Talc, also known as Magnesium Silicate, Steatite, French Chalk, Hydrated talc, is common and is the softest of all minerals. Talc is also called steatite – or, in chemical terms, magnesium silicate hydrate
Along with dolomite, and to a less extent magnesium carbonate, it is an important source of MgO flux for bodies and glazes. Dolomite and magnesium carbonate have high loss on ignitions which can produce glaze bubbles, blisters and pinholes, while talc also evolves gases it is less of a problem in this respect.
Talc is also used to produce low expansion ceramics, for example thermal shock resistant stoneware bodies. In these it acts as a low expansion flux that reduces body expansion by converting available quartz mineral, mainly in kaolin, to silicates of magnesia. Cordierite bodies used in kiln furniture and flameware (an a host of other applications e.g. catalytic converters) employ a high percentage of talc and extend this concept so that all free quartz is used up. Such bodies tend to have a narrow firing range because all the silica needs react before the body distorts.
When talc is being used as a flux in low percentages (like porcelain tile) there is need for caution where the body composition is close to a eutectic point of the two or three primary components. Small increases in temperature, firing time or minor flux content (like the talc) can prematurely vitrify the surface trapping gases being evolved within the matrix and producing bloating.
Body Maturity - Body Flux
Talc in 1-4% amounts can be used in the cone 4-10 range to effectively increase body maturity. In some case 1% will move a body down by one cone.
Body Thermal Expansion - Expansion Increase
Talc is used up to 60% in low fire artware bodies to increase thermal expansion so they fit commercial glazes.
Glaze Opacifier - Opacity
Talc is a refractory powder and can promote matteness and opacity when added to low-fire glazes.