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Artware

Potters know artware as pottery firing at low temperatures with brightly coloured glazes and decorated using decals, underglazes, lustres, etc.

Key phrases linking here: talc body, art ware, artware - Learn more

Details

News Fall 2021: The hobby casting and artware markets have been turned upside down in North America. White Texas talc is is now surrounded by a climate of legal chill (for potential asbestos content). Suppliers are working on the problem and it appears dolomite can be used as a substitute without loss of whiteness. And dolomite has no health issues. Bodies will have a lower firing shrinkage and higher porosity.

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. The ball clay imparts great dry strength for handling and the talc improves the casting, drying and glaze fit properties. 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. Bodies are very stable when fired to cone 05-06, they do not warp, even on thin pieces. And they often fire amazingly white compared to stonewares, thus bright colored glazes work well on them. This enables artists to almost completely forget about technical issues of casting, glazing and firing and focus purely on aesthetics - thus the term "Artware".

Manufacturers of modelling, throwing and sculpture bodies of this type could be adapted to plastic forming by simply adding a little bentonite to improve plasticity. Such bodies have among the best throwing properties of any clay available. The beauty of this adaptation is that the whole world of prepared glazes is then available to potters and sculptors.

The Down Side

Firing and fired properties had to be compromised to get the easy working properties. Firing is done at cone 06-04, pieces are weak, you can rip some apart with your bare hands.

These bodies are only lightly sintered, they fire white simply because there is no glass development taking place, no 'glue' is forming to cement the matrix together or bring out the color of the iron.

Glaze fit: Glaze is just not 'glued on' very well. Therefore crazing and shivering will occur with much smaller thermal expansion mismatches between body and glaze compared to stoneware.

Food Safety: Low fire glazes are potentially more soluble and prone to leaching than stoneware. Most glazes are high in frit so they inherit the food safety of the frits used. However there are many ways to combine frits and additives to make glazes unsafe, unconventional recipes, especially bright colors and crystallization effects, need thorough testing to assure food safety.

Porosity: Low fire bodies are porous, they 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.

Low fire talc bodies are a bit of a king-with-no-clothes situation, they pretend to be ceramic with fancy glossy glazes but underneath they are porous and weak and unceramic.

The Materials

Talc is used in low fire hobby bodies to increase the thermal expansion, without it it is very difficult to create glazes that do not craze. Talc is also the filler, diluting the plasticity and reducing the drying shrinkage of the ball clay. Very few talcs are white enough (most fire yellowish or even brownish), in North America only Texas talcs are suitable.

Ball clay imparts plasticity to clay bodies. Ball clays are much too plastic to use alone. Ball clays are dirty (high in iron) compared to kaolins, it is normal to seek out the whitest ball clays available, especially those that have low soluble salts content.

Bentonites are ball clays on steroids! They add a lot of plasticity in small percentages. Bentonites are dirtier than ball clays but since the amount needed is so small it is not an issue. Bentonites are used in porcelain casting bodies to add leather hard strength and shrinkage to enable them to pull away from the mold.

Whiting or Calcium Carbonate: Normally small additions of this are employed (e.g. 5%). Many people think it is used to whiten the color, but not so. Whiting reduces a phenomenon called 'moisture expansion', causing glazes that are otherwise fitting to be stretched and therefore craze. The mechanism of this could be increasing the open porosity of the fired matrix, giving water a way to more quickly channel out when pieces are heated. It is also thought to assist in the development of crystal species that increase thermal expansion.

Safety

Talc is a magnesium silicate, either fibrous (like asbestos) or platy (like kaolin). Platy talcs are the ones that are useful in ceramics and producers make great efforts to insure the purity of their products (platy only). Ball clays contain significant quartz. Dolomites are not hazardous.

The Recipe

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. It is very valuable to understand the principles of deflocculation to properly tune this recipe to your materials and water supply.

A Typical cone 06-04 Ceramic Slip

Talc 50.0
Ball Clay 50.0
Water 45.0% of dry amt
Soda Ash 0.1% of dry amt
Sodium Silicate 0.2-0.4% of dry amt

If crazing is a problem: Use more talc and less ball clay (some people use a 60:40 talc:ball clay mix for casting). Be aware that commercial glazes do not all have the same thermal expansion, some will fit better than others. Test the transparent version of their glaze first.

Depending on the type of ware you make and glazes employed, you may have the option of firing higher (thus getting an improvement in strength). Lots of commercial glazes will handle up to cone 03.

Slips, engobes: At low temperatures these may not stick to the body very well unless you tune them to fit specifically and apply them at leather hard stage.

Commercial underglazes don’t develop a glassy phase at low fire so they do not bond well with the body. Consider testing different brands, they are not created equal. Or, better yet, make your own.

Related Information

Mural made using talc body and underglazes only

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Anti-racism themed. The clay is Plainsman L213. Spectrum underglazes.

Mosaic by Sikiu Perez

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Low temperature white talc body with bright glazes.

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!

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.

How to make a ceramic time-bomb

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This mug is pinging loudly and literally self-destructing in front of my eyes! Why? The glaze is under so much compression (the inside is pushing outward, the outside inward). Spiral cracks are developing all the way up the side. Small razor-sharp flakes are shivering off convex contours. Why? I accidentally fired a low-temperate talc body at cone 6 (the glaze is the Alberta Slip base cone 6 glossy). The clay body is not overly mature, but it just has an extremely high thermal expansion (talc is added to increase the expansion to fit low fire commercial glazes, they would craze without it). Shivering is serious, it is a mismatch of thermal expansion between body and glaze. It can happen at any temperature.

Are you firing a white talc body to cone 04 for durability. You are not getting it!

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Cone 04 is higher than cone 06 but ware is still very porous and weak. While it might be able to withstand handling, it may not be able to withstand being burst by its own glaze. Or shivering. Or crazing. Consider using a terra cotta with an white engobe. Fire to cone 03. This piece has a black engobe on the outside and a glaze under compression on the inside.

The talc body looked white until I put it beside a cone 04 porcelain

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On the right is L213, a body made using Taxas talc and Kentucky ball clay. On the left is L3778H, a translucent porcelain made using Grolleg kaolin, Nepheline syenite and silica. It normally vitrifies at cone 6 but we don’t need vitrification here because this is for artware, just like L213. At cone 04 these two have similar porosities and fired strengths. The glaze is G1916QL1, a crystal clear transparent glaze. The glaze does craze after a few days on the porcelain. Increasing the firing temperature to cone 03 extends that time to 3 weeks. Cone 02 may extend it further. Other options to improve glaze frit would be to increase the silica content in the body (at the expense of kaolin) and further reduce the thermal expansion of the glaze.

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.

Inbound Photo Links



Medium fire white bodies used at cone 04. Is that smart?


Stilts not always needed when firing artware pieces with glazed bottoms

Artware and stoneware pottery mugs
Which is better for hobby functional ware? Cone 04? Cone 10 reduction?

Links

Glossary Terra Cotta
A type of red firing pottery. Terra cotta clay is available almost everywhere, it is fired at low temperatures. But quality is deceptively difficult to achieve.
Glossary Low Temperature Glaze
In ceramics, glazes are loosely classified as low, medium and high temperature. Low temperature is in the cone 06-2 range (about 1800F-2000F).
By Tony Hansen
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