Perhaps you are shocked that a material this dark and dirty (the bars are fired from cone 1 to 7 oxidation, bottom to top), would be used in porcelains. Why? Bentonites are very difficult to process. This is just raw bentonite (HPM-20), dry ground to -325 mesh (to guarantee no fired specks). That grinding does not reduce the soluble salts (that melt by cone 4) or the iron (which accounts for the dark-burning color). These undesirable properties must be tolerated (as whiteness loss) to get the plasticity supercharge 3-5% of this can impart. Why not use super-white bentonites or smectites instead? They can cost ten or even twenty times as much!
|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.
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