|Monthly Tech-Tip |
Alternate Names: Magnesium Silicate, Steatite, French Chalk, Hydrated talc
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 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. 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 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 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.
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 melt of most glazes has begun).
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 can detrimentally affect body plasticity. Vacuum pugged solves the problem. So if you need to reprocess scrap material in production and cannot vacuum pug this could be an issue.
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.
Talc has a complex commercial and litigious history in North America. Various deposits and products have been associated with asbestos.
Talc is commonly dusted on to plaster molds to ease release of cast pieces in (where difficult shapes are being made). It is spread by putting it 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.
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 than CaO does.
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.
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, the stoneware at cone 6. The former is porous and completely non-vitreous, 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, most remain unchanged. The body's COE is the additive sum of the proportionate populations of all the particles. Good luck calculating that!
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.
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.
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.
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 chart compares the decompositional gassing behavior of six materials as they are heated through the range 500-1700F. These materials are common in ceramic glazes, 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 your 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).
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.
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.
Because this glaze employs 10% dolomite instead of 10% calcium carbonate it has a lower thermal expansion and is less likely to craze. While the dolomite is contributing MgO, which normally mattes glazes, there is not enough to do it here.
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 color is so much darker in the raw form, it fires whiter!
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.
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).
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).
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.
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!
|Materials||Cimcoat 325 Talc|
|Materials||4392 Rosa Blanca|
|Materials||Pioneer 2661 Talc|
|Materials||Texas Talc 92|
|Materials||TDM 92 Talc|
|Materials||Glacier 200 Talc|
|Materials||Light Magnesium Carbonate|
|Materials||Texas Talc 286|
|Materials||Luzenac Talc 00S|
Bloating in clay bodies occurs when the firing goes high enough to seal the surface and prevent the passage of gases releasing inside.
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.
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.
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.
In the ceramic industry, cordierite is a man-made refractory crystalline material having extremely low thermal expansion.
|Temperatures||Amorphous cargon burns from Texas Talc (750-850)|
|Temperatures||Talc crystalline water vaporizes (900-1000)|
|Temperatures||Talc melts (1420C-)|
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.
Talc at Mineral.Galleries.com
Talc at webmineral.com
Luzenac: All About Talc
Talc at Wikipedia
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?
|Oxides||MgO - Magnesium Oxide, Magnesia|
|Body Maturity||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||Talc is used up to 60% in low fire artware bodies to increase thermal expansion so they fit commercial glazes.|
|Glaze Opacifier||Talc is a refractory powder and can promote matteness and opacity when added to low-fire glazes.|