Two reasons why porcelain recipes need silica

Tuesday 9th October 2018

This is 70% kaolin and 30% feldspar. The fired body looks very porcelaneous but right out of the kiln the crazing is like this. The thermal expansion of the kaolin:feldspar mix is too low, no ordinary glaze will fit it. The other issue is with the flat particle shape of kaolin. The throwing process has tended to line up the particles concentric to the centre. During drying and especially firing, more shrinkage occurs along the radius lines than along the concentrics. Ten of these were made at the same time and all ten cracked like this during the bisque firing (the drying process would have initiated the stresses). Added silica (preferably 20% or more), although having particles that are very small, acts like aggregate in concrete, strengthening the matrix and forcing the clay particles to orient more randomly.

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