Firing Temperatures, Glaze Considerations and Effects

Like many of the slab ashtrays illustrated on these pages, the method of making them is extremely simple for beginners and experts. See the detailed explanation in section #3 called Making Slab Pieces.

This slab ashtray, seen from a bottom view was made of red, stoneware clay. The word stoneware, means the clay can easily take temperatures to cone 7 (very hot) and the clay forms a stoneware nature that is almost impervious to water. Whereas the same clay fired only to bisque temperatures (like cone 06) will still absorb water and must be completely glazed and sealed if they will be immersed in water, such as washing. (You may want to see the complete page on Firing Cones and Temperatures for reference.) This ashtray was NOT fired to stoneware temperatures, and a low temperature glaze was applied, so the foot area with no glaze on it would absorb water, which could damage the ceramics from the inside if this should happen. I could have just as easily fired the clay to stoneware temperatures and this would not be a factor. But a consideration is, that the glaze does not stick to a stoneware surface very well (remember, it's almost impervious to water?). So trying to put a low fire glaze on a stoneware fired pottery is quite difficult.
The proper way to do stoneware is to fire the clay to bisque hardness, then put a high temperature stoneware glaze on it and fire it all to cone 6 or 7 and you will have a glazed stoneware piece that is waterproof and safe to use for eating or drinking. The low fire glazes MAY be hazardous and you need to read labels carefully and if it does not SAY safe for food and drink, DON'T. As a rule of thumb, the CLEAR, low-temp glaze, often used for lining the inside of eating pottery, is lead free and safe. It's the colored glazes you have to be careful of if fired to low temperatures. Fired to high temps, the lead will all burn away (but so does a lot of the color and shine).
Note that NO glaze is applied on the "foot" to keep it smooth and flat. Glaze on the bottom of the foot will run, stick to bottom of kiln and other bad things. Don't glaze the part of the piece that touches the floor. Now, with low fire pieces that will be used for eating and need to be washed, they MUST be glazed all over and put in the kiln on stilts. These are little ceramic pieces with sharp nails sticking up upon which to place the fully glazed piece. After glazing, you pop the stilt off easily, and with a high speed grinder, can smooth the little "pricks" of glaze from the bottom. This is the alternative method to handle the foot or bottom of pieces.
For purely decorative pieces, simply glaze all over and put the part of the piece that will be the bottom or the least seen/used part on the stilts. If it is the bottom of a heavy piece like a bowl or ashtray, after smoothing the "pricks" from the bottom, you can glue felt on the bottom (or use the felt that you can buy with self-stick glue on it. This will completely prevent scratching of delicate wood table tops.
FIRING TEMPERATURES, CONES and TIMES:
The first step is heating up the kiln and driving out residual moisture in the clay and glaze. Even clay and glaze that has air dried for a week, will still have some moisture in it. If you bring it up to high temperatures too fast, the escaping moisture may cause the already melted glaze on the surface to bubble and look terrible. Also, the clay will slightly change shape, and if a joint or section was not very securely attached, it will fall off at this time (like the handle to a coffee cup). So...start off on LOW fire temps with the top of the kiln propped open about an inch. After a few hours, switch to the medium temp. A few more hours and you switch to the high setting and after about 30 minutes, close the kiln tight and plug the peepholes. This will insure you drive out all the moisture before getting near the glaze melting temperatures and bring the pottery up to heat slowly to prevent cracking and breaking of the pieces.
When it is said that a piece is fired to cone 07, it means fired on high in a kiln until a cone of 07 density melts. Cone 7 is much higher temperature than cone 1, but cone 07 is a much lower temperature than a cone 1. If the cone number has a "0" in front of it, think of that as a "minus" sign. Cone 08 has a much lower melting temperature than a cone 01. So with whole numbers the higher the number, the hotter the temperature. When a "0" is in front of the number, it's a lower temperature with the higher number.

It's possible to regularly check thru a peephole until you see the cone (usually triangular and larger on the bottom than top) slump over at the top where it is melting. That means the kiln has reached the proper temperature for the glaze you desire and you turn the kiln off and let it cool slowly. The other type of cone is designed for a kiln-sitter. Here the cone is a triangular bar, the same size on both ends. It lays across two bars with a rod that rests on the top middle of the cone. This rod is connected to a kiln cut-off switch. When the cone sags in the middle, meaning it has reached proper temperature, the rod moves down and cuts off the kiln automatically. I highly recommend this type of cutoff.

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Please send any comments to M.D. Smith (mdsmith@HiWAAY.net)