|Monthly Tech-Tip |
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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.
These are cone 10R porcelains made using Grolleg kaolin, feldspar and silica. Plainsman P700 (left) has 20% G200 Feldspar. Coleman porcelain (popular among potters in the US) has 30% of the same feldspar. The Coleman porcelain is certainly more vitreous. In fact, it is too vitreous. It is much more likely to warp during firing. And, it is much more problematic with regard to plucking (lower left), foot rings glue to even kiln-washed shelves. But, notice that the Coleman porcelain is not any more translucent than P700 (it appears less so because I could not throw it as thin). So why is 30% feldspar even used? I have a theory: American kaolins do require 30% feldspar to vitrify at cone 10, but not Grolleg. it contains significant natural feldspar, so much so that far less feldspar is needed in the recipe. I think that, in the past, Grolleg was simply substituted for an American kaolin a standard recipe. P700 benefits in a special way by recognizing this different in Grolleg: It dedicates the 10% gain to kaolin, increasing body plasticity. On top of that it adds bentonite, producing a porcelain whose high plasticity would come as a shock to people used to the Coleman body.
A highly sought-after property in porcelain, fired close enough to melting take on the glass-like property of passing light. Translucent implies tendency to warp during firing.