Pyrometric cones are how we measure the heat work in a kiln. Different types of clays and glazes fire to different cone ratings, anywhere from 1500 to 2500 degrees. Here at my studio we fire to cone 8. Many people fire stoneware and porcelain to cone 6 or cone 10. Terra cotta is fired to cone 04.
Cones are small pieces of clay, shaped into a 3 sided cone, that are formulated to melt and bend over at a specific amount of heat work. You put them in the kiln and take a look at them through the peep hole in the kiln wall to see if they are bending as the kiln heats up. Typically we use several cones of different numbers in a firing, to have visual indicators during the entire firing process. We do this visual check mostly with gas and wood fired kilns. Older electric kilns have a mechanical shutoff device called a Kiln Sitter that holds a small cone and automatically shuts off the kiln when the cone bends. No checking on the kiln required until the end, to make sure it did indeed shut off. Modern electric kilns have computers that do all the fancy logarithmic calculations to achieve the same results as actual cones.
So what is heat work? Heat work is a function of temperature over time. What that means is that it takes time for the heat in the kiln to have an affect on the clay and glazes. It doesn't only matter how hot you fire the kiln, it matters how fast you fire and how long you hold temperature (if at all).
Think of the ceramic glazes as a block of ice. There are 3 different ways to melt that block of ice (glaze) in an oven (kiln):
- If you put that block of ice in an oven and bring the temperature up at a rate of 50 degrees per minute, the block of ice will be completely melted at say, 250 degrees.
- Now do it again and bring the oven temperature up at 50 degrees per minute again, but hold at 150 degrees. After a certain amount of time the block will melt, even though the oven didn't reach the 250 degrees it did before. Less temperature, same result (heat work).
- Now do it again but bring the temperature up at 100 degrees per minute until the ice block is completely melted. The oven will reach a higher temperature before the block completely melts because the temperature is climbing at a faster rate. Same result (heat work), but different temperature and time.
This confuses a lot of potters, but it's important stuff to know because firing to the wrong cone can have catastrophic results. For instance, firing to cone 5 instead of 05. Cones below cone 1 have a '0' as their first number- think of the '0' as a negative sign. So cone 01 (-1) is more heat work than cone 05 (-5). Cone 5 is a lot more heat work than cone 05, and clays that are formulated to fire at cone 05 will melt into a puddle at cone 5. That's right, the clay will melt. And it will ruin the kiln shelves and possibly even flow into the bricks and elements of the kiln wall. Big mess.
So that's how cones work. Cool, eh? Feel free to post questions in the comments.
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