Kaolin...............52%
Potters flint........35% (Sourse of Alumina)
Feldspar (potash)....13%
Matures at cone 10.
This would be fine for slip casting but not plastic enough for other forming methods. In order to improve the plasticity you can replace up to a third of the kaolin with a good ball clay. Replacing some or all of the flint with calcined alumina will result in a stronger product (up to 300% improvement in fired strenth!). The amount of feldspar determines how much glass is formed on firing, more feldspar means more glass and lower porosity. Too little feldspar will result in a weak product (unless fired VERY high). You can also try replacing the feldspar with the same amount of nepheline syenite, this will not only cause the body to mature at cone 8 but it also acts as a source of alumina to some extent. You should have no trouble in locating these materials from the ceramics supply store.
Example
The substrate was made by slip-casting the following mixture in a plaster mould: Alumina (calcined): 33.5% (alumina) G200 feldspar: 13.5% EPK kaolin: 53.0% To above mixture water was added to the amount of 25% of the total. The slip was deflocculated by adding about 1% of Darvan #7. After casting/drying it was fired to about 1300 degrees Celsius to give a very hard, strong, paper white, slightly porous ceramic body.
See Magic of Fire for more information on ceramic working and materials.
It would be highly advantageous to sand blast the finished product.
Another type of substrate that may be suitable can be fabricated using abrasive grit and polyester resin (available from car accessory shop).
Al2O2 (aluminium oxide) or silicon carbide (carborundum) is suitable and the grit size should be from about 30 to 80. Using mesh size 60 you should mix 1 part (weight) resin to 3.6 parts grit (60)
and mix well. If a grit size is not 60 you may have to add more or less resin. Then add the hardiner. This mixture should be placed into a mould that will give the finsihed product a rough surface. A mold can be made from potters clay (no need to fire, only dry it) or a sand mold. A plastic pipe will not make a suitable mold.
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