Talc
Formula: Mg3Si4O6 or 3MgO.4SiO2.H2O
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 colourless or appears white, and often it has a mother-of-pearl sheen. When it contains other substances, it can also appear light grey, green, yellow or pink.
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, talc is much 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 1-3% in middle temperature stoneware bodies it can drastically improve the maturity and melting! 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).
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.
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
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.
Out Bound Links
- (URLs)
Talc at Mineral.Galleries.com
http://mineral.galleries.com/minerals/silicate/tal... - (URLs)
Talc at webmineral.com
http://webmineral.com/data/Talc.shtml - (URLs)
Luzenac: All About Talc
http://www.luzenac.com/talc.html - (Materials - Related)
Dolomite - CaCO3.MgCO3 or CaMg(CO3)2 - Double carbonate of magnesia/calcia
Calcium Magnesium Carbonate, Raw Limestone
- (Materials - Related)
Light Magnesium Carbonate - Mg5(CO3)4(OH)2.4H2O
Hydrated Magnesium Carbonate Mineral, Hydromagnesite, Magnesium Carbonate Light
- (Materials - Related)
Magnesite - MgCO3
Mag Carb, MgCO3, Anhydrous Magnesium Carbonate
- (Materials - Related)
Pyrophyllite - Al2O3.4SiO2.H2O - A Soft Hydrous Aluminum Silicate
Pyrophillite
- (Hazards)
Talc Hazards Overview
Hazards of this material in the ceramic industry a... - (Temperatures)
Talc melts (1420C-?)
- (Typecodes)
1: GNM - Generic Material
- (Typecodes)
1: FLS - Flux Source
- (MDT - Member)
Generic
Generic for building a completely custom MDT. Only... - (MDT - Member)
Europe
Countries of Eastern Europe and former Soviet Unio... - (MDT - Member)
Australia
We are working on this database and would apprecia... - (MDT - Member)
Asia
All of Asia including Turkey, Russia, Indosnesia, ... - (URLs)
Talc at Wikipedia
http://en.wikipedia.org/wiki/Talc - (MDT - Member)
Africa
All of continental Africa. We are working on this ... - (MDT - Member)
Latin and South America
Latin America and South America. We are working on... - (MDT - Member)
New Zealand
We are working on this database and would apprecia... - (MDT - Member)
North America
The decision about what materials to include in th... - (MDT - Member)
Ron Roy
This is the traditional Ron Roy materials file. He... - (MDT - Member)
UK
We are working on this database and would apprecia... - (MDT - Member)
Crystal Glazes
These materials are specially defined for makers o... - (Glossary - Unspecified)
Cordierite Ceramics
Cordierite ceramics get their properties from the ... - (Typecodes - Type)
1: LEM - Low Expansion Material
- (Minerals - Mineral equivalent)
Steatite
Talc is also called steatite (it is a magnesium si...
In Bound Links
- (Articles - Unspecified)
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 ... - (Materials - Parent)
Nytal Talc - Hydrous Magnesium Calcium Silicate
- (Materials - Parent)
Texas Talc 286 - A talc having a rounded rather than platy particle shape.
TxTalc286
- (Materials - Parent)
Glacier 200 Talc - Montana Talc
Cyprus Glacier 200 Talc
- (Materials - Parent)
Caltalc - California talc
- (Materials - Parent)
Ceramitalc HDT
- (Materials - Parent)
4392 Rosa Blanca - Talc
- (Materials - Parent)
I.T. Talc - Hydrous Magnesium Calcium Silicate
IT Talc
- (Materials - Parent)
Pioneer 2661 Talc - White Texas Talc
Pioneer Ceramic Talc, Pioneer Talc, Pioneer 2882
- (Materials - Parent)
Texas Talc 92 - A talc having a rounded rather than platy particle shape.
TxTalc92
- (Materials - Parent)
Noor Talc - Talc
One Talc, Egyptian Talc
- (Materials - Parent)
Suzorite 325-PE - KMg3AlSi3O10F(OH) - Phlogopite mica
2882, Suzorite 325-S
- (Materials - Parent)
Sierralite Talc - Chlorite
- (Materials - Parent)
CT-30 Talc
- (Materials - Parent)
TDM 92 Talc
- (Materials - Parent)
Luzenac Talc 00S
Talc OOS, OOS Talc
- (Materials - Parent)
Talc 2C
- (Materials - Parent)
Desertalc - Originally from Johns-Manville
- (Materials - Parent)
Amtalc-C98 - KMg3AlSi3O10F(OH) - Phlogopite mica
- (Materials - Parent)
Talcron - 3MgO.4SiO2.H2O - Talc
- (Hazards - Unspecified)
Talc Toxicology
Hazards of this material in the ceramic industry a... - (Oxides - Material source)
MgO - Magnesium Oxide, Magnesia
- (Materials - Parent)
Spluga Talc
- (Materials - Parent)
Ceramitalc - Talc
- (Materials - Parent)
Cimcoat 325 Talc - OFF-WHITE TALC
- (Materials - Parent)
Roudnice Talc
- (Materials - Parent)
VanTalc - Hydrous Magnesium Calcium Silicate
- (Materials - Parent)
FinnTalc - Hydrous Magnesium Calcium Silicate
- (Library - Unspecified)
Mineralogy vs. Chemistry
Materials of the same chemistry can have very diff...
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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