October 2, 2023

Deciphering the Language of Clay Firing

Discover the specialized vocabulary potters use to discuss clay firing processes. Learn key ceramics terms for firing temperatures, stages, kilns, and more...

Stacks of pottery

Firing clay seems simple - just put it in a hot kiln. But ceramics artists and potters use a vast array of technical terms to describe critical nuances. What exactly do potters mean when they refer to firing to cone 10 in an electric updraft kiln under oxidation?

Let’s define the essential vocabulary potters use to precisely discuss clay firing processes, temperatures, kiln types, and techniques. Mastering the language is key to flawless results.

Key Stages in Clay Firing

  • Bisque firing - The initial firing cycle to harden raw clay before applying glazes. Bisque firing pyrochemically transforms clay so it will remain porous after glazing. Typical bisque temperatures are cone 04-06.
  • Glaze firing - The high-temperature firing that melts glazes onto bisqueware. Glaze firing permanently fuses the glaze to the clay body through sintering. Typical glaze temperatures are cone 6-10.
  • Reduction firing - Firing with limited oxygen to create natural, mottled glaze effects. Reduction firing requires special techniques in gas kilns to carefully control airflow.
  • Oxidation firing - Firing with full access to oxygen, allowing glazes to fully oxidize and produce vibrant colors. Oxidation is the standard for most electric kilns.
  • Atmosphere - The gaseous conditions inside the kiln during firing, depending on ventilation and oxygen levels. Atmosphere impacts glaze chemistry.
  • Heatwork - The total amount of time and temperature the clay and glazes experience during firing. More than just peak temperature.
  • Soak - Holding the kiln at a specific temperature for an extended time to allow heat to fully penetrate the clay.

Key Temperatures in Clay Firing

  • Cone - A small pyramid of calibrated ceramic material that deforms and melts at specific temperatures, indicating when critical heatwork is reached.
  • Cone 022 to Cone 14 - The cone numbers range from 022 to 14 based on the old Seger cone values. The cone number indicates the temperature at which the cone will deform or bend.
  • Cone 06 - Approx. 1830°F (1000°C), a common bisque firing temperature.
  • Cone 04 - Approx. 1945°F (1063°C), another common bisque firing temperature.
  • Cone 6 - Approx. 2232°F (1222°C), a common mid-range stoneware glaze temperature.
  • Cone 10 - Approx. 2381°F (1305°C), a common high-fire stoneware and porcelain glaze temperature.
  • Pyrometric cones - Precisely calibrated cones that deform to indicate when certain heatwork conditions have been met during firing.
  • Large cones - Also known as guide cones, these are visible cones mounted inside the kiln to manually monitor temperature progress.
  • Witness cones or pack cones - Small cones placed inside the kiln and fired with the ware to verify accuracy and to provide a record of how the firing progressed.

Remember that while cones indicate a specific temperature, they're really designed to measure heatwork, which is a combination of temperature and time. A kiln might reach the temperature associated with a certain cone, but if it doesn't maintain that temperature long enough, the cone won't bend. This is why cones are so useful for accurately gauging what's happening inside a kiln.

Kiln Types for Firing Clay

Electric kilns - Offer consistent, automated temperature control through heating elements. Limited in size but excellent for oxidation firing.

  • Gas kilns - Use propane or natural gas burners with adjustable vents. Allow more control over the atmosphere and reduction firing.
  • Wood kilns - Fueled by burning wood in a firebox. Greater temperature variability with more atmospheric effects.
  • Updraft kilns - Heat travels upward from the burners on the bottom to the chimney at the top. Heats evenly but can cause more glaze defects.
  • Downdraft kilns - Heat is pulled down through the ware toward bottom vents by the chimney. Excellent for reduction firing and raku.
  • Front-loading kilns - Kiln chamber accessed through a large front door. Allows easy loading but heat loss while firing.
  • Top-loading kilns - Kiln loaded through the top, which remains closed during firing. Retains heat and allows stacking.

Technical Terms for Kiln Parts

  • Firebox - The burner chamber where combustion occurs, like the firebox in a wood stove.
  • Flues - Channels or passages that direct hot gases from the firebox through the kiln chamber.
  • Baffle - A partial barrier used to divert and mix gases to ensure even temperatures.
  • Peephole - A small opening with a plug for looking inside the kiln during firing.
  • Spyhole - A specially sealed peek window used for safely observing firings.
  • Vents - Adjustable openings to control oxygen intake and vent exhaust gases. Used to manage atmosphere.
  • Damper - A sliding plate used to close off a flue or chimney and regulate draft.
  • Bagwall - An internal wall made of refractory bricks with openings to diffuse heat. Resembles a bag full of holes.
  • Muffle - An internal kiln chamber used for protection during special firings like saggar or raku.

Key Ceramic Materials in Kilns

  • Refractories - Heat resistant materials like bricks that line kilns to withstand high temperatures. Made from alumina, silica, or fireclay.
  • Insulation - Materials like ceramic fiber blanket used to insulate kilns for efficiency and hold in heat.
  • Kiln wash - A refractory coating painted on kiln shelves and saggars as a protective barrier. Prevents glaze drips from fusing ware to the kiln.
  • Stilts - Tiny ceramic tripods used to support wares above kiln shelves so heat circulates underneath.
  • Posts - Small pointed ceramic pillars used to separate stacking pieces so they don’t fuse together.
  • Setters - High alumina shelves, often with holes, used to support pieces being fired. Protects kiln shelves from wear.

Additional Technical Ceramics Terms

  • Clinker - Sintered lumps formed from molten ash or glaze dripping inside a kiln during firing. Can block airflow.
  • Reduction flashing - Pale rings or spots formed on clay bodies and glazes during reduction firing when oxygen briefly enters the kiln.
  • Fireclouds - Dark irregular mottled spots on ceramics caused by sparking of carbonaceous particles during firing.
  • Crawling - The uneven pooling of melted glaze, leaving bare areas of exposed clay. Caused by contamination or surface issues.
  • Crazing - A web of fine crack lines formed in glaze from too much stress between the clay and glaze.
  • Shivering - The chipping or flaking of glaze that pulls away from the clay body surface during cooling. Caused by thermal expansion differences.
  • Scumming - A crystallized crusty deposit left on ware from impurities leaching out during firing. Typically from overfiring.
  • Bloating - Deformation and swelling caused by gases expanding and trapped inside clay or glaze during firing.

That covers the key technical terms potters use to discuss clay firing processes with nuance and precision. Fluency with this specialized language is essential for mastering the firing craft.

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