| Description |
For glazes. Low temperature flux primarily used in glaze, but can be used wherever talc is called for in a formula. Lowers thermal expansion. May affect some colorants. A source of magnesia and silica in a glaze.
Talc is the most common mineral in the class of silicates and germinates and is the softest of all minerals. Talc is also called steatite – or, in chemical terms, magnesium silicate hydrate. It is the main component of soapstone. Its crystals usually develop massive, leafy aggregates with laminar particles. Ground talc is called talcum.
Talc is the softest mineral, with a Mohs hardness of 1. Its silicate layers lie on top of one another and are bound only by weak forces (residual van der Waals forces). This gives it its characteristic greasy or soapy feeling – hence the name "soapstone”. In its pure form, talc is colorless or appears white, and often it has a mother-of-pearl sheen. This sheen often appears at the surface of talc-containing slurries as they are being mix. Talcs containing impurities like carbon or iron can also appear light grey, green, yellow or pink in the raw powdered state.
No talcs have the theoretical chemistry (although some can be very close), the most common impurities are CaO (up to 8%), Al2O3 (up to 6%) and Fe2O3 or FeO (up to 2%). 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.
Some textbooks claim that talc is used as a low fire body addition to encourage conversion of excess free quartz to cristobalite to increase body expansion which reduces crazing. Ron Roy has argued that his testing indicates that cristobalite does not form at cone 04 or below. Thus, while the exact mechanism by which talc increases body expansion may not be completely evident, clearly glazes fit talc bodies and craze on non-talc ones.
Amazingly, 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.
Thus talc is truly a curious material. By itself it is a refractory powder; yet in amounts of only 1-5% in stoneware or porcelain bodies it can drastically improve vitrification! Yet cone 06-04 ceramic slips containing up to 60% talc can often be fired to cone 6 without melting or even deforming! Nothwithstanding this, other 50:50 talc:ball clay bodies will completely melt and boil at cone 6! In glazes at middle temperature raw talc is refractory, its presence tends to create opaque and matte surfaces, yet if supplied in a frit it can create wonderfully transparent glossy glazes. At cone 10 it is a powerful flux but also can be used in combination with calcium carbonate to create very tactile magnesia matte glazes (the MgO forms magnesium silicate crystals on cooling to give both opacity and a matte silky surface).
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
Talcs vary alot in their iron content (some talcs have almost zero iron, others are much higher), so if you are making a body high in talc be aware that the reason it is not firing as white as you would like might be because of the talc, not the clays. Some talcs can have significant carbon. Texas talcs, for example, have CO2 chemically bound into the dolomitic portion, this can produce 7% LOI (in addition to the crystal water LOI that burns off later).
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