|Monthly Tech-Tip |
Pure HPM-20 micro-fine bentonite fired to cone 8 (top) and cone 2 (bottom) oxidation (it is actually a mix of raw and calcined material to make it possible to make the bars). Below that is an 85% silica:15% HPM-20 bentonite mix; they are fired to cone 10 (top) and 6 (bottom); these lower bars tell us the degree of plasticity imparted but also how much the bentonite is staining a normally paper-white burning material. HPM is a very expensive micro-pulverized bentonite, but, like other common bentonites, it still has significant iron. However note that much of the color on the top bars is from the soluble salts on the surface. These salts do not appear to come to the surface in the same way when mixed with the silica. It is very common to put these relatively dirty materials into porcelains to plasticize them. Why? The alternative is a material like VeeGum, it is 10-15 times the price! Still, if only a few percent of this is added, the color is affected less than you might think.
Bentonite is a super-plastic clay. This block of it took months to dry, the material really holds on to its water! It shrunk to about half the size and, of course, broke up into many pieces in the process (because bentonite has such a high drying shrinkage). That white powder is calcium sulphate, it is soluble and comes to the surface with the water as the clay dries. The finer the manufacturer grinds the material, the more salts are liberated. In most ceramic applications for commercial raw bentonites, these soluble salts are not an issue (but the iron content certainly can be). The reason these salts can be tolerated is that bentonite is normally employed in bodies and glazes in the 1-5% range.
HPM-20 Volclay Bentonite
Bentonite can make a clay body instantly plastic, only 2-3% can have a big effect. It also suspends slurries so they don't settle out and slows down drying.