Stoneware Casting Body Recipes

Description

Some starting recipes for stoneware and porcelain with information on how to adjust and adapt them

Article

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.

Maturity: For greater vitrification increase the feldspar by 5% and reduce the clay by the same and test (vice versa for less vitreous).
Thermal expansion: Increase the silica to kill crazing, reduce to eliminate shivering (remember that ball clay contains significant quartz).
Casting rate: Choose a larger particle size kaolin or ball clay for faster casting (there are large particle ball clays promoted for use in casting, note that these might be higher in iron). Don't struggle with a slow casting slurry, it is not worth it.
Whiteness: The ball clay and kolin are both clays. Minimize ball clay in favor of white burning kaolins to whiten, but watch for losses in dry strength or ability of ware to pull away from the mold. There are many different kaolins with an array of advantages and disadvantages. The ones that are good for plastic porcelains will often impede working properties of a casting slip because of their fine particle size. For optimal whiteness remove the ball clay entirely and use all kaolin.
Translucency: Use clay materials having minimal titanium content and use more feldspar for better melting.

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

Silica	15.0
Ball clay	19.0
Kaolin	19.0
Nepheline Syenite	45.0
Talc	2.0

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

Silica	25.0
Ball clay	25.0
Kaolin	25.0
Feldspar or Nepheline Syenite	25.0

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

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.

Powder Mix	100
Water	36.0% of dry amt
Soda Ash	0.05-.1% of dry amt
Sodium Silicate	about 0.2 to 0.3% of dry amt
or
Water	36% of dry amt
Darvan	0.5% of dry amt

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.

Related Information

Here is how vigorously a deflocculated ceramic slurry should be mixed

A video of the kind of agitation needed from a propeller mixer to get the best properties out of a deflocculated slurry. This is Plainsman Polar Ice mixing in a 5-gallon pail. Although it is quite plastic compared to industrial casting slips, it has a specific gravity of 1.76, is very fluid and casts well in the hands of a potter. These properties are a product of, not just the recipe, but the mixer and its ability to put high energy into the slurry.

Links

Articles	Understanding the Deflocculation Process in Slip Casting Understanding the magic of deflocculation and how to measure specific gravity and viscosity, and how to interpret the results of these tests to adjust the slip, these are the key to controlling a casting process.
Articles	Understanding the Terra Cotta Slip Casting Recipes In North America This article helps you understand a good recipe for a red casting body so that you will have control and adjustability.
Articles	Formulating a Porcelain The principles behind formulating a porcelain are quite simple. You just need to know the purpose of each material, a starting recipe and a testing regimen.
Glossary	Deflocculation Deflocculation is the magic behind the ceramic casting process, it enables slurries having impossibly low water contents and ware having amazingly low drying shrinkage
Glossary	Glaze Chemistry Glaze chemistry is the study of how the oxide chemistry of glazes relate to the way they fire. It accounts for color, surface, hardness, texture, melting temperature, thermal expansion, etc.
Glossary	Stoneware To potters, stonewares are simply high temperature, non-white bodies fired to sufficient density to make functional ware that is strong and durable.
URLs	http://www.bigceramicstore.com/Books/slipmolds.htm Books on Casting
URLs	http://www.baileypottery.com/books/molds.htm More books on Casting

By Tony Hansen
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