|Monthly Tech-Tip |
Ovenware clay bodies have a low expansion by virtue of materials in their recipe and/or the way they are fired. But potters bend the rules.
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Ovenware clay bodies have a lower thermal expansion than typical bodies so they can withstand more sudden changes in temperature without cracking. Flameware bodies are not the same, they can withstand an open flame and demand much more compromise in working properties, strength, glaze fit, etc.
Ovenware manufacturers dedicate considerable resources to producing low-expansion bodies and matching glazes that are far more thermal shock resistant than what a typical potter can make. Still, potters have found ways to get away with using standard bodies and glazes. For example, by making sure glazes fit well (no crazing or excessive compression as proven by testing), drying and firing evenly so as not to build in stresses, avoiding highly vitreous bodies (which propagate cracks so better), bodies with quartz sand, make ware with an even cross-section, avoiding angular contours and larger sizes with broad flat bottoms and telling customers to be careful about subjecting ware to sudden temperature change.
Glaze fit is a major problem in designing an ovenware body since common glazes will certainly craze (it is much easier to make a low-expansion clay body than a glaze). Thus it is normal to compromise the lowest possible expansion on the body in order to get a reliable glaze fit. The lowest expansion glazes can be made using MgO based boron frits in glazes having the lowest possible KNaO. Glaze chemistry is invaluable to shape a compromise between low expansion and the desired appearance. Anyone serious about producing low-expansion ware needs to understand thermal expansion and be ready to change their process and endure increased costs.
There are main two mechanisms for creating a low expansion body: By firing to form a crystalline matrix that has low expansion (e.g. Corningware, cordierite) or by employing materials having particles of low expansion (e.g. mullite, pyrophyllite, petalite and kyanite) and formulating and firing in such a way that these particles are not altered. The former produces a more vitreous body and requires much more expertise and test equipment. As noted, the latter is a bit of a 'crowbar' approach and is dependent on not firing to full maturity (otherwise mineral species can be dissolved by the feldspar in the body or simply altered in crystal form and the low expansion effect is lost). This can create a bit of a 'tug-of-war' in the body since the glass that glues all the particles into a matrix will likely have a higher expansion. Obviously, ovenware bodies should have much lower free quartz content, especially the larger particle sizes, since these have very high thermal expansion. This does not just mean avoiding only ground silica, ball clays also contribute a lot of quartz.
Often, ovenware body recipes are published without any explanation about the mechanism they employ or what to do about glaze. Common recipes found in textbooks often feature a high percentage of spodumene (30%) along with some feldspar and pyrophyllite (about 10% of each) and a mix of ball clay and kaolin or stoneware clays. The object with these appears to be to create a low expansion glassy matrix in which the kaolin can convert to mullite. But if ball clays and stoneware clays are present, these contain significant amounts of quartz particles; while some may dissolve, it seems likely that much will not (a nothing in ceramics has a higher thermal expansion than quartz). Some recipes are a combination of talc and ball clay or fireclay. The intent is to create cordierite at cone 10. However, cordierite does not develop until long after cone 10 so it is no coincidence that such bodies also contain large amounts of grog (which assumes the burden of absorbing or slowing cracks). For more information on this and testing standards and labs, see the last part of the article on flameware.
A common thermal expansion-resistant non-glazed product that many potters are asked to make is pizza stones. These are fairly thin and can be diameters to 14 inches or more. Although they do not need to withstand the same shock as regular ovenwares, the flat shape makes them susceptible to cracking on the uneven heating they often are subjected to. The principles outlined above should be sufficient to create a product that will endure common use.
It can if the flame is not too big! This is a roadside stand in Mexico in 2016. Each of these "cazuelas" (casseroles) have a small flame under them to keep the food inside warm. The pedestal is unglazed. The ware is thick and heavy. The casseroles are hand decorated with under-glaze slip colors with a very thin layer of lead glaze painted over (producing a terra sigilatta type appearance, but with brush stroke texture). These have been made and used here for hundreds of years. How can they not crack over an open flame? The flame is small and it is applied in the center of the bottom so that stresses are distributed radially and symmetrically. The ware is fired at what potters in Canada or the US would consider bisque temperature. It is porous, open and able to absorb the stresses. Some of them do have small cracks, but these seem to relieve stresses and prevent more. They know these pieces are not strong, so they treat them with care in heating, handling and washing. They know the glaze will leach lead or even dissolve if they put acidic liquids in them - so they don't do that.
This is a flameware, made from a recipe promoted by a popular website. Are they serious? How could you throw this? Maybe it is possible, but we need an explanation. How could the page fail to mention how coarse this surface would be? How porous and weak ware would be? There are just so many body and glaze recipes taking screen space online, promising to be so much more than they actually area in real life!
Flameware is ceramic that can withstand sudden temperature changes without cracking. The low thermal expansion of true flameware makes craze-free glazes very difficult.
Co-efficient of Thermal Expansion
Ceramics are brittle and many types will crack if subjected to sudden heating or cooling. Some do not. Why? Differences in their co-efficients of thermal expansion.
Ferro Frit 3249
A magnesia borosilicate frit having very low thermal expansion and melting point. Invaluable in pottery as to increase the MgO in glazes to prevent crazing.
Thermal Shock Failure
A simple test any potter can do by making and firing square tiles and using a plumbing torch to see how long before they fracture.
|By Tony Hansen|
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