A lot of time, patience, experience and in most cases fun, go into introducing a new glaze to Lillstreet’s collection. Our newest edition, amber celadon, was brought to us by David Trost and Dave Bromely. Read on to find out just how it happened.
Somebody always has a glaze that they have seen, have used in the past, or were given a recipe for ('this would look great on your stuff'). Normally someone shows a formula to Karen, she considers it then gives it to Dave Bromely to check out as a possible glaze for Lillstreet. The first step in testing a glaze is just to look at the formula. Is it safe to handle by students? No barium, no lead, of course, and a few other chemicals that could be toxic. Is it a formula for a glaze that will be stable at cone 10 temperatures (2340-2377 F) which we fire our gas kilns to? The cones are a measurement of heat work, which is temperature and time together. The digital pyrometer is only reading temperature. Glaze melt is dependent on heat work (temperature and time) not just temperature. For example if you fire to cone ten according to the cone you have done the necessary heat work for the cone 10 glaze to properly melt. However if you fire to just a pyrometer reading between 2340-2377 there is the possibility that either not enough heat work was done (fired really quickly) and actually was a lower cone or too much heat work was done (fired slowly) and went to a higher cone. If the formula checks out on these criteria the next step is to test fire the glaze. Dave makes up a 100-200 gram batch, puts the glaze on a test tile and sees what happens. Normally the piece is dipped twice, the second time only on the top half, with some room at the bottom for potentially glaze runs.
David Trost brought in the original formula for the amber glaze. Karen was interested and asked Dave Bromely to test it out. The amber glaze color was good, but it ran down to the bottom of the test tile cup cup leaving a bead where the foot met the kiln shelf. Not good, but fixable. So here comes the glaze chemistry part. How do you make the glaze less runny but keep the character of the glaze? I added some clay and took out some silica. Why?
Glazes are made up of three basic parts, flux, silica, and stabilizer. Fluxes melt the mix. Silica provides the glassy surface. Stabilizers keep the glass on the pot and not on the kiln shelf. Fluxes include potassium, calcium, sodium, lithium, magnesium, borax, barium, strontium, and some others. Silica principally comes from sand or flint. Clay is the principle source for stabilizing glazes. All of these components come from mined and or manufactured materials that are readily available. Pretty simple, right? Well not really. In addition, we have the feldspars. Feldspars are naturally occurring deposits of rock that have different combination of all the above (fluxes, silica, and clay) mixed up together. Plus the clay is not just clay. Clay is mostly made up of silica, and alumina. Alumina is the stabilizing part of clay. But you can also have some fluxes and colorants (like iron) in the clay. All these materials can act differently depending on what form the chemicals are in. If you take the precise chemical composition of custer feldspar (a potassium feldspar) and create it with purified chemicals and then make a glaze with that it will not be the same glaze as one made with naturally occurring custer feldspar. Trust me. So what do you change?
Clear fluid glazes like amber celadon have the right balance of flux and silica but not enough stabilizers. To find out how much clay was needed to keep the glaze from running Dave Bromely ran a test in which he increased the clay component by 10% in five steps (that is 2% more, 4, 6, 8, and 10% more). He also decreased the silica by the same interval. He dipped test tiles in each sample and let the fire do its job. As the clay increased the running stopped but so did the shininess of the glaze. In the combination containing the largest amount of clay, the glaze was sort of flat, missing that shine. Plus there was one flat color, not the contrasting light and dark areas from the movement and pooling of the glaze that celadons' are known for. But in the middle, about 4% up, the glaze did not run and still had a decent amount of pooling. Bingo. That's where we wanted to be.
Next, Dave Bromely tested the amount of colorant (iron) in the glaze to see if we wanted darker or lighter. Just like all the other materials, different sources of iron give different colors. Dave varied the amount and type of iron using the same type of test as before. All together, it took six to eight different tests spanning several weeks to get our newest glaze nailed down for all the different high fire clay bodies we use at Lillstreet.
Since it is still a little runny, the bucket of amber celadon is in the advanced section with the Cohen red and rutile blue. Students must take classes for a full year before they can use the advance glazes- If you’re not familiar, ask your teacher.
So, next time you see Dave Bromely, say hello and thanks for all the hard work! What glazes should we introduce next?