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Talc

Formula: Mg3Si4O6 or 3MgO.4SiO2.H2O
Alternate Names: Magnesium Silicate, Steatite, French Chalk, Hydrated talc

OxideAnalysisFormula
MgO31.87%1.000
SiO263.38%1.334
H2O4.75
Oxide Weight120.44
Formula Weight126.45
Enter the formula and formula weight directly into the Insight MDT dialog (since it records materials as formulas).
Enter the analysis into an Insight recipe and enter the LOI using Override Calculated LOI (in the Calc menu). It will calculate the formula.

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).

The soapstone form of talc was first used by Indians who carved it. Coarse grade talc is used in roofing preparation. Finer grades are used in rubber, paint, steel marking pencils, soaps, lubricants, tailor's chalk (or French chalk), pigments, and it is used for talcum powder.


Mechanisms

Out Bound Links

In Bound Links


Pictures
1215U flow test, MgO is sourced from Talc (right) and from a much more actively melting MgO frit (left).

Click for 297% larger

GR10-B transparent glaze using 10% talc instead of 10% calcium carbonate. This version has a lower thermal expansion and is less likely to craze.

Click for 620% larger

Texas talc (left) and Montana talc (right). Texas talc contains some amorphous carbon. The carbon is not stand-alone, but as CO2 in the dolomitic part of the ore. It produces 7% LOI between 750-850C.

Click for 304% larger

GR10-G Silky magnesia matte cone 10R (Ravenscrag 100, Talc 10, Tin Oxide 4). This is a good example of how high MgO produces a silky matte effect.

Click for 617% larger

Example of a cone 10 transparent that is crazing badly. This is 10% calcium carbonate added to ravenscrag slip. 10% talc does not craze.

Click for 359% larger

2,5,10,15% talc added to Ravenscrag Slip on a buff stoneware fired at cone 10R. Matting begins at 10%. By Kat Valenzuela.

Click for 363% larger

The strange vitrification profile of a talc body from cone 2 to 9 and 10 Reduction.

Click for 756% larger


By Tony Hansen

XML for Import into INSIGHT

<?xml version="1.0" encoding="UTF-8"?> <material name="Talc" descrip="" searchkey="Magnesium Silicate, Steatite, French Chalk, Hydrated talc" loi="0.00" casnumber="14807-96-6"> <oxides> <oxide symbol="MgO" name="Magnesium Oxide, Magnesia" status="U" percent="31.870" tolerance=""/> <oxide symbol="SiO2" name="Silicon Dioxide, Silica" status="" percent="63.380" tolerance=""/> </oxides> <volatiles> <volatile symbol="H2O" name="Water" percent="4.750" tolerance=""/> </volatiles> </material>


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