Techno File: Talc Dave Finkelnburg

Appears in the May 2021 issue of Ceramics Monthly.

Used most often in clay bodies fired at low temperatures and glazes fired at high temperatures, talc may be one of the least understood of the highly useful raw materials available to ceramic artists.

Defining the Terms

Coefficient of Thermal Expansion: A numerical measure of the change in length of a sample per unit length per degree of temperature change. Note that the number will be different depending on whether it is expressed in metric or US customary units.

Sinter: To connect particles of material together by heating them hot enough that they bond together without melting. The resulting material is porous.

Unity Molecular Formula (UMF): A chemical formula for a ceramic recipe in which one category of ingredients is set to one. For glazes, this is most commonly the fluxes. Glass formers and modifiers are then given relative to that one mole of flux oxides.

Use and Material

Talc is a convenient and inexpensive source of silica and magnesium for both clay bodies and glazes. Its successful use, however, is widely different between the two applications.

Because magnesium and silica both have low coefficients of thermal expansion, talc can help control crazing in glazes. It is also a common ingredient in some low-fire clay bodies. However, talc does not melt in low-fire ware. Rather, it sinters enough to bond to the clay that is required to make the body plastic.

Talc may make up 40–60% of the recipe in slip-casting bodies fired to temperatures as low as cone 08. Such high-talc bodies can make weak, porous fired ceramics. Because high-talc bodies rely on a glaze coating to make them usable with liquids and are usually not durable, they are usually not suitable for most functional ware. Low-fire clay bodies with small percentages of talc typically do not present the same issues.

Not all talc is created equal. As a naturally occurring mineral, it’s never found in its pure form. Pure talc is a hydrated magnesium silicate with the chemical formula Mg₃Si₄O₁₀(OH)₂. Talc mined for use in ceramics usually also contains one or more of these oxides: calcium, iron, sodium, potassium, and aluminum. One of the most widely available talcs at present, mined in west Texas, contains about 5% calcium oxide (CaO). Calcium is a strong flux. Such a talc should be tested before using it in either a body or glaze recipe to see if the result is acceptable given the amount of calcium present.

Talc doesn’t begin to give up its water of hydration until it’s heated to about 1652°F (900°C). That’s a bit hotter than cone 010 and is reached relatively late in low-temperature firing. This explains why talc is not recommended as an ingredient in low-fire fritted glazes. Such glazes, while they melt readily in the kiln, are likely to either blister or be filled with bubbles when talc begins to give off water vapor after the glaze has melted.

Glazes fired to higher temperatures, between cones 5 and 10, are better candidates for using talc as a recipe ingredient. The amount of magnesium (as magnesium oxide, MgO) in the recipe, however, should not exceed about 0.3 on a unity molecular formula (UMF) basis in clear glazes. Higher amounts will produce opaque glazes with satin to matte surfaces.

Applications

A simple, commonly used hobby-body casting slip can be made from 50% talc and 50% ball clay plus water and deflocculants. This body is exceptionally easy to cast and has remarkably low drying and firing shrinkage. New Mexico potter Jeff Lawrence reports, “This mix makes a wonderful slip that casts beautifully in 20–30 minutes, has low shrinkage (maybe 3% to dry and another 2–3% to fired), shows excellent green strength, and is super tolerant of fast firing.”

A de-airing pugmill is required to make a throwing body using the same recipe. The high amount of talc in the body tends to cause it to break apart as it’s being worked if the body is made without the vacuum feature of a de-airing pugmill.

A Note on Current Concerns

There have been concerns for some time about asbestos or asbestiform-mineral contamination of talc. Several lawsuits focused around the material have won large dollar amounts in the last five years, arguing harm from such contamination by a large corporation selling talcum powder. A major talc mine in New York state has closed because its talc contained asbestos. Note: All current talc available to ceramic artists is reported to be free from asbestos. However, it is still ground to a very fine powder and the same dust-control and clean-up measures used for clay and silica apply equally to talc in the studio.

the author Dave Finkelnburg is a studio potter and practicing engineer. He earned his master’s degree in ceramic engineering from Alfred University.