The Most Common Mistake: Mismatched Firing Ranges
The cone system only works if your clay body and your glazes are both designed for the same cone range. Firing a cone 06 glaze in a cone 6 firing will over-fire the glaze: it runs like water, pools at the base, and fuses the pot to the shelf. Firing a cone 10 glaze in a cone 06 kiln means the glaze never properly melts and looks raw and dull.
Reading a Cone After Firing
A cone can tell you exactly what happened in your kiln:
- Still standing straight: The kiln did not reach the target cone. Under-fired: glaze is probably raw and matte.
- Bent to 90 degrees (tip touching): Perfect. Target heat work achieved.
- Completely melted flat: Over-fired. Temperature went too high or held too long.
Cone Packs
Professional potters use a cone pack: three cones of different numbers placed together in a small clay holder inside the kiln, visible through the peep hole.
For a cone 6 firing, a typical pack uses:
- Cone 5 (the guard cone: bends just before peak)
- Cone 6 (the target cone)
- Cone 7 (the safety cone: bends just after peak)
Watching cone 5 bend tells you the kiln is approaching temperature. Cone 6 bending signals it is time to shut down. Cone 7 still standing means you did not over-fire.
Pro Tip
Always match your clay body, your glazes, and your kiln schedule to the same cone. Read every product label carefully before purchasing. This single habit prevents the most common and most expensive glazing failures.
Post-Firing Cone Read
After the kiln cools:
- Inspect each cone pack and note which cone fully bent
- Compare to controller program or pyrometer reading
- Adjust next schedule only if cones disagree with the numbers
Trust the cones more than the display – they remember the whole firing, not just a moment.
The Bigger Picture
The differences between earthenware, stoneware, and porcelain are largely defined by the cone range at which each is fired. Matching clay and glaze to the correct cone is what determines whether a finished piece is properly vitrified and food-safe.
Common Mistakes and Quick Fixes
- Mixing unknown glazes in one load: Verify cone range for every glaze before firing.
- Trusting one cone pack only: Place packs where they represent the most important ware zones.
- Ignoring over-bent safety cone: Adjust the next schedule rather than repeating the same program.
- Changing program without evidence: Use cone readings first, then controller edits.
Practice Exercise
For your next three firings, keep the same load style and compare cone-pack outcomes against controller peak readings. After three cycles, tune your program once based on that data. This builds stable, repeatable firings.
