What temperature is cone 6, and why does heating rate change it

A cone measures heatwork, not a single temperature

A pyrometric cone is a slender three-sided pyramid made from a blend of ceramic materials that softens and bends in a predictable way. The Orton Ceramic Foundation, whose cones were developed in the late 1800s, describes what they measure as heat-work: the combined effect of time and temperature on the ware. That is why a cone number is not one fixed temperature. Give the kiln more time near the top and the cone bends at a lower reading; rush the end and it needs a higher one.

Orton is specific about which part of the firing matters. The final degree of bending depends on the heating rate during the last 100°C (180°F) of the firing and on the final temperature. Orton also points out that a cone bends over a fairly small temperature range, so a cone that looks much more bent than last time may represent only a few degrees of difference.

This is why cones are still used next to digital controllers. Orton calls controllers paired with thermocouples invaluable for running a firing, but notes that they cannot measure heat-work. The controller reports what its thermocouple read; the cone shows what the ware went through.

Cone 6 on the Orton chart

Orton's temperature equivalents chart gives cone 6 several numbers, depending on the type of cone and the heating rate over the last 180°F. For self-supporting cones, the kind most potters stand next to the ware, it lists 2165°F (1185°C) at 27°F per hour, 2232°F (1222°C) at 108°F per hour, and 2269°F (1243°C) at 270°F per hour. The chart labels those three rates slow, medium, and fast, and the medium figure is the one most people quote.

Large cones, which have to be set in a plaque at an 8 degree angle, read slightly differently: 2228°F at 108°F per hour and 2266°F at 270°F per hour. Small cones are listed at 2291°F at 540°F per hour, a column Orton marks as fired in a gas kiln. Self-supporting cones come with the correct height, 1 3/4 inches above the base, and the 8 degree bending angle built in, and Orton says glaze and kiln manufacturers recommend them for the best results.

The neighbours are close. At 108°F per hour the chart puts self-supporting cone 5 at 2167°F and cone 7 at 2262°F, so cone 6 sits 65°F above cone 5 and only 30°F below cone 7. In Celsius at 60°C per hour the same three cones are 1186°C, 1222°C, and 1239°C.

Why slow segments and holds move the result

Because cones respond to heatwork, anything that adds time near the top pushes them further. Orton's advice for choosing a target cone says that if your firing profile includes a soak at the maximum temperature, you should raise the target by one to two cone numbers for a one-hour soak and by two to three numbers for a two-hour soak.

So two firings that both show 2232°F on the controller can finish differently. A schedule that climbs the last 180°F at about 108°F per hour with a short hold lines up with the chart's cone 6. The same peak followed by a long hold can carry the firing a cone or two further, by Orton's rule of thumb. A much slower final climb also lowers the temperature at which cone 6 bends, as the 27°F per hour column shows.

Orton's own worked example shows how to read the chart for an unusual schedule. Firing to 1,147°C with the last 100°C at 75°C per hour, you use the nearest column, 60°C per hour, and find that 1,147°C falls above cone 2 and below cone 3, so the target cone is cone 3.

Cone 6 is not cone 06

Cone numbers run from 022, the coolest, up to 01, and then from 1 upward; Orton's self-supporting cones span 022 to 32. The leading zero changes everything. On the Orton chart at 108°F per hour, cone 06 is 1828°F (998°C) and cone 6 is 2232°F (1222°C), a gap of about 400°F. Mixing them up on a glaze label or a controller is an easy mistake with expensive results.

As for where cone 6 sits, the key on Orton's chart groups cones 4 to 12 with porcelain, china, stoneware, floor tile, and structural clay, and cones 010 to 3 with craft bodies, wall tile, glaze, and earthenware. Digitalfire's Tony Hansen describes cone 6, roughly 2200°F or 1200°C, as the medium temperature oxidation range that most potters work in using electric kilns. Glazy's documentation puts mid-fire, cone 5 to 6, at about 1180 to 1230°C, and notes that most potters bisque lower, often between cone 08 and cone 04.

How to check what your kiln actually did

The standard method is a three-cone pack. Orton describes the guide cone as the first to deform, the firing cone as the one to watch, and the guard cone as only needed when the kiln's heatwork exceeds the firing cone's range. For a cone 6 firing that means cones 5, 6, and 7 side by side.

Orton suggests starting with one cone number below and one above your target, firing them, and measuring the target cone afterwards: if it finishes between 25 and 75 degrees of bend, you picked the right cone for that schedule. Packs placed at the top, middle, and bottom of the kiln show hot and cold spots, and running them routinely gives early warning when a firing starts to change. Orton notes that kilns sometimes drift from their setpoint even when nothing seems to have changed.

The step most often skipped is writing it down. Orton recommends keeping a written log or a digital photo of each firing's cone positions as a reference for future kiln performance. A firing log app such as FireCard keeps the result photo, the cone, the schedule segments you programmed, and your notes together on one card per firing, so this month's cone pack can sit next to last month's.

Common cone 6 mistakes

Trusting the display alone. The controller reports one thermocouple's reading. Orton's point is that it cannot measure heatwork, so a witness cone is the check.

Comparing numbers from different columns. A cone 6 figure from the large-cone column, the self-supporting column, or the 270°F per hour column can differ by tens of degrees. Note which cone type and rate a number belongs to before comparing it with your own.

Ignoring the hold. A hold at peak adds heatwork. By Orton's guidance, a one-hour soak is worth one to two cone numbers.

Mixing up 06 and 6. About 400°F separates them. Read glaze labels and controller settings twice.

This guide explains how cones relate to temperature. For a particular clay body or glaze, follow the firing range its maker gives, and for operating a kiln, follow the kiln manufacturer's manual.

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