Plainsman Low Fire Bodies

L210 - Brown Earthenware			L211 - Buff Low-Middle Earthenware/Stoneware
L212 - Talc Earthenware			L213 - Talc Throwing Body
L215 - Red Talc Earthenware			Buffstone - Buff School Clay
TerraStone - Red School Clay

Contrary to what most people think, making good quality low-fire pottery requires much more expertise and experience that does stoneware. But low temperature has important advantages for those who overcome the problems:

• The energy, time, and effort to fire the kiln is only a fraction of what it is for stoneware.
• The bright colors available at lower temperatures are fabulous. In addition, overglaze brushwork stays put and holds clean edges rather than dissipating into the glaze.
• Whiter burning clay bodies are available because there is no glassy phase that brings out the color of iron.
• Bright red body colors can be achieved and the red color is maintained with a transparent glaze cover at cone 06-04 (at higher temperatures the glaze turns the body color brown).
• Ware is very stable in the kiln and warping is only a problem on very extreme forms.
• Low-fire tends to be repeatable and produce consistent ware. There is usually room for some error on firing without a significant impact on the results.
• Many low temperature frits are available and many are almost complete glazes. Thus it is quite easy to formulate a base glaze. Special purpose frits designed to adjust thermal expansion can be employed to fit a glaze.

The major disadvantages of low fire ware are:

• Ware lacks strength and hardness compared to stoneware. It is curious that many people who use low-fire white bodies describe them as 'vitreous' and 'porcelainous' when actually their surface is more akin to plaster, they have porosities around 12%, and ware can be torn apart easily with your bare hands! Of course, if you fire these bodies higher (to cone 02 for example) they are much stronger. At cone 1, L210 for example, approaches stoneware strength.
• The porous nature of the ware makes it subject to water saturation.
• Glazed ware is not usually able to withstand significant thermal shock. Thus low fire pottery is difficult to adapt to mugs, teapots, etc.
• Glaze is not nearly as firmly bonded to the body as it is at higher temperatures. This adds even more difficulty to the challenge of fitting glazes.
• Glazes are more expensive because they are usually frit based. Stains are commonly used for color and they are fairly expensive also.

Working and Drying Characteristics

In general you will find that low fire bodies are quite plastic because feldspar and other fluxes are not needed. Our L213, for example, is one of our most plastic materials. High porosity and lack of maturity are generally accepted or just ignored and good workability is expected.

Firing

There is no such thing as a vitrified earthenware body. Low fire talc bodies do not even come close to vitrifying at cone 06-04. Red bodies are significantly stronger but still do not develop high strength until cone 02. Still the advantages are compelling enough that the ceramics industry has learned to live with the fragility of the ware.

With this in mind it is important to keep warping in mind. Although the fired body may appear immature, do not overestimate its ability to withstand sagging on extremely overhung shapes (e.g. bowls with flared flat rims).

For the strongest ware consider glaze firing your clay to cone 02 or higher or bisque firing to cone 03.

Glazing

Low temperature bodies offer the advantages of low energy costs coupled with the ability to achieve bright glaze colors. However the porous body can become water-logged if it is not completely covered by a non-crazed glaze. The ware will expand slightly if this happens resulting in even more crazing.

The volume production of functional pottery at low temperature requires much more expertise in glaze mixing, glaze fit, and glaze application than it does for high temperature stoneware. There is a narrower range of raw materials available, slurries are not as easy to use, and there are more complex chemistries. However many frits are available for low fire glazes.

At the same time the hobby ceramics industry has been quite successful even though the average crafter is not knowledgeable about glaze or body mixes. Since the hobby ceramics market is standardized on a universal white burning slurry mix of approximately 50% talc and 50% ball clay, the glaze companies can be confident that their products will work for everyone; they have thus assumed the technical burden of making the glazes work with the body. A huge selection of commercial glazes is available from Duncan, Mayco, Spectrum, and similar companies. This standardization is a contrast to high fire pottery where everyone has their own glaze recipes and firing processes; and problems. Plainsman produces some bodies with added talc so that users can employ these commercial glazes.

Since there is minimal development of a clay-glaze interface that 'fixes' the glaze to the ware surface, the expansion of glaze and body must be much more closely matched than with stoneware ware. If you do not know about the theory behind matching the thermal expansion it can be researched at http://digitalfire.ab.ca/cermat/education/116.php and in a book available at www.ceramicsoftware.com/magic/index.html.

It is doubly important that glazes fit so that glaze and clay expand and contract together as the ware is heated and cooled. Furthermore glazes that do not fit will severely weaken the ware; crazed earthenware provides a breeding ground for bacteria, shivered pieces can potentially shed fine sharp flakes of glaze into food or drink.

If you put a slip or engobe between body and glaze you have a formidable challenge. Be sure that it has the same drying and fired shrinkage behavior as the body. Slips will have an even poorer body-bond than glazes do because there is no glassy phase. Thus even slight incompatibilities will result in flakes of the slip falling off the fired ware. If you would like to research the development of a good slip check the web page at www.ceramicsoftware.com/magic/slip.htm. For information on the majolica process check http://digitalfire.ab.ca/cermat/education/207.php. Again, stress-test your glazed ware by repeated two minute immersions alternating between boiling and ice water to prove the glaze and slip fit. You should also do a soaking test, leaving a specimen in water for a few days to see if crazing is induced. If your ware survives this it will service for many years.

We do not recommend that you have more than two or three base glazes (one is ideal). It is better (and cheaper) to understand how to adjust one glaze to vary its color, surface, expansion, character, etc. than to have twenty recipes that you cannot control. If you add a significant amount of Zircopax or Superpax to opacify a transparent base, you should stress test again (since these materials have low expansions and will change the glaze's thermal expansion). You can find base glaze recipes at http://digitalfire.ab.ca/cermat/education/226.php.

Glaze Recipes

Visit the web page at http://digitalfire.ab.ca/cermat/education/226.php for a good glossy and matte base glaze with lots of information on controlling the two to create any effect you want.

If you wish to use commercial glaze on our talc bodies there is an amazing variety of products from companies such as Duncan, Mayco, Spectrum. Every imaginable effect is available and the results are consistent. Although it is true that these glazes are more expensive, they are quite predictable. Most glazes come in small jars and are meant to be painted on, however this is not practical for many types of ware. Dipping glazes are also available so that you can completely immerse ware to get a quick even coating.

Plainsman Clays Ltd. 702 Wood Street, Medicine Hat, Alberta T1A 1E9 Canada

Phone: 403-527-8535 FAX: 403-527-7508