What is the Correct Firing Temperature?
This is a question that I am often asked. The answer is that there is no absolute correct temperature. It depends on too many variables.
Some of these variables are: the type of kiln (brick or muffle), the location of the door and the heating elements, the size of the kiln, the age of the kiln, the carbon you use, the amount of carbon you use, the number of times you have used it, the box you fire in (steel, fiber blanket, ceramic, ceramic cloth, lava cloth, fiberglass cloth, etc.), the number of times you have fired in the same box, and finally, the altitude.
There is a simple way to figure out the right firing temperature for your kiln, which is trial and error.
Use bronze clay to make some test pieces, 3, 6, and 16 cards thick. If you have a muffle front loader kiln, fire them at 1550°F/843°C. If you have a brick kiln, fire at 1470°F/799°C. You can refer to my instruction manual for other conditions, such as box, carbon, and length of firing.
Take note of the firing conditions in which you did your test: carbon, box, etc.
After firing check the pieces. If they look curved, blistered, swollen, or somewhat textured, it means that this temperature is too high. Lower the temperature by 20°F (10°C) and repeat the test. Continue testing until you get smooth, strong test pieces.
To check sintering, use slight pressure to try to bend the thinner piece with your fingers. If it breaks, it means the temperature was too low. Raise the temperature by 20°F (10°C) and repeat the test.
A thicker piece may not break easily, even if it is not fully sintered. To check thicker pieces, sand the surface with course sandpaper. If the piece is not sintered, you will immediately see a non-metallic mass under the surface. Again, raise the temperature by 20°F (10°C) and repeat the firing until you get a strong, fully sintered piece.
Once you have found the correct temperature, try as much as possible to stick to the firing conditions you noted prior to your testing.
Next, look at the different schedules for the other clays, and adjust them accordingly. Now you have a customized firing schedule for your kiln.
Conditions may change, of course, so it is a good idea to test sintering after every firing. Buff or sand a hidden spot of the fired piece to see if there is non-metallic matter under the surface. If there is, you haven’t lost the piece. Just re-fire, or repair and re-fire.
I have just come back from teaching a workshop in Grand Junction, Colorado. It’s high desert, 4,700 feet above sea level. Two things happened that I had never encountered before.
First, the same candle warmers that I use in my studio (at sea level) were a lot hotter. They melted plastic, while at sea level I can dry a piece with a straw in it with no problem. Also, some of the kilns were firing too hot.
This seems somehow counter-intuitive, since from what I understand, because of lower air pressure, the higher the altitude, the longer it takes to heat; cooking rice takes longer, and baking bread requires adding dry ingredients.
Since this question may be relevant to metal clay users who live in high altitudes, I would appreciate any input on the subject that I can post on this blog.
The other thing that happened, was that Baldwin’s patina darkened the copper a lot more than I am used to seeing. This may be related to the composition of the air at higher altitudes, so again, if you happen to know the reason, I would love to hear it and let other people know.