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Formula: Mg3Si4O6 or 3MgO.4SiO2.H2O
Alternate Names: Magnesium Silicate, Steatite, French Chalk, Hydrated talc
|This is the chemistry of the material (the formula compares numbers of molecules, the analysis compares them by weight). In glazes, the composite chemistry of the mix is the main factor shaping fired properties (e.g. color, melting temperature, surface). In bodies we are interested in their physical properties (e.g. shrinkage, porosity, strength), these are a product of the mineralogy of the materials.|
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.
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.
1215U flow test, MgO is sourced from Talc (right) and from a much more actively melting MgO frit (left).
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.
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.
GR10-G Silky magnesia matte cone 10R (Ravenscrag 100, Talc 10, Tin Oxide 4). This is a good example silky matte mechanism of high MgO. The Ravenscrag:Talc mix produces a good silky matte, the added tin appears to break the effect at the edges.
This liner glaze is 10% calcium carbonate added to Ravenscrag slip. Ravenscrag Slip does not craze when used by itself as a glaze at cone 10R on this body, so why would adding a relatively low expansion flux like CaO make it craze? It does not craze when adding 10% talc. This is an excellent example of the value to looking at the chemistry (the three are shown side-by-side in my account at Insight-live.com). The added CaO pushes the very-low-expansion Al2O3 and SiO2 down by 30% (in the unity formula), so the much higher expansion of all the others drives the expansion of the whole way up. And talc? It contains SiO2, so the SiO2 is not driven down nearly as much. In addition, MgO has a much lower expansion that CaO does.
2,5,10,15% talc added to Ravenscrag Slip on a buff stoneware fired at cone 10R. Matting begins at 10%. By Kat Valenzuela.
This body is made from approximately 50:35:15 ball clay:talc:silica:silica sand. These test bars are fired from cone 2 to 9 oxidation (bottom to top) and 10 Reduction and from them the porosity and fired shrinkage can be measured (shown for each bar). Notice that the fired shrinkage is pretty stable from cone 2 to 8, but accelerates at cone 9 oxidation. But in reduction this stage has not been reached yet. The same thing happens with porosity, the cone 9 bar is dramatically more dense than the cone 8 one. But in reduction, it is still porous.
GR10-C Ravenscrag cone 10R silky matte glaze (90% Ravenscrag Slip, 10% talc) produces stunning surfaces and have excellent slurry and application properties.
GR10-C Ravenscrag silky talc matte glaze on Plainsman H443 in oxidation (left) and reduction. In reduction, the iron oxide in the body and glaze darkens and melts much more. The behavior of the tin oxide opacifier is also much different.
Permeability demonstration. Texas talc (left) quickly absorbs all the water poured on top. The water is just sitting on top of the Montana talc (right) and has not permeated at all. Montana talc resists whetting of the particles much more.
Talc exhibits unique powder characteristics, a product of the particle shape and particle surface characteristics. While most powders slide cleanly from this stainless steel scoop, talc powder leaves a film. Dolomite and calcium carbonate are similar.
This chart compares the gassing behavior of 6 materials (5 of which are very common in ceramic glazes) as they are fired from 500-1700F. It is a reminder that some late gassers overlap early melters. The LOI (loss on ignition) of these materials can affect your glazes (e.g. bubbles, blisters, pinholes, crawling).
Feldspar and talc are both flux sources (glaze melters). But the fluxes (Na2O and MgO) within these materials need the right mix of other oxides with which to interact to vitrify or melt a mix. The feldspar does source other oxides for the Na2O to interact with, but lacks other fluxes and the proportions are not right, it is only beginning to soften at cone 6. The soda frit is already very active at cone 06! As high as cone 6, talc (the best source of MgO) shows no signs of melting activity at all. But a high MgO frit is melting beautifully at cone 06. While the frits are melting primarily because of the boron content, the Na2O and MgO have become active participants in the melting of a low temperature glass. In addition, the oxides exist in a glass matrix that is much easier to melt than the crystal matrix of the raw materials.
This is a cone 10 glossy glaze. It should be crystal clear and smooth. But it contains strontium carbonate, talc and calcium carbonate. They produce gases as they decompose, if that gas needs to come out at the wrong time it turns the glaze into a Swiss cheeze of micro bubbles. One solution is to use non-gassing sources of MgO, SrO and CaO. Or, better, do a study to isolate which of these three materials is the problem and it might be possible to adjust the firing to accommodate it. Or, an adjustment could be make to the chemistry of the glaze such that the melting happened later and more vigorously (rather than earlier and more slowly). The latter is actually the likely cause, this glaze contains a small amount of boron frit. Boron melts very early so the glaze is likely already fluid while gases that normally escape before other cone 10 glazes even get started melting are being trapped by this one.
Out Bound Links
Calcium Magnesium Carbonate, Raw Limestone
Hydrated Magnesium Carbonate Mineral, Hydromagnesite, Magnesium Carbonate
Mag Carb, MgCO3
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In Bound Links
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