeship, stop reading now! The next section of this blog post contains tips and technical details directed at potters (chemists might be interested as well).

Here’s a basic formula for a crystalline glaze:

-50% Frit 3110 (A Frit is a combination of minerals that have been heated until they turned to glass, and then ground back into a powder. It works to lower the melting temperature of the glaze.)
-25% Silica (Silica, also known as flint, is what sand is made of. It melts and turns to glass.)
-25% zinc oxide (this is used to create crystals.)

Most crystalline glazes are variations on that theme, with other minerals sometimes added in for different effects. My base recipe also includes titanium and alumina. When fired, the silica and zinc in the glaze combine with one another chemically and form zinc silicate crystals.

The formula I provided above creates a white crystalline glaze. To color the glaze, you add one or two colorants in a concentration almost always below 10%. Some minerals you might use for color include iron, copper, cobalt, and nickel. Most of these colorants are toxic in their powdered form, so it’s important to wear a respirator when mixing.

Mixing glazes is a lot like baking a cake. Once the dry stuff is sitting together in a bowl, you add the liquid measure (water, in this case) and combine. I use a hand-held immersion blender to mix my glazes, and I don’t sieve.

Next, you brush the glaze on (or dip, but I make small batches so I brush). I do three coats with most glazes- four or five if the glaze has a higher metal content for complicated reasons.

Finally, I find a pedestal that matches the diameter of the foot of the piece that I’ve glazed, and I use a mixture of Elmer’s glue and kaolin (a mineral found in porcelain clay) to glue the piece to the pedestal. Crystalline glazes don’t have any alumina in them, and alumina is used in most glazes to make the glaze stick to the side of the pot instead of running everywhere. However, alumina also inhibits crystal development. The result of not including alumina is a really runny glaze that needs a pedestal below it to catch excess glaze that runs off the pot. Otherwise, you ruin your kiln shelves. (Even with proper pedestals, it seems like I’m constantly ruining kiln shelves.)

Next the piece goes in the kiln (with a bunch of other pieces) and goes through the firing cycle. My kiln has a very precise computer built into it, so I program it to run exactly the schedule that  I want. I’ll fire the first 1800 degrees at 308 degrees per hour, and then decrease the speed to 108 degrees per hour. Finally, I let the kiln “soak” at the peak temperature (about 2350F) for 15 minutes. Next I drop the temperature rapidly and let it sit at 2050F for two hours, and then 1950F for 30 minutes.

There are four major factors that affect the kinds of crystals you will get in a crystalline firing. Varying just one of these factors by a small margin can create totally different results which is awesome and infuriating. The important factors are:

1. Firing schedule: For example, I can decrease the soak that I give the kiln by as few as nine degrees, or vary the time, or vary the speed of the approach to peak temp, and my results change drastically.
2. Glaze formula: Changing proportions of glaze ingredients creates big appearance changes, for obvious reasons.
3. Glaze thickness: It’s counterintuitive, but a thicker glaze application gets me fewer crystals, and vice versa. A really thin coat will give me millions of rough, ugly crystals all over the pot’s surface and a coat that’s too thick will result in no crystals.
4. Clay body: Crystalline is almost always made with porcelain clay, but different kinds of porcelain have different properties and can create different results.

The day after I start the kiln, I unload it. I wait until it has cooled to three or four hundred degrees, and then I pull the pieces from the kiln one at a time and submerge them in cold water. Subjecting them to such severe thermal shock helps make the joint between the piece and the pedestal crack apart seamlessly. This is the part of the process where pieces are often broken, but the cold water technique has been working well for me. After running the pieces under water I tap the pedestals on the ground and they usually separate from the pots.

The final step involves grinding the bottom of the piece to a smooth finish. I use a bench grinder first, then a silicon carbide sanding disc that’s attached to a throwing bat. Once the piece is totally smooth on the bottom, I wash it thoroughly and it gets photographed, priced, and listed on etsy.

This technique is perfect for me: It requires a lot of precision, cataloging, and experimentation, and I like that about it. It’s also a nice change from the more creative, artistic, fly by the seat of your pants work that happens at the wheel. Thanks for reading this far!

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Categories: Uncategorized | 3 Comments

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3 thoughts on “Crystalline Glazing

  1. Hi……….

    This post is really nice and informative. The explanation given is really comprehensive and informative. I am feeling happy to comment on this post.

  2. Hi………

    Thanks for sharing your info. I really appreciate your efforts and I will be waiting for your further write ups thanks once again.

    Glazing India

  3. Hi………………

    Thanks for sharing your info. I really appreciate your efforts and I will be waiting for your further write ups thanks once again.

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