May 29, 2009, Newsletter Issue #346: Recipe: Terracotta Casting Body
Tip of the Week
Terracotta Casting Body
Low fire red bodies can be very strong, I have measured a cone 1 red burning bodyīs strength at 10,000 lbs/square inch, thatīs as strong as many cone 10 porcelains? Low fire iron bodies turn brown when they vitrify, the red color depends on stopping well short of vitrification. Many terra cotta bodies begin to brown out suddenly above cone 1 and many even lower. Even if the body fires red, if it is burned too hot transparent glazes will flux the surface enough to turn it brown.
You can cast with 100% Redart (a widely available clay in North America) and get a nice product. However it may be too red and could use better working, drying and handling properties. Try 25% of a large particle size kaolin, 25% feldspar, and 50% Redart. If this is deflocculated properly it casts like a dream. Use the above proportions for water, clay, and deflocculant for a starting point.
Donīt be to worried about high porosity. Many commercial wares are 6-8% and yet they are very strong. Just make sure the glaze fits (can survive a hot water:ice water test) and strengthens the ware.
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