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The very whitest porcelains are made from New Zealand kaolin. However, while Grolleg kaolin does not fire quite as white, it requires up to 10% less feldspar to produce a vitreous porcelain (it contains natural feldspar). That 10% less spar can be made up in kaolin, imparting better workability and dry strength to the body (and Grolleg is known for its dry strength). Assuming that 25% silica is needed for glaze fit, one only needs to discover what blend of feldspar and kaolin in the remaining 75% achieves the desired degree of vitrification (e.g. we like zero porosity just-reached at cone 6). We found 25% nepheline was too vitreous (pieces warped) and at 20% porosity was not yet zero. While the Grolleg version fires a little darker, the better workability imparted by the extra kaolin makes up for that. The plasticity needed for good throwing requires the addition of bentonite (4% for NZK and 3% for Grolleg). Both of these can be made into casting bodies by reducing the amount of bentonite (~ 1% for NZK, 0.5% for Grolleg). Do your testing to discover the % of bentonite needed for the leather hard to pull away from a mold without cracking but not take too long to cast.
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