Pyrometric Cones for Kilns: 2026 Buying Guide
Pyrometric cones tell you what actually happened inside a kiln, not what the digital display claims happened. This guide covers which cone types and ranges to stock for pottery, glass, and heat-treating work, and how to read them correctly in 2026.
TL;DR
- Pyrometric cones for kilns confirm actual heat-work inside the chamber; digital controllers only estimate it — trust the bend, not the readout.
- Self-supporting cones need no plaque; standard cones require a cone pack or kiln sitter to stand at the correct angle.
- Cone 06 bends near 1828°F, cone 6 near 2232°F, and cone 10 near 2345°F — buy cones matched to your clay or glaze, not to convenience.
- Cone packs degrade in humid storage and stop reading true — replace stock yearly through 2026 and beyond.
Cone temperature reference
1828°F
Cone 06 bending point
Low-fire glaze and bisque
2232°F
Cone 6 bending point
Standard mid-range stoneware
2345°F
Cone 10 bending point
High-fire stoneware and porcelain
Why this matters
A kiln controller reads a thermocouple, not the ware. It tells you the chamber hit a set temperature, but it can't tell you whether the glaze or clay body actually vitrified, because vitrification depends on time at temperature as much as peak temperature itself. That combination is called heat-work, and pyrometric cones are the only direct physical measure of it.
ProKilnSupply stocks Olympic, Jen-Ken, Evenheat, and Aim kilns built around precise digital controllers, but every one of those controllers benefits from a witness cone as a cross-check. Before you test fire a new kiln safely, set a cone pack in the load so you have a baseline reading against the controller.
Who this is for
This guide is for hobbyist ceramicists firing a home electric kiln, studio owners running production loads, school ceramics programs verifying safe and consistent firings, and glass artists checking slump and fuse cycles. It does not cover heat-treating ovens for knife making, since those rely on pyrometers and soak-time logs rather than witness cones.
What to look for in pyrometric cones for kilns
Self-supporting vs. standard cones
Self-supporting cones have a wider base and stand on their own directly on a kiln shelf, no plaque required. Standard cones need a cone pack, clay plaque, or kiln sitter to hold the correct 8-degree lean angle. Get the wrong style and the cone either falls over mid-firing or reads inaccurately because it's standing straight up.
Cone number matched to firing range
Every clay body and glaze has a published cone rating — cone 06 for many low-fire glazes, cone 6 for most mid-range stoneware, cone 10 for high-fire stoneware and porcelain. Buying the wrong cone number tells you nothing about whether your actual glaze matured.
Witness cone placement
Place cones where you can see them through a peep hole, and place more than one per load on large kilns. A single cone in the center of a full kiln misses the temperature variance between the front, middle, and back of the chamber, which can run 20-50°F apart in an unevenly loaded kiln.
Large vs. small cones
Large cones are built for mechanical kiln sitters that physically trip a shutoff lever when the cone bends. Small cones are for witness packs viewed through a peep hole on kilns with digital controllers. Mixing these up means the cone either can't trip your sitter or is too big to read cleanly through a small peep hole.
Brand consistency
Orton is the standard reference brand in the US, and its published cone chart is what most glaze and clay manufacturers calibrate their firing ranges against. Switching brands mid-studio, or mixing lots, introduces bend-temperature drift you can't account for.
Storage and shelf life
Cones are made with a binder that absorbs moisture. Stored in a damp studio corner for a year or more, that binder swells and the cone can bend early or late, giving you a false reading exactly when you need an accurate one.
Cone selection by firing type
As a kiln-only specialist, ProKilnSupply recommends pairing a witness cone set with your controller's firing schedule rather than relying on the digital readout alone.
Cone 6 stoneware (the standard pick) — stock a spread of cone 5, 6, and 7 for any cone 6 stoneware firing in an electric kiln. The three-cone spread catches under-firing and over-firing on either side of your target. Buy — this is the most common firing range in home studios in 2026 and the spread is cheap insurance against a ruined glaze load.
Cone 10 stoneware and reduction — for cone 10 stoneware firing, stock cone 9, 10, and 11. High-fire loads run closer to element and refractory limits, so the wider spread matters more here than at cone 6. Buy if you're running gas kilns or high-fire-rated electric kilns; Skip the cone 10 set if your kiln's element life data caps out below cone 8.
Porcelain — porcelain has a narrower vitrification window than stoneware clay bodies, so when firing porcelain in an electric kiln, tighten the spread to cone 5, 6, and 7 rather than the wider stoneware set. Buy the tight spread; a single cone number isn't enough margin for porcelain warping risk.
Controller cross-check — anytime you calibrate a kiln controller, run a cone alongside the calibration firing. If the controller says the target was hit but the cone hasn't bent, the thermocouple is reading high and needs recalibration. Buy a small cone stock just for calibration checks — this catches drift before it ruins a full glaze load.
Glass fusing and slumping — most glass kiln programs run on hold-time schedules rather than a single peak temperature, so witness cones are used less often here than in ceramics. Skip cones for glass work unless your process calls for a specific witness point; rely on the programmable controller schedule instead.
What to avoid
- Buying only large sitter cones when your kiln has a digital controller. Sitter cones are built to trip a mechanical shutoff lever — a controller-based kiln has no lever to trip, so the cone just sits there unused.
- Off-brand cones for a school or production studio. Inconsistent bend calibration is a minor annoyance for a single hobbyist glaze test and a real problem when a program needs the same firing result across 20 students or a full production run.
- One cone in a fully packed kiln. A single witness cone tells you the temperature at one point in the chamber. Full loads need cones front and back to catch shelf-to-shelf variance.
“If the controller says cone 6 and the witness cone hasn't bent, trust the cone.”
Verdict comparison
| Cone type | Best for | Setup needed | Verdict |
|---|---|---|---|
| Standard small cones (04-10) | Witness packs mid/back of load | Cone pack or clay plaque | Buy for any glaze or clay firing needing verification |
| Self-supporting cones | Quick shelf spot-checks | None | Buy for small kilns or single-shelf loads |
| Large sitter cones | Mechanical kiln shutoff | Kiln sitter mechanism | Consider only if your kiln uses a physical sitter, not a digital controller |
| Off-brand/no-name cones | Casual budget monitoring | N/A | Skip for schools and production studios needing repeatable results |
ProKilnSupply's phone and text support team can help match a cone spread to your specific kiln model if the manufacturer's firing chart doesn't cover your glaze line directly.
FAQ
What are pyrometric cones used for in a kiln?
Pyrometric cones measure heat-work — the combination of temperature and time — inside a kiln during firing. They bend at a known point to confirm the load actually reached the intended maturity, which a digital controller alone cannot verify.
What cone number should I use for cone 6 glaze firing?
Use a spread of cone 5, 6, and 7 witness cones for cone 6 glaze firing. The spread shows whether the load under-fired, hit target, or over-fired relative to the middle cone.
Are self-supporting cones better than standard cones?
Neither is universally better; self-supporting cones are faster to set up since they need no plaque, while standard cones in a cone pack give a more controlled lean angle for precise reading. Studios often use self-supporting for quick spot-checks and standard packs for critical glaze firings.
Can I rely on my kiln controller instead of cones?
No — a controller reads a thermocouple's temperature, not the ware's actual heat-work. Pyrometric cones for kilns remain the direct physical check that the load matured as intended, and pairing both catches thermocouple drift early.
How many witness cones should I place in a load?
Place at least two cones in a full kiln load — one near the front and one near the back — since temperature can vary 20-50°F between zones in an unevenly loaded chamber. Small test loads can get by with a single cone.
Do glass kilns need pyrometric cones?
Most glass fusing and slumping schedules run on programmed hold times rather than a single peak temperature, so witness cones see less use in glass work than in ceramics. Some glass firing schedules still specify a witness point for confirmation.
How long do pyrometric cones last in storage?
Cones stored dry and sealed hold their rated bend temperature for years, but humidity swells the binder and skews the reading. Replace stock yearly if your studio storage isn't climate-controlled.
What's the difference between cone 6 and cone 10?
Cone 6 bends near 2232°F and covers most mid-range stoneware glazes, while cone 10 bends near 2345°F and covers high-fire stoneware and porcelain. The roughly 110°F gap changes which clay bodies and glazes will properly vitrify.
One last thing
Heat-work, not peak temperature, is the whole reason cones exist. A kiln fired fast can hit the exact target number on the display and still under-fire the glaze, because the ware didn't spend enough time at temperature to finish vitrifying — a cone catches that, a controller readout doesn't. Set two cones per full load, front and back, and call or text ProKilnSupply's support line before your next firing if you're not sure which cone spread fits your kiln model.