Measuring Heat Work, Not Just Temperature
A thermometer tells you the temperature inside your kiln at a single moment. But firing pottery is not just about reaching a temperature; it is about heat work: the combination of temperature and time that determines how much a glaze has melted. Pyrometric cones measure heat work directly.
What a Cone Is
A pyrometric cone is a small triangular pyramid made from ceramic materials designed to bend and melt at a specific, carefully calibrated amount of heat work. You place cones in the kiln before firing. When the cone bends to a 90-degree angle (its tip touching the base), the target heat work has been achieved.
The Orton Cone System
The most widely used cone system is the Orton scale. The numbering is counterintuitive:
- Cones prefixed with 0 count upward in temperature (cone 022 is the coolest, cone 01 is next, then cone 1 and upward).
- Cones without a prefix (cone 1 through cone 14) count upward in temperature.
In plain terms:
- Cone 022–06: Low-fire range. Used for earthenware and commercial glazes. Typically 600–1000°C.
- Cone 06–1: Mid-low range. Many commercial underglazes peak here.
- Cone 6: Mid-fire. Very common for stoneware and most studio pottery today. Around 1220°C.
- Cone 10: High-fire. Traditional stoneware and porcelain. Around 1300°C.
Cone Habit Checklist
For each new firing:
- Confirm clay and glaze cone ranges match
- Set a 3-cone pack (guard, target, safety)
- Log which cone bent to what angle in a firing notebook
Building this habit makes your firings more predictable over time.
The Bigger Picture
The pyrometric cone system was developed in the late 1800s by Edward Orton Jr., whose work led to the founding of the Orton Ceramic Foundation at Ohio State University. Understanding heat work is closely tied to the concept of vitrification, the process by which clay becomes dense and glass-like at high temperatures.
Common Mistakes and Quick Fixes
- Reading only digital temperature: Always place witness cones to validate heat work.
- Confusing 06 and 6: Double-check labels before loading any glaze or clay body.
- No firing notes: Log cone behavior every firing for trend tracking.
- Assuming all kilns fire the same: Cone packs reveal your kiln's real behavior, not just the controller target.
Practice Exercise
Run a controlled firing with a cone pack in top, middle, and bottom zones. Compare bend angles after cooling and record differences. This gives you a real map of your kiln's heat-work distribution.
