Firing is the most critical stage of the ceramic-making process, and though it takes far less creative energy than the previous stages, it certainly requires a level of know-how that isnt typically innate.
All clays and glazes are created to mature at specific temperatures, and any variance can lead to unsatisfactory results in ceramic durability or color. If fired too high, clay can deform or even melt and can result in glaze runoff; if fired too low, your pieces will be dry, rough, and potentially unsolidified.
In order to help you achieve the best possible results with your kiln, weve put together this guide describing the temperatures at which to fire each clay body and type of glaze. Below are our suggestions categorized by temperature, from lowest to highest, and the most important details youll want to know when firing each ceramic material and glaze.
Historically, low-fire has been the most commonly used firing range due to limitations in kiln technology. Though kilns are now capable of much more complex, high-temperature processes, the low-fire range continues to be popular due to the fact that it allows ceramic artists to use a variety of colorants that either burn off or become unstable at higher temperatures. Here are the important details to note about low-fire materials and glazes:
The most common low-fire clay body is earthenware, which is highly plastic (easily worked) and typically wont shrink, warp, or sag excessively. Terracotta is one of the most popular types of earthenware.
Earthenware contains iron and other minerals which cause it to reach optimum hardness between 1745 (950) and 2012 (1100). The average firing temperature at which low-fire materials reach maturity is 1940 (1060).
Since earthenware is softer than other clay bodies, it seldom becomes fully vitrified, meaning it will be porous, absorb liquids, and be less durable. A separate glaze layer will also be apparent. The color of low-fire clays after emerging from the kiln depends largely on the content of mineral impurities in the clay, but they can become brown, red, orange, buff, medium grey, or white after firing.
In general, low-fire glaze colors are more varied and brighter than mid- or high-fire glazes, but they can appear rather harsh and raw-looking. Fired even lower than their clay bodies, very low-fire glazes, like luster glazes (metallics, iridescents) and overglaze enamels, are often applied after a higher-fire glaze firing, and are best suited for firing between Cone 018 and Cone 016. A burnished low fire clay bisque for sawdust firing also occurs within this range.
With the increased availability of electric kilns, mid-range firing has increased in popularity among potters, especially as artists become more concerned about energy and fuel usage. Most electric kilns can comfortably reach this range without severely decreasing their lifespan or that of their elements.
Typically, mid-range clay is stoneware, a plastic clay that is often grey when moist. Getting its name from the dense, rock-like nature of the clay body when fired, stoneware is typically combined with other clays to modify it, such as ball clays which might be added for plasticity. It is important to note that stoneware is divided into two types - mid-fire and high-fire - and this section of the guide will be referring specifically to mid-fire stoneware.
Like low-fire bodies, mid-range stoneware is relatively soft and porous and has a clearly separate glaze layer after firing. However, a mid-range firing results in increased durability of the ware as well. When fired, stoneware ranges in color from light grey to buff, to medium grey and brown.
Mid-range glazes typically mature between Cone 4 and Cone 6, and most commercial underglazes have a maximum temperature of Cone 6. These glazes are more durable, still offer a fairly extensive color range, and though not quite as harsh as low-fire glazes, can still be quite bright.
Though some stoneware is intended for mid-range firings, other types of stoneware mature at higher temperatures and result in a different ceramic end product. Glazes intended for high-fire procedures are also quite different.
High-fire stoneware is very similar to mid-fire stoneware in terms of ingredients, and may similarly be modified through adding other types of clay bodies (such as fire clays, which raise the maturation temperature). However, high-range bodies have either more refractory elements than mid-range stoneware clays, less fluxing agents, or a combination of the two.
The average firing temperature for high-fire stoneware is 2381 (1305). However, anywhere from 2305 to 2336 (1263 to 1326) may be appropriate depending on the specific clay used and desired effect.
When fired, high-fire stoneware becomes hard, vitrified, and non-absorbent. It is extremely durable, especially compared to both low-fire and mid-range ceramics. Additionally, a body-glaze layer will form between the clay body and the glaze, and though firing color will vary, depending on the process, the finished product can be light grey, buff, medium grey, or brown.
However, the color range is limited due to the varying effects of oxidation and reduction on glaze colorants. Though there are still a few coloring oxides in this high temperature range that can produce a rich color, the palette is much more limited.
The clay bodies that require the highest firing temperatures are kaolin clays, which are most commonly used for porcelain. Though porcelain has similar requirements to other high-fire clays, here are some of the important differences youll want to note if using this ceramic material:
Named after a hill in China from which it was mined for centuries, kaolin is the purest form of clay and is the foundation of all porcelain clay bodies. Though pure kaolin clays can be fired, often they are mixed with other clays to increase both workability and lower the firing temperature, so if using a kaolin-based clay body, be sure to note how pure your material is, as this will change the required temperature.
As a clay body, porcelain is known for its hardness, extremely tight density, whiteness, and translucence in thin-walled pieces. Another difficulty with porcelain bodies is that they are very prone to warping during drying in the kiln.
When fired, porcelain becomes a hard, vitrified, non-absorbent clay body, very similar to high-fire stoneware. It also develops a body-glaze layer formed between the clay body and the glaze. The absence of any iron, alkalies, or alkaline earths in the molecular structure of kaolin not only dictate its high-fire requirements, but are also responsible for its most identifiable characteristic: its white color.
Similar to high-fire stoneware, glazes used for porcelain are limited in color variety and intensity. Most glazes intended for high temperature firings will be lighter, less brilliant, and generally fewer in number and variety.
We hope this guide has been useful in helping you to determine what temperature best suits your projects! Along with this guide, be sure to always check the labels of any glaze or clay you buy in order to confirm temperature requirements. Happy firing, and if you have any questions that have yet to be answered, please dont hesitate to contact us!
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