Each ceramic glaze should be fired to a specific temperature range. If fired at too low a temperature, the glaze will not mature. If the temperature goes too high, the glaze will become too melted and run off the surface of the pottery. For success, a potter must know the correct temperature range at which their glaze becomes mature.
When potters talk about ceramic firing ranges, they are usually referring to the three most common: low-fire, mid-range, and high-fire ranges. In regards to glazes, we need to add two other ranges: very low-fire and lower mid-range firing ranges.
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Very Low-Fire Range
- from cone 022 (approx. 1112⁰F - 605⁰C)
- to cone 013 (approx. 1566⁰F - 850⁰C)
This range is usually used for luster glazes and very low-firing overglazes. Ware must be fired at least once at a higher temperature first, in order for the clay body to mature. The ware will often not only go through a bisque firing, but also a higher temperature glaze firing. Very low-fired overglazes and lusters are then applied to the already fired primary glaze. The ware is returned to the kiln for a very low temperature firing in order to fuse the overglazes.
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- from cone 012 (approx. 1623⁰F - 882⁰C)
- to cone 02 (approx. 2048⁰F - 1120⁰C)
The low-fire range has historically been the most commonly used firing range. In the past, this was mainly due to limitations in kiln technology. However, low-fire temperatures allow potters to use a variety of colorants that either burn off or become unstable at higher temperatures.
Low-fired ware can present some difficulties, including
- the clay body may remain overly porous
- low-fire glaze colors can appear rather harsh and raw-looking
- the high percentage of flux or stronger-acting fluxes used can result in a softer, less durable glaze, and
- many of the traditional glaze materials used in this range are quite toxic in their raw state.
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Lower Mid-Range Range
- from cone 01 (approx. 2079⁰F - 1110⁰C)
- to cone 3 (approx. 2134⁰F - 1145⁰C)
The lower mid-range is one of the most overlooked, yet perhaps one of the potentially most exciting, of the temperature ranges. Within this range, most earthenware and other low-fire clay bodies actually mature to their strongest and most durable state. At the same time, many of the colorants that are available at lower temperatures are still used within the lower mid-range temperatures.
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- from cone 4 (approx. 2167⁰F - 1165⁰C)
- to cone 7 (approx. 2264⁰F - 1210⁰C)
This range is being used more and more as potters become more concerned about energy and fuel usage. Another factor has been the availability of electric kilns that can comfortably reach this range without severely decreasing the kiln's and the kiln elements' lifespans.
Other advantages to firing in the mid-range include
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- ability to adjust and use stoneware clay bodies to this range
- in turn, mid-range stoneware bodies increase the durability of the ware
- mid-range glazes are also more durable than those fired at lower temperatures, and
- there is still a fairly extensive color range available.
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- from cone 8 (approx. 2305⁰F - 1260⁰C)
- to cone 14 (approx. 2530⁰F - 1390⁰C)
This range includes the stonewares and porcelains. Glazes and clay bodies are dense and durable; however, the color range is limited. Because of the varying effects of oxidation and reduction on glaze colorants, the few coloring oxides that are viable at this range can still produce a rich, if much more limited, palette.