Ceramic Pyrometric Cones

One of the trickiest and also one of the most exciting things about making ceramics is that once you put things in the kiln to be fired, you cannot see what has happened until they have gone through the firing cycle and have cooled down again, typically at least 12 hours later. There is no window that can withstand the kind of heat involved, and opening the door would be disastrous – the thermal shock would break the ceramics. Old kilns had no time or temperature controls so these little cones were developed and then commercially produced as a neat visual way of checking whether heat was consistent from firing to firing, and from the top of the kiln to bottom.

Cones also show you when an object or glaze has 'reached maturity' – the desired combination of temperature and the time spent at specific temperatures. The cone is placed next to a peephole in the kiln door so you can check its progress. Ceramic cones are numbered to correspond with what temperature they will soften and droop at, in increments of 20˚ C. They are precisely made from ceramic materials with different ratios of added flux, a melting agent that makes the clay soften at increasingly lower temperatures. Cones respond to the amount of heat-energy the ceramics have absorbed, rather than just temperature or time. They also behave differently according to the oxygen levels in the kiln environment, as do the ceramic glazes they are designed to mimic.

Often, three consecutively numbered cones will be used in a firing for accuracy. Their triangular elongated pyramid form also helps you to tell quite accurately whether the ceramics will have 'matured': the thinner tips will soften and droop first and eventually the whole cones will slump, with the tip curling to the base. Some are self-supporting with a triangular base which won’t melt, and some, (like these) need to be squashed into a soft clay base to keep them upright and prevent them sticking to the kiln shelf.