Pyrometric cone

A pyrometric cone looks like a small tapered spike of clay, usually a few centimeters tall, but its job is to measure something a thermometer cannot, the combined effect of temperature and time known as heat work. Cones are numbered, with a large range spanning from very low fire numbers into the low teens, and each cone formulation is engineered to soften and bend over at a specific, repeatable amount of accumulated heat, not simply at one fixed temperature reached instantly.

This matters because two kilns can both reach the same peak temperature on a controller display and still fire very differently, one climbing fast and holding briefly, another climbing slowly and soaking longer, producing different results in the glaze even though the number on the display matched. Placing witness cones, angled slightly, inside the kiln or visible through a peephole shows the actual heat work delivered to that spot in that firing, which is far more reliable evidence than trusting an electronic controller alone, especially as kiln elements age and lose some power.

Most home studios use small self supporting cones or cone packs for a quick visual check through the peephole, and match cone numbers to whatever a clay body or glaze data sheet recommends rather than guessing. A common beginner mistake is confusing cone numbers with a simple hotter is higher scale in a casual way, when the practical habit that matters is simply firing to the cone number specified for the clay and glaze in use, and watching that cone bend rather than trusting temperature alone.