Tuesday, June 8, 2010

Cone geekitude

The kiln--filled to the gills with pots by Teresa, Rob, Lisa, and myself, in anticipation of this weekend's wood firing--is clicking away on the porch, slowly making its way toward that magic number, "cone 06." "Cone 06" is more or less ceramic-speak for ~1824oF--in this case, a comfortable mid-range bisque firing.

Why the word "cone"? In theory, my kiln automatically measures temperatures with thermocouples, but in practice, thermocouples and computer controls are only so accurate. If I really want to know that the kiln is doing its job, I have to use pyrometric cones--slender little pyramids of specially formulated clay. The cones are designed to melt after a given amount of heat work.*

Industry conventions number cones from 022-01 and 1-42, although most studio potters don't fire any lower than 019 (~1249oF) or much higher than cone 11 (~2361oF). Cones were originally numbered from 1 to 20, until people started inventing cones that measured values lower than cone 1. Cone manufacturers could have taken the same bold initiative displayed by the physicists who said "to hell with -273.15oC, for goodness' sake let's just call absolute zero zero Kelvin (after our pioneering colleague William Thomson, 1st Baron Kelvin)."** Alas, instead of resetting the scale, cone manufacturers chose to put zeros in front of the cone numbers and to count backwards from 01 to 022, so potters are stuck with this odd pseudo-negative labeling convention.

To get a sense of how well my kiln's computer is measuring heat work, I included a cone pack in this firing. The cone pack includes three pyrometric cones: 07, 06, and 05. If the kiln reaches cone 06, the 07 cone should be bent over; the 06 cone should be about halfway down; and the 05 cone should just be starting to bend. If I had put multiple cone packs into the kiln, I could observe differences in heat work in different parts of the kiln.

I had a cone pack in the previous bisque firing too, but I forgot that in cone-speak, 07 is a smaller value than 05. Order matters.

*Heat work is a function of not only temperature but also time. Consider the difference between a tray of brownies baked at 325oF for 40 minutes and a batch baked at 275oF for 90 minutes, or [oh dear] 325oF for 5 hours. Time matters. Of course, the depth of the brownie batter also makes a difference, as does the density with which pots are packed into the kiln.

**That's a liberal paraphrase.

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