Tuesday, March 10, 2009

Big 'Ole Gas Firing

These are some of the things that came out of the big gas kiln at the college on Monday. These two porcelain mugs I used a clear glaze over and their kiln neighbor fumed red onto each of them. I love how they came out. I'm keeping them both! The brown and white coil bowl I formed by hand (see earlier post) and the other is a small dish I made using a piece of lace for texture.

I was asked to explain the difference between several stages of clay so here goes. Greenware is clay that has been shaped into a mug or bowl or whatever, but it is has not been fired. If it sits out in the rain, it will dissolve.

Bisque is clay that has been fired at a low temperature. It is porous. It is not mature (vitrified -- turned into glass). We bisque our ware in order to shrink it a bit so that when we apply glaze and fire, the glaze 'fits' the piece. Sometimes if we glaze and fire to maturity, the glaze will shiver or craze ... we can fire from greenware to maturity, but it is a firing that is very long. Sometimes I do this with beads if I have a rush order. But the normal thing to do is to fire a greenware so that it can be further sanded and then glazed.

A glazed piece is a piece that has been glazed and fired. It is not always 'mature', ie., vitrified. I usually vitrify all my pieces. Some artists don't (and that's okay). For example, different clay matures at different temperatures. Cone 6 is about 100 or so degrees cooler than cone 10. If I am using cone 10 clay and I glaze fire to cone 6 with a cone 6 glaze, the piece is foodsafe, but it is not mature. If the piece chips, then it has to be thrown away because the clay body is revealed, and bacteria can get into those cracks and cause illness.

However, If I take cone 6 clay and fire it to cone 10, the piece will bloat and crack, and may even melt onto the shelf of the kiln. So there is nothing good that comes from that. But if I take a cone 6 clay and fire it to cone 6, or a cone 10 clay and fire it to a cone 10, then if the piece ever gets chipped, I do not have to throw it away if I don't want to. I can continue to eat or drink from it. It also has a better 'feel' if it is a mature/vitrified piece. That's my opinion that is.

Why Cone 10 or Cone 6? It is the rare potter who fires their electric kiln up to cone 10. Why? Because that extra 100 degrees (or so) are really tough on the coils that heat up in the kiln. Think of toaster coils and how they heat up to make toast. Well, that's kinda the same thing with an electric kiln. Cone 10 firings just make nicer looking pieces because the hotter a piece gets, the better the shine of the piece, the richer the colors.

Cone 6 firings are great, but cone 10 firings are great, too!

Same thing goes for gas vs electric ... the gas firings typically take a lot of the oxygen at some point from the firing. Electric firings are full of oxygen ... you can get some nice colors, but they are not reduction colors.

Wood firings are the ultimate OH MY firing experience. The firing usually lasts a couple of days and someone has to keep throwing wood in the entire time. The results are fantastically unique. Salt firings are something I haven't quite figured out. Raku is something I'm only experimenting with this semester.

Anyway, I hope that explanation helps a bit.

No comments:

Post a Comment