Art 425 Ceramics
12 May, 2014
Cone 5-6 Glaze Test Results - Spring 2014
My exploration of glazes this semester focused on cone 5-6 glazes, as after I finish my degree here at Boise State I will be teaching art in a school setting. My access to a kiln most likely will be limited to an electric kiln – either at school or in my private studio, and firing at a lower temperature will be both more economical and more environmentally friendly.
I designed my work from Project 1 with the idea that they would be fired at cone 6, but the first glaze recipe I tested did not give the results I desired. The first glaze test used different colorants with a base glaze. I used a glaze recipe I found in the Ceramics Arts Handbook Series book, Cone 5-6 Glazes edited by Bill Jones. The glaze recipe I used is called Odyssey Base, and contains 20 parts gerstley borate, 10 parts whiting, 30 parts nepheline syenite, 10 parts EPK/kaolin, and 30 parts silica. I used test tiles made with two different cone 10 clay bodies – Death Valley and Clay Art White, and fired them in both oxidation and reduction at cone 6.
I tested twelve different colorants, including a line blend with chrome oxide. The main colors I was interested in were the greens to blues. In the oxidation firing, the copper carbonate 3%, the chrome oxide 0.15% and 0.5%, and the cobalt carbonate 0.75% gave me colors which I liked, especially the copper which gave a pale, translucent teal color. The chrome greens and cobalt had rich colors, but the texture of the glaze, especially with the chrome, became somewhat grainy. This was not as noticeable on the small test tiles, but when I used these test glazes on larger pieces, the graininess was more evident over a larger surface. Based on the information I learned from the John Britt video, I would guess that this grainy effect was caused by the chrome, which acts as a stiffener because of its refractory properties.
Because I was not satisfied with the results from the first glaze tests (and because one of the main ingredients for that recipe was temporarily out of stock), I used one of the pre-mixed class glazes for my second glaze test, which was a triaxial blend. I decided to try adding the chrome and cobalt colorants to a base of G-13 glaze, thinking that I would get some nice opaque teal colors form the mix of the two colorants that would also be lightened by the white base. As I was mixing the colorants with the glaze before firing, it looked as if the colors would mix as I expected, but when they were fired, I discovered the blends with the chrome turned a muddy brownish color rather than shades of green. I was not able to determine exactly why this happened. My research showed that zinc will turn chrome a brownish color, and that magnesium will make chrome muddy, but I was not able to find a reason for why the glaze colors in my test reacted the way they did. I will continue to research this.
Still searching for a cone 5-6 glaze for my pieces, I tested several of the cone 6 class glazes which use mason stains for the colorants. While the resulting colors were consistent in both reduction and oxidation firings, I did not care for the crayon-like colors the stains gave. The lighter Castile Blue was not bad, but the glaze was very runny. Adding a refractory to this glaze might help with the runniness. Another possibility would be to make the runniness work with the piece by creating deliberate channels in the form to direct the flow of the glaze.
After these tests, I turned again to the internet to seek out a glaze recipe that would highlight the textures on the surfaces of my Project 1 pieces. I found two possible recipes that had the blue-green colors I was looking for, and settled on a recipe called Lynne’s Glossy Grey from The Perkins Center for the Arts. It is a glossy glaze with lots of movement, and it looks great on textured surfaces where it breaks nicely. The base for the glaze is: 50 parts Gerstley borate, 30 parts silica, 15 parts EPK/Kaolin and 5 parts titanium; with the following colorants added: 2.5% soda ash, 2.0% copper carbonate, 1.0% red iron oxide and 0.75% cobalt carbonate.
The glaze looked great on both red and white clay bodies, changing color from rich warm browns to gray-blue flashes in areas where the surface of the piece caused it to pool. I sprayed the glaze on my tall coil pot after glazing the inside with Meyers clear. The over-sprayed area inside the top of the vase’s neck showed streaks of blues and grays. On the white clay the colors ranges from golden honey brown to blues and greens.
I learned a lot from my glaze tests this semester, and though most of the tests I tried did not result in a glaze I would use in further work, the final glaze (Lynne’s Glossy Grey) is one that I will continue to use for my work, and one I will use in my own classroom when I am a teacher. I will continue to research other glazes so I can put together a palette of glaze colors for my future personal and classroom use.