Talc
Mg3Si4O6
Chemistry
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Volatiles
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Significant Temperatures
- 1420: Melts
Links to Other Materials
- Dolomite - Related
- Light Magnesium Carbonate - Related
- Magnesium Carbonate - Related
- Pyrophyllite - Related
- Cache
Hazards
- Talc
Miscellaneous
- Family: Flux Source
- Region: North America
- Mined At: Unspecified
- Raw Mineral: Yes
- Generic: Yes
Notes
CAS NO: 14807-96-6
Standard hydrated talc (3MgO.4SiO2.H2O) is one of a family of insoluble mineral magnesium silicates. It is a soft, greasy, granular or fibrous mineral. 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.
No talcs have the theoretical chemistry, the most common impurities are CaO (up to 8%) and Al2O3 (up to 6%).
Talc is a common mineral and occurs in large beds of crystalline schists together with serpentine, dolomite, and chlorite. Talc 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 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 formed by the conversion of magnesium-bearing igneous rocks by various geologic means. High quality deposits form through hydrothermal alteration of dolomitic stone by Si/Mg-bearing fluids or by contact metamorphism of dolomitic rock by granite. Tremolite or actinolite containing rocks can also be converted to talc by Si-bearing fluids.
Geologically talc is closely related to chrysotile asbestos and soapstone.
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, talc is much less of a problem in this respect.
High temperature magnesia matte glazes employ MgO from talc and magnesium carbonate to form magnesium silicate crystals on cooling to give both opacity and a matte silky surface.
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. While the ease of glaze fit associated with talc bodies appears to confirm this, Ron Roy points out that his testing indicates that cristobalite does not form at cone 04 or below and that his dilatometer curves show no cristobalite in such bodies.
Talc is also used to produce thermal shock resistant stoneware bodies where 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 extend this concept so that all free quartz is used up. Such bodies tend to have a narrow firing range because all the flint needs react before the body distorts.
Talc is a curious material in that, by itself it is a refractory powder; yet in amounts of 1-5% in middle temperature stoneware bodies it can drastically improve the maturity and melting. In ceramic slips, where 50% is often used, it produces a body that melts suddenly by cone 4. In glazes at middle temperature talc does not participate much in the melt and its presence tends to create an opaque silky matte surface, at cone 10 it is a powerful flux.
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.
(Richard Willis)
Mineral, of the ideal form 3MgO, 4SiO2, H2O with a typical empirical analysis of MgO 31.7%, SiO2 63.5%, H2O 4.8%
Hardness: 1 Density: 2.7-2.8 Insoluble in acids
Member of the clay-making mineral group pyrophyllite-talc. Popular magnesium earth for clay mixes of all temperatures. Talc “porcelain” clays, composed principally of steatite, have been created for special purpose products such as cast figures, pots, objects-of-art, etc., where the firings will be in the 1200ºC range, and, frequently, where a high-fire crackling is chiefly desired. Popularly used in the more pure form of “steatite”
Properties
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.
Linked Articles
Low Fire White Talc Casting Body Recipe
The classic white ball clay talc casting and modelling recipe has been used for many years. It is a dream to use as long as you are aware of the problems and risks.
URLs
- The Mineral Talc - http://mineral.galleries.com/minerals/silicate/talc/talc.htm
- Talc Mineral Data - http://webmineral.com/data/Talc.shtml
- Luzenac: All About Talc - http://www.luzenac.com/talc.html
Suppliers
- Generic
Authors
- Tony Hansen (Owner)
XML
<?xml version="1.0" encoding="UTF-8"?>
<material name="Talc" descrip="Mg3Si4O6" generic="1" rawmineral="1" searchkey="Magnesium Silicate, Steatite, French Chalk" loi="0.00">
<families>
<family name="Flux Source"/>
</families>
<regions>
<region name="North America"/>
</regions>
<oxides>
<oxide symbol="CaO" name="Calcium Oxide, Calcia" status="" percent="6.000" tolerance=""/>
<oxide symbol="MgO" name="Magnesium Oxide, Magnesia" status="" percent="28.000" tolerance=""/>
<oxide symbol="Al2O3" name="Aluminum Oxide, Alumina" status="" percent="2.000" tolerance=""/>
<oxide symbol="SiO2" name="Silicon Dioxide, Silica" status="" percent="58.000" tolerance=""/>
</oxides>
<volatiles>
<volatile symbol="" name="" percent="5.500" tolerance=""/>
</volatiles>
<references>
<reference name="seealso" reason=""/>
<reference name="seealso" reason=""/>
<reference name="seealso" reason=""/>
<reference name="seealso" reason=""/>
</references>
<hazards>
<hazard name="Talc"/>
</hazards>
<temperatures>
<temperature temperature="1420" notes="Melts"/>
</temperatures>
<suppliers>
<supplier name="Generic" country="" url="" label=""/>
</suppliers>
<notes>
<note>CAS NO: 14807-96-6
Standard hydrated talc (3MgO.4SiO2.H2O) is one of a family of insoluble mineral magnesium silicates. It is a soft, greasy, granular or fibrous mineral. 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.
No talcs have the theoretical chemistry, the most common impurities are CaO (up to 8%) and Al2O3 (up to 6%).
Talc is a common mineral and occurs in large beds of crystalline schists together with serpentine, dolomite, and chlorite. Talc 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 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 formed by the conversion of magnesium-bearing igneous rocks by various geologic means. High quality deposits form through hydrothermal alteration of dolomitic stone by Si/Mg-bearing fluids or by contact metamorphism of dolomitic rock by granite. Tremolite or actinolite containing rocks can also be converted to talc by Si-bearing fluids.
Geologically talc is closely related to chrysotile asbestos and soapstone.
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, talc is much less of a problem in this respect.
High temperature magnesia matte glazes employ MgO from talc and magnesium carbonate to form magnesium silicate crystals on cooling to give both opacity and a matte silky surface.
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. While the ease of glaze fit associated with talc bodies appears to confirm this, Ron Roy points out that his testing indicates that cristobalite does not form at cone 04 or below and that his dilatometer curves show no cristobalite in such bodies.
Talc is also used to produce thermal shock resistant stoneware bodies where 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 extend this concept so that all free quartz is used up. Such bodies tend to have a narrow firing range because all the flint needs react before the body distorts.
Talc is a curious material in that, by itself it is a refractory powder; yet in amounts of 1-5% in middle temperature stoneware bodies it can drastically improve the maturity and melting. In ceramic slips, where 50% is often used, it produces a body that melts suddenly by cone 4. In glazes at middle temperature talc does not participate much in the melt and its presence tends to create an opaque silky matte surface, at cone 10 it is a powerful flux.
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.</note>
<note>Mineral, of the ideal form 3MgO, 4SiO<FONT SIZE=2><SUB>2</SUB></FONT>, H<FONT SIZE=2><SUB>2</SUB></FONT>O with a typical empirical analysis of MgO 31.7%, SiO<FONT SIZE=2><SUB>2</SUB></FONT> 63.5%, H<FONT SIZE=2><SUB>2</SUB></FONT>O 4.8%
Hardness: 1 Density: 2.7-2.8 Insoluble in acids
Member of the clay-making mineral group <B><I>pyrophyllite-talc.</I></B> Popular magnesium earth for clay mixes of all temperatures. Talc &#147;porcelain&#148; clays, composed principally of steatite, have been created for special purpose products such as cast figures, pots, objects-of-art, etc., where the firings will be in the 1200&ordm;C range, and, frequently, where a high-fire crackling is chiefly desired. Popularly used in the more pure form of &#147;steatite&#148;
</note>
</notes>
</material>
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