Standard hydrated talc is one of a family of insoluble mineral magnesium silicates. It is a soft, greasy, platy or fibrous mineral. Talcs useful in ceramics are considered to be the "high aspect ratio" (very platy) grades. Geologically talc is closely related to chrysotile asbestos and soapstone. The mineral itself can range in color from white and silver-white to apple-green and even black. It shines with a pearly luster and its cleavage is basal perfect and it can be easily cut. Clay bodies containing talc also have a soft, soapy feel and talc in glazes tends to form a film on the surface of the slurry.
Platy talc, like kaolin, is highly useful in coatings, especially paint (in fact, it is much more widely used than kaolin). Talc particles can be self suspending, making them an asset to producing a slurry. Unlike kaolin, talc particles are hydrophobic. But strangely, they still disperse well in water.
Talc mineralogy and chemistry is complex and the subject of academic study. Talc is a common and occurs in large beds of crystalline schists together with serpentine, dolomite, and chlorite. It is most often found in metamorphic rocks in a foliated or granular compact form known as steatite or soapstone. Thus, commercial talcs normally contain traces of iron, aluminum, and calcium; and they vary widely in composition and particle physics (e.g. size/shape/surface properties) from one geological deposit to another (and even within the same deposit). Some talc deposits are quite uniform, consisting almost entirely of platiform (flat) talc crystals, others can be a conglomeration of crystal types and fibers and many different related minerals involved in their formation.
Talc is an interesting example of how materials of the same chemistry can have very different firing behavior. At low temperatures it is refractory, it is often added to clay bodies to increase their thermal expansion (its ability to do this relates to the fact that it does not melt, but retains its mineral character). However talc is composed of SiO2 and MgO, both of which are low expansion oxides, yet together in the mineral talc the expansion is high. Talc will matte low temperature glazes, simply because it is stiffening the melt. Yet, at middle temperatures (e.g. cone 6) a small amount of talc (e.g. 2-4%) can greatly improve body maturity, several cones can be gained. However, if more talc is added, the effect is lost. Explaining such behavior involves interactions, mineralogy, chemistry and the physical presence of the material. At high temperatures, the talc does dissolve in the glaze melt liberating the MgO as a powerful flux that behaves as chemistry suggests it should. At cone 8, plenty of boron is needed to coax talc to decompose. However, talc can also be fired (at temperatures well above cone 10) to form cordierite crystals (thus a man-made mineral), cordierite is very refractory and used to make kiln furniture.
Talc is the subject of litigation (connection of the fibrous types to development of mesothelioma). Hundreds of thousands of tons a year are used in a wide range of industries, it is considered indispensable in the production of so many products essential to modern society.
Dolomite, as a rock, is called "dolomitic limestone". It is a carbonate, similar to limestone, but h
Limestone forms by sedimentation, of coral and shells (biological limestone) or by the precipitation
Talc at Wikipedia
What is Talc at SpecialtyMinerals.com
Talc micrograph at minersoc.org
|By Tony Hansen
Follow me on