|Monthly Tech-Tip |
The reason for this rather strange-looking wrinkling, experienced by one potter, is not obvious. But we are suspicious that the issue occurs at the application stage, and that substituting the calcined kaolin for raw kaolin in the recipe will thus resolve it. The G1214Z matte base glaze contains a high percentage of clay, kaolin to be specific. That can increase drying shrinkage enough for cracking to occur, leading to crawling. To reduce shrinkage we supply the kaolin as a mix of raw and calcined. The original recipe called for 36 kaolin, we supplied that with 20 EPK and 14 calcined kaolin. But that only adds up to 34, not 36. Why? Kaolin experiences a 12% LOI (weight-loss on firing). That means that 14 calcined kaolin supplies the same oxides as 16 raw EPK (14/0.88=16). Would this glaze work with 36 kaolin? For most people it would. As long as the bisque is porous enough to absorb water quickly, the glaze slurry does not have too much water and glaze is not applied too thickly. Adding a small amount of CMC gum, e.g. 0.1%, would also help if cracking occurs.
G1214Z - Cone 6 Silky Matte
This glaze was born as a demonstration of how to use chemistry to convert a glossy cone 6 glaze into a matte.