How much feldspar should be used in a Grolleg porcelain?

Monday 26th November 2018

These are insight-live.com/glossary/125">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.

Pages that reference this post in the Digitalfire Reference Library:

Grolleg Kaolin, The translucency of Polar Ice porcelain, Serious cracking in a crystalline-glazed P700 Grolleg porcelain. Why?, Four Cone 6 (1200C) Casting/Throwing Porcelain Recipes (Grolleg and #6 Tile)

This post is one of thousands found in the Digitalfire Reference Database. Most are part of a timeline maintained by Tony Hansen. You can search that timeline on the home page of digitalfire.com.