A kiln firing schedule specifies how temperature rises from ambient to target temperature, holds at critical thresholds, and cools to a safe opening temperature. The schedule directly affects whether work survives the firing intact, whether glazes mature correctly, and whether the clay body reaches its intended density and strength.

Two firings are standard in most studio ceramics: a bisque firing that converts greenware to a porous, hardened state, and a glaze firing that melts and fuses the glaze onto the bisqueware. Both have distinct temperature profiles, and errors in either — particularly in ramp rates during critical transitions — are the most common cause of cracked, bloated, or crawled work.

Understanding Cones

Ceramic cones are the primary temperature reference system in studio ceramics. A cone number corresponds to a specific heat-work value — the combination of temperature and time at temperature that ceramic materials respond to. Orton witness cones, when placed in a kiln, bend at a defined heat-work point and provide visual confirmation that a firing has reached its target.

The cone scale runs from cone 022 (approximately 586°C) up through cone 10 (approximately 1285°C for standard witness cones) and beyond for industrial applications. Home studio ceramics work primarily within the following ranges:

  • Cone 022–018: Enamel and luster overglaze firings
  • Cone 06–04: Bisque firings for most clay bodies; glaze firings for earthenware
  • Cone 6: Mid-fire glaze firing for stoneware and porcelain (most common home studio target)
  • Cone 10: High-fire glaze firing for stoneware and porcelain in gas or wood kilns

Cone vs Temperature: Not a Fixed Relationship

The temperature at which a cone bends depends on the rate of temperature rise. A fast ramp will reach the cone's bend point at a higher thermometer reading than a slow ramp. Published cone temperature equivalents assume a standard rise rate of approximately 60°C per hour. Kilns firing faster require higher thermometer targets to reach the same heat-work value.

Bisque Firing Schedule

The bisque firing converts clay from its greenware state — still containing chemically bound water — to a porous, hardened bisqueware that can accept glaze. The critical challenge is removing water in stages without the thermal shock that cracks pieces.

Stage 1: Moisture Removal (Ambient to 120°C)

Even thoroughly dry-looking clay contains residual moisture. Below 100°C, this moisture evaporates as steam. At 100°C, water boils and turns to steam rapidly. If the kiln rises through this range too quickly, steam pressure inside dense clay sections can cause cracking or explosive spalling. A standard safe ramp for this stage is 50–80°C per hour, with a 30-minute hold at approximately 110°C to ensure complete steam release before continuing.

Stage 2: Chemical Water Burnout (350–600°C)

Between approximately 350 and 600°C, organic materials in the clay — carbon compounds and organic binders — burn out and produce carbon dioxide and other gases. These gases must exit through the porous clay before the body begins to sinter and close. If the kiln rises too quickly in this range, carbon becomes trapped inside the clay body, producing a characteristic grey-black core in the fired piece. A ramp of 100–150°C per hour with kiln vents open is standard through this range.

Stage 3: Quartz Inversion (573°C)

At 573°C, quartz in the clay body undergoes a rapid crystalline inversion — a reversible change in silica crystal structure that occurs both on the way up and on the way down in a firing. This inversion is accompanied by a small volume change (approximately 2% expansion on the way up, contraction on the way down). Moving through this transition too quickly, particularly during cooling, causes dishes and large flat pieces to crack from thermal stress. A ramp of 100°C per hour through this range, or a brief hold just below 573°C, is standard practice.

Stage 4: Temperature to Bisque Target (600°C to Cone 06)

Once past 600°C, the clay body is structurally stable enough to rise more quickly. A ramp of 150–200°C per hour is common from 600°C to the bisque target of cone 06 (approximately 999°C). A 15-minute soak at the top temperature ensures heat-work is even throughout the kiln load before cooling begins.

Cooling from Bisque

Allow the kiln to cool naturally from the bisque top temperature with the lid and vents closed until approximately 200°C. Do not open the kiln above 100°C — thermal shock from cool room air entering a hot kiln causes cracking, particularly in porcelain and thinly thrown stoneware. Opening temperature is typically 60–80°C, at which point pieces can be handled safely.

Cone 6 Glaze Firing Schedule

The glaze firing takes bisqueware — already ceramic, no longer containing structural water — to the temperature at which the glaze melts, flows slightly, and bonds to the clay body. The critical parameters are reaching the correct heat-work at the top temperature and cooling slowly enough through the quartz inversion to prevent dunting (thermal-shock cracking).

Ramp to Temperature

A typical cone 6 glaze firing schedule for a home electric kiln:

  • Ambient to 120°C: 80°C/hr (ensures any residual moisture in bisqueware is expelled slowly)
  • 120°C to 600°C: 150°C/hr (kiln controller medium ramp)
  • 600°C to 1100°C: 100°C/hr (slower final approach)
  • 1100°C to 1220°C (cone 6): 60°C/hr
  • Hold at 1220°C: 10–15 minutes

Cooling from Cone 6

After the hold, allow the kiln to cool naturally with the elements off. Do not add forced air cooling or open vents above 100°C. The quartz inversion at 573°C on the way down is the critical moment — if pieces survive the rise, they are most vulnerable to dunting during this cooling inversion. Large, thick, or flat pieces are at greatest risk.

Cone 10 Reduction Firing in Gas Kilns

Cone 10 reduction is a different firing process available only in gas kilns (or specialised salt and soda wood kilns). Reduction refers to a low-oxygen atmosphere created by partially restricting the kiln's air supply, which causes iron and other metal oxides in glazes and clay bodies to behave differently than they do in the oxidising atmosphere of an electric kiln.

Reduction firing produces characteristic effects: celadon glazes, iron saturation glazes (tenmoku), the spotted and mottled surfaces of shino, and the flame markings of natural-ash glazes. These effects cannot be replicated in electric kilns, which is why potters seeking them specifically build or access gas kilns even when electric firing is more convenient.

For Canadian home studios, gas kiln installation requires compliance with local building codes, gas line permits, and — for outdoor propane kilns — specific setback requirements from structures. Urban locations in Toronto, Vancouver, and other major cities often restrict or complicate residential gas kiln installation.

Raku and Specialty Firings

Raku is a fast-fire, low-temperature process (typically cone 06 to cone 04) in which pieces are removed from the kiln while still glowing and placed in a reduction chamber — traditionally a metal trash can with combustible material. The rapid thermal change and localised reduction produce crackle glaze surfaces and carbon-darkened clay where glaze is absent.

Raku is not food-safe. The thermal shock of the process leaves microcracks in the glaze and clay, and the clay body does not vitrify at the low firing temperature. Raku pieces are decorative objects. The firing process is typically conducted outdoors and requires protective gloves, face shields, and fire-resistant clothing.

Soda and salt firing introduce sodium compounds (sodium bicarbonate or sodium chloride) into a hot kiln through the burner ports. The sodium vaporises and forms a glassy sodium silicate skin directly on the clay surface, producing characteristic orange-peel texture. Both require gas kilns and significant ventilation due to the fumes produced.

Kiln Firing Log Practice

Maintaining a firing log — date, clay body, glaze combinations, schedule used, and results observed — is one of the most practical habits in studio ceramics. Kilns behave slightly differently over time as elements age and the kiln interior absorbs heat differently after many firings. A log makes it possible to identify when a schedule that produced good results one year starts producing different results, and to diagnose the cause systematically rather than through guesswork.

The Digital Fire reference database maintains detailed technical notes on kiln firing, glaze chemistry, and clay body behaviour that complement the schedule guidelines here with deeper scientific context.

Further Reading

Last updated: May 4, 2026 — Blue Willow Ceramics Inc., Toronto, ON