The paint grinder or color maker will select the barytes he requires by assaying for fineness and whiteness (absence of color) principally. It was the custom with some barytes manufacturers to add minute quantities of ultramarine blue to the pulp to disguise the yellow cast in some of their goods, but discriminating color makers objected to the blued article, because when using it with certain colors as an extender the addition proved anything but beneficial. And paint grinders who used barytes as extender for white pastes did not favor it for the reason that it was an easy matter for them to do the blueing if such was deemed advisable.
To test barytes for fineness, it is mixed with spirits of turpentine until the mixture is rather liquid and spread with a spatula upon a strip of glass, when it must not scratch the glass nor show any coarse particles, but should present a uniformly smooth film, which after the evaporation of the turpentine will also indicate the whiteness of the material. Of course, the test is best made in comparison with an adopted standard. The test for absence of color by placing small hillocks of dry barytes on a piece of white paper and pressing down the hillocks with paper laid on top is sometimes misleading and not conclusive enough for the paint grinder or color maker, and this also applies to the practice of rubbing out the dry powder on paper with a spatula.