This is a low fire fritted stoneware fired to cone 03. But it still has about 4% porosity. The green underglaze is not developing enough glass to bond well with the body surface. Repeated blows to the surface by a hammer are chipping off chunks of glaze/underglaze at the bond with the body. This is not happening with the other underglazes. The green underglaze is obviously more refractory than the others and should be reformulated.
A ceramic compound meant to be applied to green or bisque ware and covered using a transparent overglaze. There are good reasons to make your own underglazes if you are in production.
In ceramics, the zone of adherence between glaze to the underlying body is called the clay-glaze interface. The integrity of this interface is important to strength and functionality.
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