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Stoneware Casting Body Recipes
Section: Clay Bodies, Subsection: Formulation
Some starting recipes for stoneware and porcelain with information on how to adjust and adapt them
Here are a couple of starting recipes for casting stoneware bodies. It is assumed, of course, that you might need to make adjustments. Remember that property adjustments are always on a trade-off basis. If you want a super white body, for example, you are going to have to give up some working properties.
Once you get a feel for why each of the materials is in a porcelain or stoneware body you will find that balancing all of the factors involved will lead you toward only one possible recipe. For example, you need enough silica so glazes will not craze and enough feldspar to vitrify the body. That leaves the remainder for clay which you can mix and match for your needs. If two researchers are given a clear description of the properties needed in a porcelain, the way it will be fired and the set of materials to work with they will come up with the same recipe.
Cone 6 Porcelain Clay Slip
Talc is not a strong flux at cone 6, but for some reason small amounts can have a marked effect on the development of maturity (only 1% can reduce the porosity by 1%). At cone 6 it takes a lot of feldspar to vitrify the body, nearly twice as much as for cone 10. This recipe will deflocculate to 1.8 specific gravity and will fire to produce vitrified ware that is extremely strong and durable compared to typical artware. It has a long firing range and can produce ware of excellent translucency with clays of low titanium content.
Cone 10 Whiteware Clay Slip
At cone 10 much less feldspar is needed. Thus there is more room for clay and silica in the recipe. If you decide to reduce the ball clay in favor of kaolin, remember that the former contributes quite a bit of quartz and thus glazes might craze. To compensate you might have to increase the silica to 30.
Deflocculation refers to the magic process of creating a slurry with only half the amount of water that would normally be necessary (see the link to an article on this topic). Generally you measure the water and put it into a container, start your propeller mixer and add most but not all of the deflocculant. Then you add the powder body mix. The slurry becomes very heavy so you need a powerful mixer than can run for hours. Getting the last bit of powder to mix in can take some time so patience is required.
Note that water and deflocculant amounts are examples, you need to know how to measure specific gravity and viscosity to adapt their amounts to your materials and water (see the article on deflocculation for more information).
Comparison to Low Fire Artware Casting Bodies
I emphasize again that these bodies are not as robust as a ball clay:talc low fire casting slip used in the hobby casting industry. These bodies will present more mold release problems, green strength will be lower, and you simply will not be able to use some of the molds that work well with talc slip. Also, it is important to have molds that have smooth surfaces; rough porous surfaces provide too many places for the slip to 'hang on' and you will have even more release problems. If necessary do a dusting of talc on problem mold areas where pieces will not release. If pieces do not have the strength to shrink away from the mold without pulling apart consider using a plastic kaolin (e.g. #6 Tile). These will greatly increase the strength (don't assume a kaolin is plastic because people tell you it is, slurry up, dewater and evaluate the ones you have to determine which is most plastic).
Brown Burning Casting Bodies
It is generally best not to use iron oxide additions, they tend to gel the slurry. Rather, employ a high iron clay like the American Redart, Banta or Carbondale materials in place of some or all of the ball clay and kaolin.
When Money is No Object
If you are willing to spend extra money you can greatly improve your casting slip. For example, it is amazing how much zirconium silicate additions can whiten the fired product. Likewise, the use of micro-fine calcined alumina instead of silica will drastically increase fired strength and whiteness and reduce thermal expansion. The use of a body frit will enable you to make a lower firing body. Blue stain can be added to brighten color or a decolorizer like antimony can be added to cancel the browning effect of iron. If you have a kiln that will go to cone 20 you can remove most of the feldspar and add other things that impart properties you want. If you know alot about firing and have a kiln in which you can control the gases in the atmosphere you can do some amazing things. There is no real limit to creativity in developing porcelain recipes. Dental porcelain, for example, is a mix of nearly 100% frit, it has almost no plasticity. However it vitrifies at below the bisque temperatures an average potter would use.
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