Kaolinite is the closest thing we have to pure clay mineral. Clays that have been deposited at or near their site of alteration. They typically are contaminated by rocks and mineral particles that must be removed by processing. The cleanest and lowest iron clays are from this type of deposit. Kaolins are primary clays.
A Grolleg based porcelain (right) vs. #6 Tile Kaolin based porcelain
Both of them employ raw bentonite to augment the plasticity and both have about 50% kaolin in the recipe. The Grolleg body requires more bentonite (because it is much less plastic than #6 tile). In spite of the fact that the raw bentonite has a high iron content and it darkens the color, the Grolleg porcelain is still much whiter.
The white one feels smoother, but it is actually far coarser. Why?
Large particle kaolin (left) and small-particle ball clay (right) DFAC drying disks demonstrate the dramatic difference in drying shrinkage and performance between these two extremes (these disks are dried with the center portion covered to set up a water content differential to add stresses that cause cracking). These materials both feel super-smooth, in fact, the white one feels smoother. But the ultimate particles tell the opposite story. The ball clay particles (grey clay) are far smaller (ten times or more). The particles of the kaolin (white) are flatter and lay down as such, that is why it feels smoother.
Fired color of a New Zealand based porcelain compared to other bodies
The whitest test bar here is a New-Zealand-kaolin-based cone 6 porcelain (employs VeeGum for plasticity). Immediately to the left of it are three North American-koalin-based bodies using standard bentonites. The bar to is right in a Grolleg based body that uses a standard bentonite rather than a white burning one. All are plastic.
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