Cone Guide
A pyrometric cone is a slender pyramid of glaze-like material engineered to bend at a precise amount of heat work — the combined effect of time and temperature. A pyrometer reports how hot the kiln is right now; the cone reports what the ware actually received. This guide shows how cones bend, how a cone pack is read, how the numbering works, and the values that matter in the studio.
Mounted at an 8° tilt.
≈ 15 – 25 min from maturity.
Tip at 3 o'clock.
Tip level with the base — properly fired.
Past the target — slumped flat.
Left: one number cooler. It falls first and announces that the finish is near.
Center: the target. The firing is done when its tip reaches the level of the base.
Right: one number hotter. If it bends, the firing went too far.
The scale runs 022 up to 01 (cool side), then 1 up to 14 (hot side). The two sequences meet in the middle.
Cone 06 is "minus six": the higher a zero-number, the cooler; the higher a plain number, the hotter.
The classic mix-up: cone 06 ≈ 998 °C, cone 6 ≈ 1222 °C — 224 degrees and two different worlds of ware apart.
Built-in base, set at 8°. The most reproducible witness cone — the values in the table below.
The classic witness cone, wadded into a clay plaque at 8°. Same readings, more care in mounting.
Stiffer and shorter — bends later than a large cone. Made for the mechanical kiln sitter, not for reading nuance.
Straight rods for kiln-sitter shutoffs: they sag, the lever drops, the kiln switches off.
Place the pack level with the ware and visible through a peephole — never against the elements or in the direct flame path.
Wear shade-rated kiln glasses to read cones at temperature — never look into a glowing kiln unprotected.
From first bend to maturity takes only about 15 – 25 minutes: once the guide cone falls, stay with the kiln.
Trust witness cones over the pyrometer: thermocouples drift with age, and heat work is what the ware actually feels.
Table values: Orton self-supporting cones at a final ramp of 60 °C per hour, read at tip level with base. Firing slower bends the same cone at a lower temperature, faster at a higher one.
Numbering, clock positions and the 8° mounting angle follow the Orton system, the de-facto studio standard; European Seger cones use their own scale.