What does cone 6 mean in ceramics?

Cone 6 refers to a pyrometric cone that deforms at approximately 2232°F (1222°C) at a typical firing rate of 108°F per hour rise. It is the standard mid-fire temperature used by most commercial studio glazes. Clay bodies, glazes, and kilns rated to cone 6 are formulated to mature at this temperature. The cone number does not specify an exact temperature but a heat-work combination: rate of temperature rise matters as much as final temperature.

What is the difference between low fire, mid fire, and high fire?

Low fire covers approximately cone 022 to cone 1 (roughly 1063°F to 2109°F). Mid fire covers cone 2 to cone 7 (approximately 2124°F to 2295°F). High fire covers cone 8 to cone 14 (approximately 2305°F to 2489°F). Most home electric kilns and commercial studio glazes target mid fire, specifically cone 6. Low fire includes earthenware and underglaze work. High fire requires kilns rated to cone 10 or above.

What is a witness cone?

A witness cone is a small self-supporting pyrometric cone placed inside the kiln during firing. When the kiln reaches the target temperature range, the cone softens and bends. After firing, the angle of the bent cone indicates whether the firing was too cool (cone still upright or barely bent), correctly fired (cone bent to approximately the 3-to-4 o'clock position), or overfired (cone bent flat or deformed past the target). Witness cones confirm what actually happened in the kiln, independent of the controller's programmed temperature.

Why does ceramic firing use cone numbers instead of degrees?

Temperature alone does not fully predict how glazes and clay bodies mature. Firing rate affects the result: a kiln climbing at 300°F per hour reaches cone 6 at a lower thermometer reading than a kiln climbing at 100°F per hour. Pyrometric cones respond to the combined effect of temperature and time, what the ceramics community calls heat work. A cone measures heat work, not just heat. Two firings that reach identical thermometer readings but at different rates will produce different results on work and glazes; two firings that bend the same cone will produce consistent results.

Do I need witness cones if my kiln has a digital controller?

Using witness cones periodically is good practice even with a digital controller. Thermocouples drift over time and can read inaccurately; elements wear and affect duty cycle; a thermocouple that reads 2232°F may not actually be producing cone-6 heat work in all zones of the kiln. Placing a cone pack in different locations of the kiln once or twice a year confirms that the controller's programmed cone matches actual kiln performance.

Pyrometric Cones Explained: Numbers, Temps, Witness Cones

A pyrometric cone is a small elongated pyramid made from ceramic materials that soften and deform at specific combinations of temperature and time. The cone number indicates when it bends. Placing a cone inside the kiln during firing gives an independent read of whether the firing achieved its intended heat work, separate from whatever the controller’s thermocouple reported.

Most studio pottery in electric kilns targets cone 6, a mid-fire temperature around 2232°F (1222°C) at a standard firing rate. Understanding what cone numbers mean, how the numbering system works, and how to read witness cones is the baseline knowledge for consistent kiln results.

The cone numbering system

Orton Pyrometric Cones cover a temperature range from cone 022 (the lowest) through cone 14 (the highest). The numbering system has a quirk: cones in the “0” series count downward (cone 022 is cooler than cone 06), while cones in the positive series count upward (cone 1 is cooler than cone 10).

The boundary between the 0 series and the positive series falls between cone 01 and cone 1. Cone 01 and cone 1 are different cones, and they fire at different temperatures.

Standard temperature equivalencies (Orton, at 108°F/hr rise):

Cone	Temperature (°F)	Temperature (°C)
022	1063°F	572°C
018	1263°F	684°C
06	1828°F	998°C
04	1945°F	1063°C
02	2079°F	1137°C
1	2109°F	1154°C
4	2167°F	1186°C
6	2232°F	1222°C
8	2305°F	1263°C
10	2381°F	1305°C
12	2419°F	1326°C

Source: Orton Ceramic Foundation, Pyrometric Cone Temperature Equivalency Chart. Temperatures at 108°F/hr (60°C/hr) final firing rate. Verified June 2026.

These temperatures are at a specific firing rate. A faster rate shifts cone maturation to a higher thermometer reading; a slower rate shifts it lower. The cone itself is the standard, not the thermometer.

Ceramic glaze test tiles and glaze recipe boards at a ceramics studio — Glaze recipes are formulated for a specific cone range. A recipe labeled "cone 6" contains fluxes that become active at approximately 2232°F. Firing the same recipe at cone 04 (1945°F) produces an underfired, matte result; firing at cone 10 (2381°F) produces runny or overfluxed surfaces. (Photo: geishaboy500, CC BY 2.0 via Flickr)

Low fire, mid fire, high fire

Low fire (approximately cone 022 to cone 1): Earthenware clay bodies mature in this range. Low-fire glazes are typically more colorful and brighter than high-fire glazes because many colorants are stable only at lower temperatures. Porcelain and stoneware are not fired in this range; they do not vitrify at these temperatures. Bisque firings for all clay types fall in this range (typically cone 06 to cone 04 bisque).

Mid fire (approximately cone 2 to cone 7): Most commercial studio glazes sold in the US are formulated for cone 6. Mid-fire stoneware and porcelain bodies mature in this range. The combination of practical kiln temperatures, wide glaze palette, and body durability makes cone 6 the most common target in home and community studio settings.

High fire (approximately cone 8 to cone 14): Traditional stoneware and porcelain bodies reach full maturity in this range. High-fire reduction kilns (gas and wood) typically target cone 10 or cone 12. Most home electric kilns are rated to cone 10, though the majority of electric kiln use targets cone 6. Firing electric kilns at cone 10 regularly shortens element life compared to cone-6 use.

Three pyrometric cones bent to different angles after firing — The whole idea in one photo: cones slump when they have absorbed enough heat-work, so the angle they bend to reads the firing. *Tinux via Wikimedia Commons. CC0.*

Witness cones: reading the result

A witness cone is placed inside the kiln before firing to confirm that the target cone level was actually reached. Self-supporting witness cones stand upright in a small clay pad or commercial holder; traditional regular cones require a commercially made stand to hold them at the correct 8-degree lean.

After firing and kiln cool-down:

Cone still upright, no bending: the kiln did not reach the target cone level. The firing was too cool or too short.
Cone tip barely touched down (2 o’clock position): slightly underfired. Most glazes in this state are matte or underdeveloped.
Cone bent to 3 to 4 o’clock position (tip nearly touching the base): correctly fired. This is the standard “mature” position for Orton cones.
Cone fully flat, tip buried in the base: overfired. The kiln reached or exceeded the target level significantly.

Placing three cones in a pack (one cone lower, the target cone, and one cone higher) gives a more complete read. If the lower cone is bent and the target cone is bent to 3 o’clock with the upper cone standing, the firing hit the window precisely. If all three are flat, the kiln significantly overfired.

Ceramics workshop with ware boards holding unfired pottery awaiting the kiln — Unfired greenware and bisqueware are the two main load types. Greenware fires at bisque temperature (typically cone 04), which drives off chemical water and burns off organic matter. Bisqueware fires again at glaze temperature (cone 6 for mid-fire work) to mature the glaze. Each stage has its own target cone. (Photo: Mariusz Raniszewski, CC BY-SA 4.0 via Wikimedia Commons)

Self-supporting vs. regular cones

Self-supporting cones (also called Large Orton Cones) have a triangular base that allows them to stand upright without a holder. They are the most common format for studio use. Self-supporting cones come in the same temperature range as regular cones and are available individually or in boxes of 50.

Regular cones are slender without a base and must be set in a cone holder at an 8-degree lean (the standard firing angle). They are used in commercial production settings and by studios that prefer the traditional format.

Both types are manufactured by Orton to the same temperature specifications. The choice is practical rather than technical.

Controller cones vs. witness cones

Skutt’s KilnMaster and L&L’s Bartlett Genesis controller both offer a cone-fire mode that programs the kiln to reach a target cone level. The controller fires to the programmed cone based on what the thermocouple reports. A witness cone checks whether that reported temperature matched actual heat work in the kiln.

Thermocouples drift gradually after many firings and may read higher or lower than actual temperature. Elements wear and cycle at different duty percentages over time. In a 1-zone controller like the KilnMaster, one thermocouple reading represents the whole kiln; actual temperature at the top and bottom of the kiln varies. In a 3-zone controller like the Bartlett Genesis, three independent readings provide better coverage.

Periodic witness cones catch thermocouple drift early. A cone pack placed in the top, middle, and bottom of the kiln once or twice a year shows whether the controller is producing uniform results through the full stack of work.

Fired ceramic bowls and mugs showing mature cone-6 glaze surfaces — Properly fired cone-6 glazes show their intended surface character: the flux chemistry has activated, the colorants have developed, and the glaze has adhered firmly to the bisque body. Underfired glazes look matte and powdery; overfired glazes can run and pool at the base. (Photo: Robert Collins, Unsplash License)

Common cone levels and what fires there

Cone	Common use
06-04	Bisque firing for all clay types; low-fire glazes
04-02	Low-fire earthenware; low-fire commercial glazes; luster overwash
2-4	Low mid-fire; some commercial studio glazes
6	Standard mid-fire; most commercial studio glazes; mid-fire stoneware and porcelain
8-10	High-fire; traditional stoneware and porcelain; most electric kilns rated to this range
10-12	Gas/wood kiln range; some cone-10 electric kilns

For the kilns reviewed on this site, the maximum temperature ratings are:

Skutt KM-1027: Cone 10
L&L e23T: Cone 10
Skutt KM-1018: Cone 10
Skutt KM-818: Cone 10
Paragon Caldera: Cone 10

All five kilns reviewed here are rated for the full range of studio firing temperatures through high fire. Most electric kiln work targets cone 6.

For kiln controller details that work with cone-fire modes, see the kiln controller guide. For firing schedule structure at different cone levels, see kiln firing schedules. For complete model specifications, see individual model reviews linked from the first kiln buying guide.