•The secret to cool bodies and glazes is a lot of testing.
•The secret to know what to test is material and chemistry knowledge.
•The secret to learning from testing is documentation.
•The place to test, do the chemistry and document is an account at https://insight-live.com
•The place to get the knowledge is https://digitalfire.com
Most often the term stoneware refers to a high fired (about 1200C+) ceramic clay:feldspar:quartz blend that is semi-vitreous (not translucent and not zero porosity). To appreciate the scope that stoneware can encompass it is helpful to contrast it with porcelains (this description is for people who make stoneware, not users): Stonewares fire darker colors, are less dense (have higher porosity), are less vitreous, the fired surface is less homogeneous, the fired shrinkage is much less and the fired strength is lower. The working properties of stonewares are more robust, they dry with fewer cracks yet are more plastic, they often have coarser texture (although they can be very slick and smooth), they can be fired more rapidly and are more stable and warp-resistant in the kiln. Stonewares are less expensive, often dramatically so. 'Whiteware' is a subset of stoneware, it fires white like porcelains but is not zero-porosity (thus not highly vitreous).
Stonewares can be glazed to create ware that approaches the strength of porcelains. Stoneware bodies are thus more practical for the vast majority of ware made by industry, potters and hobbyists. Industrial stonewares are typically glazed with opaque and colored glazes, the body is fine particled. Stonewares used by potters and traditional potteries generally employ less refined materials, even fireclays, they can fire brown, grey, buff or off-white. They also commonly have some fired-speckle-producing impurities and some particulate material, such as sand or fine grog, that impart texture.
Stonewares are noted for their excellent working properties, this is because they are ball-clay-based rather than kaolin-based (although they can contain some kaolin). Ball clay levels can be as high as 70% in some super-plastic versions, this is possible because many ball clays are quite vitreous at higher temperatures and they contain significant natural quartz (meaning that only small additions of feldspar and/or quartz are needed). Some natural stoneware clays have sufficient levels of plasticity, quartz and feldspar that they can be used as stoneware with no additions (Plainsman Clays mine such materials from the Whitemud Formation in Saskatchewan, Canada).
When stonewares employ more coarsely ground clay materials they often have a minimum safe density. That means that during final stages of firing bloating will begin before zero porosity is attained - this happens because feldspars are densifying the matrix while certain larger particles are still decomposing and generating gases. Fired maturity studies (measuring fired shrinkage and porosity over a range of temperatures) are important to establish a compromise firing temperature that produces adequate strength while having a margin against bloating.
The quartz particles in stonewares form a skeletal structure that imparts strength during firing. Voids between the particles find feldspars melting and bonding the structure and the crystal transformation of kaolinte to needle-like mullite (porcelains take these processes further). So the formulation of stoneware is mainly about creating a matrix where this will happen while at the same time producing a body that is easy to form and dries without problems. Formulating stonewares is about being able to measure their physical properties (plastic, dry and fired) and adjust recipes accordingly (e.g. the DFAC, SHAB, LDW tests). Stoneware bodies need enough quartz so glazes do not craze, enough feldspar so that they develop the desired fired density (1.5% porosity, for example) and enough plasticity for the forming process (but no so much that drying problems arise). Throwing bodies, for example, typically have 5-7% drying shrinkage (any more than than will often mean drying cracks). Plasticity is easily achieved either by selecting more plastic clays or adding bentonite (often as little as 1% can make a difference). When fired color is desired, iron-containing clays can be employed (these are often very plastic).
Low temperature bodies having strengths that rival high temperature stonewares can be made simply by employing fluxes that melt there. Terra cotta clays, for example, contain natural melters that will quickly densify and mature bodies as low as cone 02. Frits can be added to make stoneware at almost any temperature (if price is no object).
Brown and buff stoneware clays compared to a porcelain at 1300C in reduction
Courtesy of Plainsman Clays.
Body frits are very useful
Stoneware at cone 02? Yes. These test bars are fired to cone 02. The top body is 50:50 Redart and a silty raw material from Plainsman Clays (named 3D) plus some bentonite and 1% iron. The bottom one also has 5% Ferro Frit 3110. The porosity: The bottom one is 3%, the top one 8%. So each 1% frit reduces the porosity by 1% in this case.
Stonewares often dry alot better than porcelains
The mugs on the left are made from a moderate plasticity stoneware clay. On the right: A highly plastic porcelainous whiteware type body. On the left, there is not the slightest hint of a crack anywhere. Right: 3 s-cracks on the bases, 2 handles have separated completely at the bottom and all have small cracks at the join. It is little wonder that stonewares are preferred by potters, they produce much fewer rejects.
Cone 6 iron stoneware cross section close-up with glaze
The glaze is well melted, but the interfacial zone with the body is wider than terra cotta but much narrower than for porcelain. The body is developing glassy phases as does porcelain and stoneware and its color has changed from red to brown. However it is possible to add a frit and glass-bond the particles at cone 02 (at much higher cost of course). Not surprisingly, glazes must be more closely tuned to match the thermal expansion of the body for lower temperatures (since they are not stuck on as well).
Is Lincoln 60 really a fireclay? Simple physical testing says...
Materials are not always what their name suggests. These are Lincoln Fireclay test bars fired from cone 6-11 oxidation and 10 reduction (top). The clay vitrifies progressively from cone 7 upward (3% porosity at cone 7 to 0.1% by cone 10 oxidation and reduction, bloating by cone 11). Is it a really fireclay? No.
A Weeks stoneware crock rolling machine
These were used in the early 1900s to make crocks up to 60 gallon size in the Medalta Potteries factory in Medicine Hat, Alberta, Canada. The metal barrel was fitted with a wooden base, then lined with paper. Then the clay was rolled up the side and surface finished. The barrel was then dropped and the crock removed.
Surface treatment affects glaze speck development in jiggered stoneware
Notice the inside of this large transparent glazed cone 6 stoneware bowl. There is a concentration of specks on one part because that area was sponged at the leather hard and dry stages to smooth surface problems that happened during the jiggering process. These specks are normally driven below the surface during forming.
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By Tony Hansen