Matte Crystalline Pieces
The most difficult and challenging pieces pots that I have ever made are the matte crystal porcelain ones. Even after twenty years of unceasing pursuit of and experimentation with them, the matte crystalline glazes continue to elude me, refusing to yield any kind of consistent performance. A piece may take as many as a dozen firings to be completed or destroyed (about one year of firings for me). They do not respond to logic. Nevertheless, I love them and cannot stop working with them, and I have been able to solve a number of problems associated with them.
My glazing process is to first apply glaze to the inside, then apply wax-resist to the inside of the piece (if I'll be dipping the outside) so that the outside glaze will not overlap the inside. For the first firing, I usually dip the outside, using a tool constructed from a plumbing supply shop (read on).
A major problem with matte crystallines is the fact that any imperfections in the glaze surface will be magnified in the final piece. What I'm aiming for is a surface that doesn't show process marks (e.g., lines formed in the glaze by pouring, dipping, or spraying, or finger marks in the glaze) so in the early days of my crystalline experimentation, I was frustrated by glaze application as well as formulation. My Eureka experience came while surfing a plumbing supply shop one day, when I discovered Fern-Co bushings. I was able, using these bushings, some aluminum inserts, and a vacuum pump, to develop a system that allowed me to hold a pot by its foot using a vacuum. The beauty of this method is that it eliminates process marks. I don't need to touch the surface of the pot to dip it into the glaze; it is held by vacuum to this interchangeable set of tools.
Once the pot has been dipped into the glaze, I measure the thickness of the glaze on the piece and adjust accordingly. This brings us to another major problem that was solved by another piece of equipment, the glaze micrometer.
I realized, through trial and error, that my glazes were sensitive to thicknesses of one-thousandth of an inch, and that, for the piece to have a chance of success, the glaze must be applied to within three-thousandths of an inch of the ideal. I thinned down the glazes, but needed a tool to measure the thickness of the raw glaze on the pot. Eyeballing the glaze was not sufficiently precise, nor was scratching through the raw glaze with a pintool or using any of the commonly available thickness depth gauges. Finally, I modified a very sensitive Starrett gauge to measure the thickness of the glaze surface to within a half-thousandth of an inch. This tool is probably the most valuable in my studio, and I use it every time I glaze.
After glazing the pieces are loaded into the kiln. The placement of the pots in the kiln greatly influences their final result. I've learned that certain parts of the kiln are better for certain glazes. Much of the variation has to do with atmosphere. I fire in a reducing atmosphere, and the flame ebbs and flows in a liquid way around the pots in the kiln; it lingers and pools in some places more than in others. Heating and cooling also varies in different parts of the kiln. Certain glazes like to be fired hotter or cool more slowly than others, and I've found places in my kiln to put pots with those glazes.
I fire to a temperature of around 2350 degrees Fahrenheit (what potters call cone 10). After one firing, the matte crystallines are almost never successful, but the glaze has a "used" quality and is usually not improved by refiring as is. Applying more glaze helps improve the odds, but it's difficult to apply glaze after the piece is already fired, because the surface is vitreous. To solve this particular problem, I developed the following process: I carefully grind down the roll of extra glaze around the foot, heat the piece to 350 degrees Fahrenheit, remove the hot piece from the kiln, and spray more glaze on it while hot. The glaze dries almost immediately without running. The pot is then fired again to cone 10 and, if it is again not successful, the glaze is ground down and the piece is heated, sprayed and fired still again, and so on.
This inelegant glazing process is still evolving, but it is the best I have come up with to date. I'm hooked on the glazes and compelled to continue trying to work out the process. When a matte crystalline piece emerges from the kiln successfully, I believe that there is nothing more beautiful in the ceramic kingdom. The way the crystals form is so organic and mysterious that I am sometimes just at a loss for words to describe them. The glazes seem to reference the microcosmic and macrocosmic processes that we find happening all around us, and this reminds me of who I really am, and what it is that I'm actually doing here. I find them very contemplative, and if I can bring a bit of that sanctuary into someone's life, then I feel the pieces are a success.
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