How to use witness cones

A witness cone sits in the load and bends when the ware has received the right amount of heat-work. Place a series at the top, middle, and bottom of the kiln, mounted at 8 degrees, and read the bend after firing. A properly fired cone bends until its tip is level with its base.

Publicado el 22 de julio de 2026 · Fuente: Orton Ceramic Foundation, self-supporting cones

A pyrometer tells you how hot the air is next to a thermocouple. A cone tells you what the ware actually received, which is temperature and time together. Those two readings stop agreeing the moment an element ages, a thermocouple drifts, or a schedule runs slower than the factory table assumed.

Cones are cheap, they take a minute to set, and they are the only instrument in most studios that measures the thing which actually matures clay and glaze.

Why use witness cones instead of the pyrometer?

Because clay does not respond to temperature alone. It responds to heat-work, meaning how hot and for how long. A cone integrates both, which is why Orton publishes its temperature equivalents against a heating rate for the last 180 F of the firing rather than as single fixed numbers.

The spread is large enough to matter. A self-supporting cone 6 bends near 2165 F when the last stretch runs at 27 F per hour, near 2232 F at 108 F per hour, and near 2269 F at 270 F per hour. Fire slowly and the cone goes down more than 100 F below the number most charts quote.

On top of that, a thermocouple measures one point in the kiln, usually near a wall, and it ages. A cone in the load reports on the load. When the controller says the firing reached cone 6 and the cone on the middle shelf is barely leaning, the cone is right.

Cones do have limits worth knowing. They are made from carefully controlled compositions and bend in a repeatable way over a fairly narrow range, but they are not laboratory instruments and two cones will differ by a degree or two. They also respond to atmosphere, which matters in a gas kiln. None of that undermines the point: a cone in the load is still the closest measurement most potters will ever get to what the pots experienced.

Self-supporting or standard cones?

Self-supporting cones are the easier choice for witness work. Orton makes them at the correct height, 1 3/4 inches exposed above the base, with the 8 degree bending angle already built in, so they stand on any flat surface with no plaque and no wad of clay. Standard and small cones need mounting to read accurately.

Large standard cones have to be set in a plaque or a clay wad at 8 degrees from vertical with the correct height exposed, because both the angle and the height change how far the cone travels before its tip reaches the base. Get either wrong and the cone reads early or late by a useful fraction of a cone. Small cones fit the sensing rod of a kiln sitter and also work as miniature witness cones where shelf space is tight, mounted at the same 8 degree angle.

One more variety is worth knowing. Orton recommends iron-free cones for reduction firings in the cone 010 to 3 range, because reducing atmospheres and atmospheres containing sulfur oxides can change how a regular cone behaves. Gas firers should check which type they are buying.

Where should I place witness cones?

In the ware setting, where they see what the pots see. To survey the kiln, place a series at the top, middle, and bottom, and in a wide kiln at right, center, and left as well. Comparing the bend at each position is how you learn the character of your own kiln.

• Set cones on a flat, clean part of the shelf, ideally visible through a peep, and always clear of any glaze that might run.
• Leave room around each cone so it can bend fully without touching a pot, and keep it away from direct contact with the elements.
• Use a three-cone series where it helps: a guide cone one number cooler, the target cone, and a guard cone one number hotter. The guide warns you the firing is arriving, the guard tells you it went past.
• Put a cone at every height on every firing until you know the kiln. After that, one cone in the coolest spot you found is often enough for routine work.
• Kiln wash the shelf under the cone or set it on a spare tile, because cones stick where they land.

How do I read the bend angle?

A properly fired cone bends until the tip is level with the base, a smooth hook of roughly 90 degrees with the tip pointing at the shelf. Less than that means the load fell short. A cone flat on the shelf with the tip touching down means the firing went past. Read the tip, not the middle of the curve.

Cones bend in a recognizable way. They move slowly at first, and once the tip reaches about the halfway point, the three o'clock position, the rest happens quickly. Orton notes that a cone typically takes 15 to 25 minutes to bend once it has started, which is why watching through a peep near the end of a firing is worth the trouble.

• Standing straight or barely leaning: well short. The load did not get the heat-work.
• Bent to about three o'clock, tip level with the shoulder: close, perhaps half a cone short.
• Tip level with the base, a smooth hook: properly fired. This is the target.
• Tip touching the shelf, cone flat or puddled: over-fired. Expect extra shrinkage, possible bloating, and slumping in flat forms.
• Guard cone down as well: the firing went at least a full cone past target. Look at the shutoff and the hold before the next load.

How many cones do I need per firing?

One as a minimum, three positions while you are learning the kiln, and a three-cone series in each position when you are chasing a problem. That sounds like a lot of cones until you price them against a shelf of ruined pots. Most studios settle at one or two positions per firing once the kiln is known.

A three-cone series is a guide, a target, and a guard, set side by side in the same plaque or standing in a row. The guide cone, one number cooler, goes down first and tells you the firing is arriving. The target cone is the one you fire to. The guard cone, one number hotter, should still be standing. If it has bent, the load went past.

Bisque firings deserve cones too, even though bisque feels forgiving. A bisque that runs cool leaves ware too absorbent, so glaze goes on thick and crawls, while a bisque that runs hot leaves the surface too tight to take glaze evenly. A single cone 04 or cone 06 witness cone on a middle shelf will tell you which is happening long before the glaze firing does.

Keep the fired cones. A shoebox of cones labeled with the date and the shelf position is a firing log you can hold, and it makes patterns obvious. Elements aging over a year show up as cones that lean a little less each month, which is a far earlier warning than a firing that finally fails to reach temperature.

What does an early or late cone tell me about my schedule?

An early cone means the load received more heat-work than the program intended, and a late cone means less. Both point at the same short list of causes: thermocouple drift, aging elements, a final ramp faster or slower than assumed, or a hold that is doing more than you thought.

If the cone goes down before the controller reaches the target temperature, the kiln is delivering more heat-work than the number suggests. A slow final ramp does that, and so does a long hold, because time counts. Orton notes that a one to two hour soak is generally enough to bend the next cone number, and a four to six hour soak can take you two cones further. A hold is a temperature increase in disguise.

If the cone is still standing when the controller shuts off, the load ran cool. Elements lose power as they age and the kiln struggles over the last stretch, which shows up as a firing that used to land and now falls half a cone short. Slowing the final 200 F is the first thing to try, because it adds heat-work without asking the kiln to reach a higher number.

Cones in different positions telling different stories is a distribution problem instead. A bottom cone short and a top cone over means the load, the shelf spacing, or the elements are uneven. Whatever the pattern, write it down. Set the schedule in the firing schedule builder, fire it, and let the cones edit it.