Chris Collins

Tools · fabrication

Kiln and firing calculator

Orton cone chart at 15, 60 and 150 °C per hour; a firing-schedule builder with a graph, total time, cone reached and energy cost; clay shrinkage and absorption with size-to-make; plaster and water for moulds; and slip specific gravity, water to add and deflocculant.

Added

Works with

How to use it

Five calculators for the kiln room, in °C, mm and grams or °F, inches and pounds (the choice is remembered).

How to use it

  1. Cone chart. Orton’s temperature equivalents for regular self-supporting cones at 15, 60 and 150 °C per hour (27, 108 and 270 °F per hour) over the last 100 °C of the firing. Pick a cone and a final rate to read its temperature (between the chart’s rates the tool interpolates in log(rate)), or enter a peak temperature to see which cone it reaches.

  2. Firing schedule. Build segments of rate, target and hold (rate 0 = “full”, as fast as the kiln goes), or load a preset: bisque cone 04, low-fire cone 06, cone 6, cone 6 drop-and-hold, cone 6 slow cool for mattes, cone 10, and a crystalline crash-and-hold. The tool draws temperature against time, adds up each segment’s ramp and hold, and estimates the cone reached from the peak and the rate into it. Press ⛶ Full screen (or F) for a large graph; Esc leaves. Below, the energy: kiln kW × powered hours × average duty cycle, and the cost at your price per kWh.

  3. Shrinkage. From a line scratched on a wet test bar, measured dry and fired: drying, firing and total shrinkage, the wet size to make for a wanted fired size, and how much to scale a model or mould — with a link that opens the mold maker’s slip-casting mode with your shrinkage. Water absorption from dry and saturated weights.

  4. Plaster. Pick a box, cylinder or tube (a cottle round a model), its size and the consistency (water per 100 plaster by weight; 70 for USG No. 1 Pottery Plaster): the plaster and water to mix, with a waste allowance, and what the dry mould will weigh. The same model as the mold maker.

  5. Slip SG. Weigh a known volume of slip or glaze: its specific gravity, dry content (Brongniart’s formula), the water to add (or powder) to reach a target SG, and a deflocculant dose as a share of the dry weight.

The maths

  • Schedule: each segment takes |target − previous| ÷ rate hours, plus its hold. “Full” assumes about 500 °C/h heating and 150 °C/h cooling for the time estimate. The cone reached comes from the peak and the final heating rate; a hold adds heat-work — Orton’s rule of thumb is that a 1–2 hour soak bends the next cone and 4–6 hours the one after.

  • Shrinkage: drying = (wet − dry) ÷ wet, firing = (dry − fired) ÷ dry, total = (wet − fired) ÷ wet; (1 − drying)(1 − firing) = 1 − total. Wet size = fired ÷ (1 − total).

  • Absorption = (saturated − dry) ÷ dry × 100 (boil 5 hours as in ASTM C373, or soak 24 hours).

  • Plaster = V ÷ (c/100 + 1/2.65); water = plaster × c/100 (the set volume is the water plus the plaster’s own volume, specific gravity ≈ 2.65). One cubic foot at 70 takes about 58 lb of plaster and 40.6 lb of water.

  • Slip: SG = grams ÷ ml. Water to add to M grams at s₁ to reach s₂ = M (1 − s₂/s₁) ÷ (s₂ − 1). Dry content (Brongniart) = (W − V) × p ÷ (p − 1), p ≈ 2.5–2.65.

The tests check the chart against Orton’s published values (cone 6: 1222 °C at 60 °C/h, 1243 °C at 150 °C/h; cone 10: 1285 / 1305 °C; cone 04: 1063 / 1077 °C), that the °F columns are the °C columns converted, the schedule arithmetic, shrinkage, plaster and slip formulas.

Limits

  • Cones measure heat-work, not temperature; the chart is for Orton’s regular self-supporting cones in electric kilns in air. Iron-free, large and small cones, and reduction, differ. Put cones in the kiln — the chart and a thermocouple do not replace them.

  • Preset schedules are common starting points, not recommendations for your clay, glaze or kiln. “Full” times and duty cycles are guesses; cooling is natural and depends on the kiln and load.

  • Energy is a rough estimate from rated power and an average duty cycle.

  • Shrinkage and absorption are only as good as your test bars; typical shrinkages in the hints are for orientation.

  • Plaster and slip numbers assume the stated specific gravities; brands differ.

Related: glaze chemistry (UMF, expansion, blends), mold maker (plaster slip-casting moulds and their case moulds), thermal expansion and heat, chemistry, epoxy and resin pour calculator.

Sources: The Edward Orton Jr. Ceramic Foundation, Orton Pyrometric Cone Chart (temperature equivalents, ©2001); USG, No. 1 Pottery Plaster data (normal consistency 70); Digitalfire Reference Library (Tony Hansen) on shrinkage, specific gravity and deflocculation.

Linking to an example

Share link copies a URL with your settings. Segments are written rate/target/hold in °C, °C/h and minutes, separated by commas, with full for as fast as possible — for example ?seg=100/600/0.

Parameter Meaning

seg, start

Firing segments; start temperature (°C)

kw, duty, price

Kiln power (kW), average duty (%), price per kWh

rate, cone, fpeak

Cone chart: final rate (15, 60, 150 °C/h), cone, a peak temperature (°C)

wet, dry, fired, target

Shrinkage bar lengths and the fired size wanted (mm)

abd, abw

Absorption: dry and saturated weights (g)

shape, d1, d2, d3

Plaster: box, cyl or ring and its dimensions (mm)

consist, waste

Plaster consistency; waste allowance (%)

sw, sv, sg2, ps, defl

Slip: weight (g), volume (ml), target SG, powder SG, deflocculant % of dry

units

metric or imperial

Presets (?example=<name>):

example= Shows

bisque

a cone 04 bisque

cone6-dh

cone 6 glaze with a drop and hold

crystalline

a crystalline firing: crash to 1080 °C, hold 4 hours

porcelain-mug

shrinkage bar and the wet size for an 85 mm fired mug

mould-plaster

plaster for a 180 × 120 × 90 mm mould at consistency 70

casting-slip

a casting slip at 1.82 thinned to 1.78

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