Lithopone is the most miscible of white pigments and can be readily ground fine on any good stone mill, one having esopus stones being best for the purpose. When ground in oil in large quantities, the chaser and roller mill apparatus is best and most productive, while for grinding bases for flat wall finishes, gloss whites, etc., the stone mill is preferable and for some of these the mills are best when water cooled. It is, for many purposes in certain industries, simply mixed to paste form without grinding in oil and then thinned down to the liquid form with such vehicles as serve the purpose. The paint grinder who supplies lithopone white of normal quality in the paste form will find that with the chaser apparatus 12 pounds of a good refined linseed oil to 88 pounds pigment will produce a good workable material, while on stone mills he will require 14 pounds and 86 pounds, respectively. It would be unwise for him to purchase one of the lower grade lithopones, such as blue seal, yellow seal or black seal that contain less zinc sulphide than the green seal or red seal. It must be noted here that some of the manufacturers on the other side of the Atlantic brand the normal lithopone red seal and others green seal, so that either of these may contain 30 per cent zinc sulphide, while the other seals used may denote any percentage from 12 to 24 per cent The paint grinder, when purchasing only the normal quality, does not require any of the other brands, because in mixing he can readily add the quantity of barytes required to reduce the percentage of zinc sulphide, when he has an output for goods of that class. This will avoid carrying an assortment of the material and he will not pay for its manipulation and carriage of the goods.
A caution that will not be amiss, is to be very particular before mixing lithopone in oil, to see that it is dry, not necessarily bone dry, but so that it does not cling to the scoop or shovel, but leaves those tools fairly clean. The dry material, as is the case with zinc oxide, should be stored in a dry place, as moist lithopone does not mix well with oil and much less so with the grinding vehicles that are used in lithopone bases for flat wall finishes and inside gloss whites. Here, as in all other lines of paint making, constant care and supervision is the great need for obtaining good results.
When an apparatus has been used for grinding white lead it is natural that it should be most thoroughly scraped and cleaned before lithopone is to be mixed and ground on same. This applies also when the change is reversed. When it comes to a chemical analysis of the goods, even fractions of 1 per cent may be found and lead to a rejection of the goods with the incident loss of money and reputation. In fact, when it becomes necessary to mix and grind more than one pigment on the same apparatus, it pays to be careful.