HPM-20 micro-fine bentonite fired from cone 1 to 7 in oxidation. This bentonite is expensive compared to others and it used for the guarantee that there are no speck producing particles. However it is still high in soluble salts (that melt by cone 4) and is very dark burning in color. It is not unusual to put 3-5% of this (and other dirtier bentonites) into Grolleg porcelain bodies (where whiteness is supposedly important).
|Materials||HPM-20 Volclay Bentonite|
A common problem with dry and fired ceramic. It is evident by the presence of a light or dark colored scum on the dry or fired surface.