In 9th grade in high school, my "home" teacher was our chemistry teacher. I can't remember her name, but I can clearly see this short and plump lady cheerfully rolling into the classroom with absolutely no desire to teach chemistry to the just-as-disinterested students in the room. We were bored with anything school related. She, on the other hand, was on a mission. A nationally ranked bridge player, our chemistry teacher had only one thing in mind--to turn all her students into bridge enthusiasts. Her project was a total success. By the middle of the school year, my classmates skipped recess and free (and not so free) periods, moved around tables and, arranged into groups of four, hovered completely absorbed over their sets of cards. I never took to bridge. I did not learn the game, nor did I learn any chemistry.
I've always worked in community studios, where the staff is responsible for glazes, so I have limited experience with anything glaze related except the actual dipping of the pot into the glaze bucket. The chemistry of glazes, the process of mixing glazes, testing glazes to achieve specific characteristics, and manipulating glaze responses to various firing environments -- about all these, I gathered over the years bits of information here and there. Glazes that have iron do this...Glazes that have cobalt do that...I wish I could say that I have developed a systematic understanding of the characteristics of the glazes in the studio, so I could use them effectively and creatively. Really, I can barely remember the names of the elements (bridge vs. high school chemistry...)
I like these boxes. The blue and white glazes created a fluid surface and the red a white resulted in a pretty cool speckled surface. What happened here?
People in the studio (Hyde Park Art Center) are always gathering near the shelves where pieces fresh from the kiln are placed. Everyone is excited not only to see their own work, but also all the other glazed pieces. "Oh, look at this one. This is gorgeous, What glaze is it here?.." "This glaze came out very differently from last time..." and so on. People are generous in sharing the combinations they used and there is a communal expansion of our familiarity with the glazes we have in the buckets at the studio--how they work in combinations, the successful and what we should avoid.
In the two boxes in the pitcures I put a matte white glaze (called Mammo White) over a shiny glaze that tends "to run". Runny glazes are the enemy of the kiln shelf and studio budget. Kiln shelves are expensive and runny glazes melt down the sides of pottery until they stick to shelves and ruin them. Runny glazes have to be used carefully. The blue and red glazes I used for these boxes have harmed many kiln shelves....The white matte glaze is not runny. It is actually very stable. It turns out, as we collectively discovered in our studio, that a combination of the stable white and any of the runny glazes creates a fluid and speckled surface, as if the white is fighting the shiny, pushing up against its tendency to flow down toward the shelf. The opposing forces of the glazes produce beautiful results. In the vase below, on the other hand, I used a runny glaze alone over the white vase decorated with black slip (a paste of colored clay). There was no opposing force, and the glaze reached the shelf (see the glaze bump on the bottom). I should have known better...
Chemistry -- So it is time for me to begin learning about glazes. Glazes are made of four basic parts. Silica is found in the earth and it can turn into glass if heated to 3100 degrees F. That is very high temperature--even ceramic kilns don't get so hot. Then there is a flux material that lowers the melting point of silica, allowing it to turn into a glaze (or glass). There are many flux materials and each behaves differently in the glazing process. It is good to know how various fluxes relate to characteristics of different glazes. The third component is a refractory -- a material that remains strong even in high temperatures and stiffens the glaze, helping it stick to the pot, rather than run down with gravity and ending up on the kiln shelf. Finally, there are materials that give the glaze its color--usually these are metallic oxides and other variations, such as transparent vs. opaque. The interaction between the four components influences the appearance and feel of the glaze. Other factors are also at play--the kiln environment, the chemical composition of the clay body--but clearly it makes sense to know more how the very basic ingredients of glaze interact. I'm on it.