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Talc

Alternate Names: Magnesium Silicate, Steatite, French Chalk, Hydrated talc

Oxide Analysis Formula
MgO 31.87% 1.00
SiO2 63.38% 1.33
H2O 4.75%n/a
Oxide Weight 120.44
Formula Weight 126.45

Notes

Talc is common and is the softest of all minerals (Mohs hardness of 1). 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.

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 mixed. 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/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 (although it gasses quite late, often after glazes have begin to melt).

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. And certain talcs fit glazes better than others.

Amazingly, talc is also used to produce low-expansion ceramics, for example, thermal shock-resistant stoneware bodies. In these, it acts as a low 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 (and 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 to react before the body distorts.

Thus talc is truly a curious material in bodies. By itself, it is a refractory powder, yet in amounts of only 1-3% in stoneware or porcelain bodies it can drastically improve vitrification! Yet adding these same low percentages to some zero-porosity highly vitreous bodies does cause them to warp, blister or over-fire. And some cone 06-04 ceramic slips containing up to 60% talc can be fired to cone 6 without melting or even deforming (50:50 mixes can even go to cone 10). This is obviously a material that needs to be thoroughly tested for each application.

Low-temperature casting bodies made from a 50:50 mix of ball clay and talc were made for many years for the hobby ceramics market. These produced a white burning material, albeit a porous one, that works well for casting and, with a little addition of bentonite, for throwing also. Over time a whole industry developed to supply glazes to fit them for firing at cone 06-04.

Around 2020, most body manufacturers discontinued the use of talc. Legal issues surrounding implications that it contributes to mesothelioma appeared to shake up the talc supply industry, producers literally disappeared! Manufacturers likewise were either worried about this or could not get supplies. However, talc is still classified only as an irritant on the backs of original container bags. A shift to using dolomite instead has happened and users are working to adapt to the changes it brings, including cost. In terms of liability, as of 2022 talc is still employed in many applications, including ceramics (usually ones that don't require high whiteness). Large suppliers with state-of-the-art lab capabilities claim to be able to leverage these to ensure that products are safe to use, both from a legal and health perspective. Talc use in glazes is likely to continue, percentages are much lower and glazes do not generally find their way onto floors where traffic and sweeping would put them into the air, they are cleaned up in the wet state at work levels.

Talc is a curious glaze material also. 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). This being said, where transparency is needed it is generally best to source MgO from a frit (since talc loses its water of hydration quite late in the firing, after melting of most boron glazes has begun).

When talc is being used as a flux in low percentages (like porcelain tile) there is a 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 can detrimentally affect body plasticity but vacuum pugging often solves the problem. So if you need to reprocess scrap material in production and cannot vacuum pug this could be an issue.

Talcs vary a lot 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.

Talc has a complex commercial and litigious history in North America. Various deposits and products have been associated with asbestos.

Talc is commonly dusted onto plaster molds to ease the release of cast pieces (where difficult shapes are being made). It is spread in a loosely woven cloth bag or using micro-fibre gloves. When talc is used in consort with in-mold ductwork and compressed air, shapes can be cast that would otherwise be impossible.

If you are desperate to substitute this material in a clay body or glaze and need help, just purchase a group account at Insight-live.com and we can work together to get it done (sourcing the Li2O from a frit or spodumene). We will make a step-by-step video of the initial calculation and then work together in your account to track trials, adjustments and retrials.

Related Information

How to matte Ravenscrag Slip at cone 10 by adding talc

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

Frits work much better in glaze chemistry

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The same glaze with MgO sourced from a frit (left) and from talc (right). The glaze is 1215U. Notice how much more the fritted one melts, even though they have the same chemistry. Frits are predictable when using glaze chemistry, it is more absolute and less relative. Mineral sources of oxides impose their own melting patterns and when one is substituted for another to supply an oxide in a glaze a different system with its own relative chemistry is entered. But when changing form one frit to another to supply an oxide or set of oxides, the melting properties stay within the same system and are predictable.

Texas talc (left) and Montana talc (right)

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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. Even though the powder color is so much darker in the raw form, it fires whiter! But there is more going on here. On paper, both contain about 0.5% Fe2O3. But the iron species in the two talcs are different. In Texas talcs, the iron is part of the crystal lattice. But, in the Montana material, that 0.5% Fe2O3 is an external iron oxide mineral species, a physical contaminant. While the Montana material fires much darker because of this that iron seems to have little affect on the color of the raw white powder.

The unexpected reason for this crazing can be seen in the chemistry

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Glaze chemistry explains the crazing

The 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? This is an excellent example of the value of 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 COE of the whole way up. And talc? It contains SiO2 (so the SiO2 is not driven down nearly as much) and its MgO has a much lower expansion than CaO does.

The strange vitrification profile of a talc body

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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.

RavenTalc silky matte on the outside of a cone 10R buff stoneware mug

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GR10-C Ravenscrag cone 10R silky matte glaze closeup (on Plainsman H550 stoneware). The recipe is 90% Ravenscrag Slip (roast:raw combo) and 10% talc. The inside of this piece just has pure Ravenscrag (raw:roast).

Difference between oxidation and reduction! GR10-C matte on Plainsman H443

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Same body, same glaze. Left is cone 10 oxidation, right is cone 10 reduction. What a difference! This is a Ravenscrag-Slip-based recipe on a high-fire iron stoneware. In reduction, the iron oxide in the body and glaze darkens (especially the body) and melts much more. The behavior of the tin oxide opacifier is also much different (having very little opacifying effect in reduction).

Permeability demonstration of Texas and Montana talcs

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Texas talc (left) quickly absorbs all the water poured on it. Montana talc (right) resists whetting of the particles much more, the water is just sitting on top and has not penetrated at all.

Talc leaves a film on this stainless steel scoop

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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.

LOI is not important? Think again!

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A chart showing weight-loss vs firing temperature for common ceramic materials

This chart compares the decompositional off-gassing (Loss on Ignition) behavior of six materials used in ceramic glazes as they are heated through the range 500-1700F. It is amazing that some can lose 40%, or even 50% of their weight on firing. For example, 100 grams of calcium carbonate will generate 45 grams of CO2! This chart is a reminder that some late gassers overlap early melters. That is a problem. The LOI (% weight loss) of these materials can affect glazes (causing bubbles, blisters, pinholes, crawling). Notice talc: It is not finished gassing until 1650F, yet many glazes have already begun melting by then (especially fritted ones). Even Gerstley Borate, a raw material, is beginning to melt while talc is barely finished gassing. And, there are lots of others that also create gases as they decompose during glaze melting (e.g. clays, carbonates, dioxides).

Frits melt so much better than raw materials

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Frits, talc, feldspar melting

Feldspar and talc are both flux sources (glaze melters), they are common in all types of stoneware glazes. But their fluxing oxides, Na2O and MgO, are locked in crystal structures that neither melt early or supply other oxides with which they like to interact. The pure feldspar is only beginning to soften at cone 6. Yet 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! The frits progressively soften, starting from low temperatures, both because they have been premelted and have significant boron content. In both, the Na2O and MgO are free to impose themselves as fluxes, actively participating in the softening process.

Orange-peel or pebbly glaze surface. Why?

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An orange peel textured glaze

This is a cone 10 glossy glaze. It has the chemistry that suggests it should be crystal clear and smooth. But there are multiple issues with the materials supplying that chemistry: 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. A study to isolate which of these three materials is the problem might make it possible to adjust the firing to accommodate it. But probably not. The most obvious solution is to just use non-gassing sources of MgO, SrO and CaO (which will require some calculation). There is a good reason to do this: The glaze contains some boron frit, that is likely kick-starting melting much earlier than a standard raw-material-only cone 10 glaze. That fluid melt may not only be trapping gases from the body but creating a perfect environment to trap all the bubbles coming out of those carbonates and talc. The Aero chocolate bar of glazes!

A low fire talc body lacks plasticity when slip-mixed, but not when pugged

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This clay was slurried in a mixer and then poured onto a plaster table for dewatering. During throwing it is splitting when stretched and peeling when cutting the base. Yet when this same clay is water-mixed and pugged in a vacuum de-airing pugmill it performs well. One might think that the slurry mixer would wet all the particle surfaces better than a pugmill, but it appears the energy that the latter is putting into the mix is needed to develop the plasticity when there is a high talc percentage in the recipe.

Talc powder is floating on top of the water and will do so for some time

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Talc particle surfaces do not wet as easily. Other mineral powders (like feldspar, silica, even clay, will wet and sink immediately). Yet even after 30 minutes this still had not submerged. Pugging clay bodies containing talc can be difficult for this reason. Laminations can be a problem even with small percentages of talc.

Talc:Ball Clay bodies have incredible casting properties

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This bowl is 13cm across yet has a wall thickness of less than 2mm and weighs only 101g! It released from the mold with no problems and dried perfectly round. But it has a key advantage over stonewares and porcelains: When this is fired at cone 04-06 it will stay round!

High thermal expansion talc body cannot be COE-calculated

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Talc is employed in low-fire bodies to raise their thermal expansion (to put the squeeze on glazes to prevent crazing). These dilatometer curves make it very clear just how effective that strategy is! The talc body was fired at cone 04 and the stoneware at cone 6. The former is porous and completely non-vitreous and the latter is semi-vitreous. This demonstrates something else interesting: The impracticality of calculating the thermal expansion of clay bodies based on their oxide chemistry. Talc sources MgO and low fire bodies containing it would calculate to a low thermal expansion. But the opposite happens. Why? Because these bodies are composed of mineral particles loosely sintered together. A few melt somewhat, some change their mineral form, many remain unchanged. The body's COE is the additive sum of the proportionate populations of all the particles. Good luck calculating that!

Talc was making this glaze "puffy" - here is how I fixed it.

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Fixing a puffy pottery glaze

The recipe on the right employs talc to source the MgO needed to create a silky matte texture. However, talc is a late gasser, potentially able to produce micro-bubbles in the glaze after it has begun melting. The reduction in the definition of contour edges on the tile in front and the reduction in melt fluidity tipped me off. Using my account at insight-live.com I did the calculations needed to source the MgO using dolomite instead, producing recipe G3933E. While dolomite has a far higher LOI than talc it starts releasing the gasses of its decomposition much earlier and finishes well before talc. The mug in the back confirmed my suspicions, firing with a much smoother surface (and with far better definition of the incising).

The amazing power of a 1% talc to a vitrify a stoneware body

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1% talc added to a clay body

These two unglazed pieces are the same clay fired to the same temperature. But the one on the right has 1% talc added. Greying of the straw color is a characteristic visual change when this clay body goes from cone 5 to 6. It is accompanied by a 1% drop in porosity (according to our SHAB test). This tiny amount of talc has another detectable benefit: It is also acting as a grease, the body has a noticeably smoother feel and better workability on the wheel. Talc is refractory, this curious firing benefit only happens in small amounts, beyond about 3% it has no effect. Beyond that it makes the body more refractory.

Links

URLs http://mineral.galleries.com/minerals/silicate/talc/talc.htm
Talc at Mineral.Galleries.com
URLs http://webmineral.com/data/Talc.shtml
Talc at webmineral.com
URLs http://www.luzenac.com/talc.html
Luzenac: All About Talc
URLs http://en.wikipedia.org/wiki/Talc
Talc at Wikipedia
URLs https://golchaminerals.com/
Golcha Minerals of India - Talc Supplier
Materials Dolomite
An inexpensive source of MgO and CaO for ceramic glazes, also a highly refractory material when fired in the absence of reactant fluxes.
Materials Light Magnesium Carbonate
A refractory feather-light white powder used as a source of MgO and matting agent in ceramic glazes
Materials Magnesite
Materials Pyrophyllite
Materials Nytal Talc
Materials Texas Talc 286
Materials Glacier 200 Talc
Materials Caltalc
Materials Ceramitalc HDT
Materials 4392 Rosa Blanca
Materials I.T. Talc
Materials Pioneer 2661 Talc
Materials Texas Talc 92
Materials Noor Talc
Materials Suzorite 325-PE
Materials Sierralite Talc
Materials CT-30 Talc
Materials TDM 92 Talc
Materials Luzenac Talc 00S
Materials Talc 2C
Materials Desertalc
Materials Talcron
Materials Spluga Talc
Materials Amtalc-C98
Materials Ceramitalc
Materials Cimcoat 325 Talc
Materials Roudnice Talc
Materials VanTalc
Materials FinnTalc
Materials Sepiolite
Materials Steatite
Materials Benwood Talc 2213
Materials Natural Talc C-98
Materials Cimtuff 9115 Talc
Materials Magris Jetfil H 290 Talc
Materials Silverline 303 Talc
Hazards Talc Hazards Overview
Talc is invaluable in the ceramics industry, it is used as a glaze and body ingredient and as a parting a release agent in various processes. Is it safe?
Hazards Talc Toxicology
Temperatures Talc melts (1420-)
Temperatures Amorphous cargon burns from Texas Talc (750-850)
Temperatures Talc crystalline water vaporizes (900-1000)
Typecodes Generic Material
Generic materials are those with no brand name. Normally they are theoretical, the chemistry portrays what a specimen would be if it had no contamination. Generic materials are helpful in educational situations where students need to study material theory (later they graduate to dealing with real world materials). They are also helpful where the chemistry of an actual material is not known. Often the accuracy of calculations is sufficient using generic materials.
Typecodes Flux Source
Materials that source Na2O, K2O, Li2O, CaO, MgO and other fluxes but are not feldspars or frits. Remember that materials can be flux sources but also perform many other roles. For example, talc is a flux in high temperature glazes, but a matting agent in low temperatures ones. It can also be a flux, a filler and an expansion increaser in bodies.
Typecodes Low Expansion Material
Materials used to make bodies requiring low expansion (e.g. flameware, refractories). The individual particles of these materials have low expansion. Some of theme even expand at certain temperature ranges.
Oxides MgO - Magnesium Oxide, Magnesia
Glossary Cordierite Ceramics
Cordierite is a man-made refractory low thermal expansion crystalline solid that forms at very high temperatures (in the right mix of kaolin and talc).
Minerals Steatite
Talc is also called steatite (it is a magnesium silicate hydrate). It is the main component of soaps

Mechanisms

Body MaturityTalc 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 ExpansionTalc is used up to 60% in low fire artware bodies to increase thermal expansion so they fit commercial glazes.
Glaze OpacifierTalc is a refractory powder and can promote matteness and opacity when added to low-fire glazes.
By Tony Hansen
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