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.
When you buy a low temperature white burning body from a ceramic supplier you are buying what the industry calls a 'talc body'. Talc bodies are basically 50% ball clay and 50% talc (variations are discussed below). If your supplier says the recipe is a secret, let it pass, everyone is entitled to their beliefs. On the other hand, some suppliers have gone through a lot of effort to choose the best ball clay and talc available in their area and they might be putting in a small amount of other things. The ball clay in this body gives it great dry strength for handling and the talc imparts the good casting, drying and glaze fit properties (more on that in a minute). The use of this type of body started in the hobby casting industry and it became a standard on which the prepared glaze industry could rely. They created all of their products to fit this type of body. This standardization was a big reason for the success of what the commercial ceramic manufacturing industry would regard as an unorthodox and problematic recipe. However hobby cast ware has even thicknesses and is fired relatively slowly in top loading kilns so the body does not suffer process related failures that large industrial users would encounter (firing cracks, glaze crazing). Almost anyone can cast it successfully and very difficult shapes can be made because of its high wet and drying strength. It has both a fast casting rate and good strength (qualities not normally found together). Talc bodies are also very stable when fired to cone 05-06, they do not warp, even on thin pieces. So again, hobbyists with no knowledge of ceramic manufacture can make complex overhung shapes and never even think about warping issues. Another factor is that talc bodies often fire amazingly white compared to stonewares and thus bright colored glazes work well on them.
Manufacturers of modeling, throwing and sculpture bodies figured out that this 50:50 talc:ball clay recipe could be adapted to plastic forming by simply adding a little bentonite to improve plasticity. Such bodies have the best throwing properties, for example, of any clay available. The beauty of this adaptation is that the whole world of prepared glazes is then available to potters and sculptors.
There has to be some downside to this. There is.
Firing and fired properties had to be compromised to get the easy working properties. Firing is done at cone 06, that is a very low temperature, there is no getting around it. Pieces are weak, you can rip them apart with your bare hands easily. They are completely unsuitable for functional uses. You can fire higher to get more strength but you will not find commercial glazes to fit. And if you fire too high (e.g. cone 02) the ware can become brittle with some ball clay:talc combinations (others can survive to cone 6 amazingly).
Talc bodies fire white simply because there is no glass development taking place, no 'glue' is forming to cement the matrix together (that is why pieces are weak). However if you fire a talc body higher the color darkens dramatically because glass development brings out the color of the iron.
These bodies have 50% ball clay, there is no getting around it. Ball clay by itself will shiver almost any glaze on the planet. Ball clay contains lots of free quartz, each quartz particle 'throws tantrums' when heated and cooled through 1060F (sudden expansion and contraction of up to 5%). If you have enough of these particles they impose their character on the whole matrix. In addition, micro-cracks radiate outward from each grain of quartz and the degree of cracking varies geometrically with grain size. Micro-cracks become mega cracks if firing is too quick or pieces have too much variation in thickness.
Glaze fit: Glaze is just not 'glued on' very well. Therefore crazing and shivering will occur with much smaller differences in body and glaze thermal expansion than in stoneware where there is a highly develop clay/glaze interface.
Food Safety: Low fire glazes are much more soluble and prone to leaching and they are not nearly as hard a strong. But they look good, right!
Porosity: Low fire bodies are porous, up to 15% air space! Than means they will soak up water like a sponge. If a piece stays wet it can breed bacteria. If the water gets between two glazed walls it can explode a piece when it turns to steam during heating.
So, low fire talc bodies are a bit of a king-with-no-clothes situation (complete with a crowd of people so absorbed with the whiteness that they overlook the problems with the whole process). These bodies pretend to be ceramic with fancy glossy glazes but underneath they are porous and weak and there unceramic.
Talc is used in low fire hobby bodies for several reasons:
It increases the thermal expansion. Without the talc it is very difficult to create glazes with a low enough thermal expansion to prevent crazing on typical clay bodies made from clay, feldspar and quartz. So adding talc was a no-brainer. You must realize however that this creates possible problems. If the body expands more on heatup (and therefore contracts more on cool down) then you are going to have more firing cracks if there are gradients in the ware (variations in thickness that cause variations in temperature across the cross section during heat up/cool down).
Talc is white and therefore a good major ingredient in a white burning body. I should correct that. Some talcs are white and some fire very yellowish or even brownish. Therefore, if you are formulating your own body, try many different talcs to find the whitest one. However take into consideration that the whitest one might not be the most consistent one. A good policy is to use two or more talcs in a recipe to dilute the effects of changes in one of them.
Another thing to keep in mind is that there are a lot of differences in the mineralogies of different talcs. It is quite remarkable how different they can behave in terms of melting, expansion, color, working properties. One thing to be especially aware of: Some talcs are much more temperature volatile than others and will cause a body to liquefy if over fired too much, others will be more temperature stable.
Ball clay imparts plasticity to clay bodies. It is much too plastic to use by itself, thus it is mixed with other materials. Talc is an ideal complement to ball clay to create a dryable body that still has excellent working properties (one of the primary reasons is its particle shape). People used to using porcelains and stoneware are often surprised at the feel of a talc body.
High-ball clay bodies are not used in industry because of drying and firing cracks and glaze shivering (from the high quartz content), but in the hobby casting market it works because of the 50% talc to dilute the problematic effects of high ball clay. So if you use talc bodies, you just need to be aware that you are using a body that industry could not use because you are willing to shepherd it through the process slower and get a weaker more porous fired product than what they are willing to tolerate.
Ball clays are dirty (high in iron) compared to kaolins. However they are so much more plastic that the iron is considered tolerable. However in talc bodies it is normal to use the whitest ball clays available, especially those that have low soluble salts content (which can produce an off-color scum on the fired surface).
Since ball clays are a natural mined product they are usually just blended, ground and sold. That means there is variation in their chemistry and mineralogy (and also particle size if the manufacturer is not vigilant). Thus it is best if your talc body employs more than one ball clay to dilute the effects of changes in any one of them. Ask your supplier if this is the case (if you are using a prepared clay body).
Bentonite is ball clay on steroids! It adds a lot of plasticity for a little addition. Bentonite is also dirty (high in red burning iron), the more you use the darker the body will fire (white bentonites are not plastic enough and they are too expensive, forget about them for this use). Do not put bentonite in the casting version of this body, the casting rate will slow down dramatically.
Whiting or Calcium Carbonate: Many talc bodies have small additions of this (e.g. 5%). Many people think it is used to whiten the color, but not so. Whiting reduces a phenomenon called 'moisture expansion' that occurs in low firing porous bodies when they soak up water. This expansion causes a glaze that was otherwise fitting to be stretched and therefore craze.
The ideal firing circumstance is that every section of a piece be at the same temperature throughout the firing. When this is true you can fire very fast. However if the kiln is not heating evenly, for example, part of a piece will get ahead of the rest on heatup. As the piece goes through critical temperatures at which sudden expansion occurs the variation in temperatures across it will translate into waves of expansion moving across it. When these waves hit weak or thinner sections, something has to give.
If a piece has widely varying thickness across its cross section, then on cool down of the kiln the thicker sections are going to lag behind. As above, waves of expansion change will travel across the piece and crack it.
As if the above are not enough to worry about, remember that talc bodies are made from materials susceptible to this issue. Together they create a body designed have a high expansion. It sounds like a recipe for cracks for the unwary. If it were not for its open and porous nature to stop the majority of microcracks (at the nearest pore) talc bodies would be much more prone to cracking.
So if you are forming ware using a technique that produces wide variations in thickness (e.g. throwing, pressing) be wary.
Talc is a fibrous magnesium silicate. So is asbestos. They often occur together geologically. These types of mineral particles like to get stuck in your lungs. Ball clays contain lots of very small quartz particles that can be sharply angular and they like to get very deep into the lungs and get stuck there also. Millions of these stuck particles cause silicosis and asbestosic. Thus hobby casting bodies are a mixture of the two most potentially dangerous materials in ceramics. So caution is needed, be aware.
Can you test incoming body stocks for problems? Yes, you must. The low fire process has so many advantages but remember: it has costs. The biggest single issue is likely glaze fit, crazing and shivering are so much more common than with stoneware. You really should glaze test pieces and then stress them using a boiling water:ice water test. At the first sign of a problem adjust your glaze accordingly. If you are using a commercial glaze contact your supplier to make sure they have a strategy to do this (e.g. mixing two base glazes, using an additive). If you make your own glaze use a selection of high and low expansion frits so you can trade them off against each other to adjust expansion to adapt.
Check incoming material for large quartz grains. You can do this by simply washing some of the material through a 325 screen. Larger quartz particles will be immediately evident. Their presence can give you an alert to call the supplier and to watch your firing.
Do a test firing for color. If it is darker this could be an alert that your supplier has changed talcs. If it is lighter or darker they might have changed ball clays. This could mean that other working and firing characteristic could be affected.
Use this recipe as a starting point. For the casting version start using the amount of water and deflocculant shown. Note that slip casting recipes that show water and deflocculant do so in terms of the total clay content. Thus 45% water means that for 100 pounds of clay you need 45 lbs of water.
45.0% of dry amt
0.1% of dry amt
0.2-0.4% of dry amt
For the modeling version add bentonite (up to 5%). Use the most plastic bentonite you can find, do not worry about the fired color, 5% of even a dark brown firing bentonite should not change the color of the body much.
Use more than one ball clay and talc if you can (to insulate changes).
If crazing is a perennial problem, then use more talc and less ball clay (some people use a 60:40 talc:ball clay mix for casting. However if you are using the modeling version with added bentonite, think carefully before cutting the ball clay to 40%, it is needed for plasticity.
Consider adding some whiting to prevent moisture expansion and a small amount of barium carbonate (0.3%) to precipitate soluble salts. If you are adventurous, consider swapping some of the ball clay for kaolin, it is a much whiter, more water permeable but less plastic clay.
If you want to fire higher for more strength beware that the body will be darker in color (there is still lots of iron in even the cleanest talcs and ball clays and higher temperatures form glasses that reveal its presence).
Ware is normally very resistant to warping when fired at the typical cone 06-04. But do not get crazy, drastically overhung pieces can warp. Of course there is a big disadvantage to stability in the kiln: at these low temperatures ware is very weak and porous, thin ware can often be torn apart with your bare hands. You can burn to cone 03-02 or maybe even as high as 1 (but no higher) and get huge increases in strength but you will not be able to use commercial glazes, you will have to formulate your own (the frit companies can help you with a recipe, get them to give you a low and high expansion frit so you can trade them off for each other to adjust thermal expansion if needed). Don't even think about firing to cone 6, talc:clay mixes can melt suddenly and completely ruin your kiln. In addition, they can produce a very brittle matrix. In addition, from 1100 C on talc decomposes and releases H20 that cause lots of bubbles in glazes.
Slips, engobes: At low temperatures these may not stick to the body very well at all. If the thermal expansion of your glaze and clay body are not matched well, the glaze will simply part ways with the body at the slip or engobe buffer layer. Underglazes should be OK as long as they melt well, are not too thick and they have a matching thermal expansion.
Talc bodies are fine as long as you use them within the universe for which they were designed. All those flashy commercial glazes are pretty compelling. If you do not have any compunction to understand how glazes or clay bodies work then don't worry, be happy.
But if you working outside the 'hobby ceramics box' then beware. You need to know the limitations of what you are using. If you run into problems it is likely not the fault of the body manufacturer. He is assuming that you know the talc body and the world it is in the center of. If you are at the edges of that world then you are on your own.
On the other hand you might be wanting to dig deeper with the objective of making stronger ware. Think about firing to at least cone 02 (where stoneware strength can be achieved using terra cotta bodies). Learn to make your own glaze that you can adjust and control. Lots of prepared low fire bodies will fire to cone 02 easily. If you mix your own clay body then you are in an ideal position to change to a different recipe that will produce stronger and more functional ware.
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.
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By Tony Hansen