Pyrometric cones, also sold as kiln cones, ceramic cones or firing cones, are the potter's oldest measuring tool and still one of the most useful. They are small pyramids made from ceramic materials, blended so they soften and bend at a known amount of heatwork. Put one on a shelf next to your pots, and after the firing its bend tells you how much heat the pots really received.
The word pyrometric simply means fire-measuring: Merriam-Webster defines a pyrometer as an instrument for measuring temperatures too high for a mercury thermometer, and the kiln pyrometers potters use today read a thermocouple. A cone measures something slightly different, as the next section explains. When cones are set on a shelf just to check a firing, potters call them witness cones.
This guide is research-based. We compared Orton's published material (Orton makes the cones most US potters use), kiln maker manuals and kiln maker support pages; we did not fire test kilns ourselves. Sources are listed at the end.
How do pyrometric cones work?
Clay and glaze do not mature at a single temperature. They respond to heat over time: a pot held at a slightly lower temperature for longer can end up as mature as one fired hotter and faster. Potters call this heatwork. A thermocouple only reads temperature, so on its own it cannot tell you how much heatwork the ware absorbed.
A cone can. Orton explains that its cones were founded on a simple idea: a blend of ceramic materials compounded in exact proportions should behave like the ceramic ware being fired. As the kiln heats, the cone's materials start to soften, and gravity pulls the tip over. The cone's final bend depends on the heating rate during the last 100°C (180°F) of the firing and on the final temperature, according to Orton.
That is why the same cone has different temperature equivalents on Orton's chart. Cone 6, for example, bends at about 2,165°F with a 27°F per hour finish, 2,232°F at 108°F per hour and 2,269°F at 270°F per hour. Our pottery cone chart lists every cone from 022 to 10.
How cone numbers work
Orton numbers cones from 022 up to 42 across its full range. Self-supporting cones cover cone 022 to cone 32, from 1,087°F to 3,106°F (586°C to 1,708°C) according to Orton. The numbering has two halves:
- Numbers with a leading zero (022, 021 and so on to 01) get hotter as the number gets smaller. Cone 022 is the coolest, cone 01 the hottest of this group.
- Numbers without a zero (1, 2, 3 and up) get hotter as the number gets bigger.
Most home potters use only a few: cone 06 to 04 for bisque and low-fire glaze, cone 5 and 6 for mid-fire stoneware, and cone 10 for high fire. See how hot a kiln gets for what each range is used for.
Types of pyrometric cones
| Type | How it stands | Size or height | Typical use |
|---|---|---|---|
| Self-supporting cones | Built-in base, set on any flat surface | 1 3/4 in exposed, 8 degree angle built in | Witness cones on shelves in home and studio kilns |
| Large cones | Needs a plaque or clay pat | Mounted at 2 in | Witness cones, older studio practice |
| Small cones (junior) | Lies across the sitter rods | 15/16 in | Kiln sitter shutoff devices |
| Pyrometric bars | Lies across the sitter rods | Bar shape | Kiln sitters |
| Iron-free cones | Self-supporting or large | As above | Reduction firings from cone 010 to 3 |
Orton says the advantage of self-supporting cones is that they are already the correct height and angle, which removes the most common source of error with large cones: mounting them at the wrong height or tilt. For most home potters, self-supporting cones are the right choice.
Small cones and bars belong to the kiln sitter, a mechanical switch that turns the kiln off when the small cone bends. Orton's chart gives small cones their own temperature column, so do not swap values between types. For reduction work between cones 010 and 3, Orton recommends its iron-free series, since regular cones in that range contain iron oxide.
What is a witness cone?
A witness cone is any cone placed in the kiln just to watch the firing, not to control it. It sits on a shelf among the ware, and you read it after the kiln cools (or through a peephole during the firing). Skutt's KilnMaster manual recommends using self-supporting witness cones on every firing.
Witness cones answer three questions a controller cannot:
- Did the ware reach the right heatwork? The bend shows the result where the pots sat.
- Is the kiln even? Cones at the top, middle and bottom show hot and cold zones.
- Is the thermocouple drifting? If your controller says cone 6 but the witness cones say cone 5, the sensor or the program needs adjusting. See kiln thermocouples and kiln controllers.
How to set up a three-cone pack
Orton's own advice for choosing cones is to pick one cone number above and one below your target and fire all three together. Potters often call these the guide, firing and guard cones.
- Choose your three cones. For a cone 6 glaze firing use cones 5, 6 and 7. For a cone 04 bisque use 05, 04 and 03.
- Line them up in order. Set self-supporting cones in a row on a small scrap of bisque or a cookie, coolest cone first, all leaning the same way.
- Place the pack with care. Skutt suggests about 2 in from the kiln wall and 2 in from the tip of the thermocouple. Keep them away from glazed pots in case a cone slumps.
- Repeat on other shelves. Orton suggests placing the three-cone series at the equivalent of top, middle and bottom, and right, center and left, when you are checking how evenly a kiln fires.
- Fire, cool and read. Do not move or open anything until the kiln is cool.
If you can see a cone pack through a peephole, wear proper eye protection rated for kiln viewing. Hot kiln interiors give off intense light and infrared, and the peephole plug and surrounding jacket are hot enough to burn.
How to read cones after a firing
Orton notes that a cone bends over a fairly small temperature range even though the bend looks dramatic, and says that if the target cone's final position is between 25 and 75 degrees of bend, you have chosen the right cone for your kiln's profile. In everyday studio use, potters read a three-cone pack like this:
| What you see | What it usually means | What to do |
|---|---|---|
| Guide cone flat, target cone bent with tip near its base, guard cone upright or just starting | Right on target | Keep the program |
| Guide cone bent, target cone barely moved | Under-fired | Raise the temperature offset, add a short hold or check elements |
| All three cones well bent | Over-fired | Lower the offset or shorten the hold |
| Packs on different shelves disagree | Uneven kiln | Adjust loading; zone control or element checks may help |
Keep a simple log: the program, the date, which shelf each pack was on and a photo of each pack. After a few firings you will know your kiln's personality. Fired cones are single use, because their materials have already melted and deformed.
Cones vs controllers: do you still need cones?
Yes, at least from time to time. A digital controller fires by temperature and builds in an assumed heating rate for each cone setting, which works well when the thermocouple is accurate and the elements are healthy. Thermocouples drift and elements weaken with use, and neither change shows on the display. A witness cone is cheap insurance against both.
Many potters put a cone pack in every glaze firing and an occasional pack in bisque firings. If you use a kiln sitter, witness cones also confirm that the small sitter cone shut the kiln off at the right point. The kiln cone packs page covers ready-made packs.
Common mistakes with pyrometric cones
- Mounting large cones at the wrong height or angle. This changes when they bend. Self-supporting cones avoid the problem.
- Placing cones right in front of an element or the thermocouple. They then read a hot spot, not the shelf.
- Using regular cones in reduction between cones 010 and 3. Orton recommends iron-free cones there.
- Reading only one cone. A single cone tells you less than a three-cone pack.
- Setting cones on a bare shelf. A slumped cone can stick; set packs on a scrap of bisque or a washed area.
- Mixing up 06 and 6. They are about 400°F apart.
Safety notes
Do not reach into or open a hot kiln to look at cones. Use the peepholes with eye protection, and wait until the kiln has cooled before unloading. The kiln jacket can get hot enough to burn skin, so keep the clearance your kiln manual requires. Have a licensed electrician install or check the kiln circuit.
Which cones to start with
For most home potters, two boxes cover the basics: the bisque cone and the glaze cone, plus one either side of each. Orton sells self-supporting cones in boxes of 25.
Pair the cone 04 box with cones 05 and 03 for bisque, and use the Orton cone 6 self-supporting cones with 5 and 7 for mid-fire glaze firings.
What you need
Cones only help if your kiln can reach the cone you need. Our best kilns for home pottery guide compares maximum cones and controllers, and our kiln shelves and furniture guide covers the shelves and posts your cone packs will sit on.
Sources: Orton Ceramic Foundation pyrometric cones page, self-supporting cones page, FAQ topics and Temperature Equivalents chart (heating rate, mounting heights, iron-free cones, cone range, box size, three-cone advice, placement and bend guidance); Skutt KilnMaster operating manual (witness cones on every firing, placement 2 in from wall and thermocouple); L&L Kilns knowledgebase on firing with witness cones; Merriam-Webster definition of pyrometer. Specs checked October 2026.