It is essential to success in grinding white lead to select linseed oil that if raw must be well settled, so as to be perfectly free of foots, and when refined or bleached is free from traces of acid or alkalies, while when boiled oil is wanted as the vehicle, it must be of the old fire or kettle boiled process, in which borate of manganese is the drying medium, so as to keep the white lead from discoloration. When the white lead is to be used for exterior work, or when the manufacturer places one brand only on the market, the pigment should be mixed with pure raw linseed oil only and so ground to the proper consistency.
For inside use or for use in car and carriage painting, where pure white effects are required and turpentine is the principal thinner, the vehicle should be bleached or refined linseed oil only. Boiled linseed oil should be used in grinding white lead only when so specified. Raw oil produces a stringy lead, so termed by the painters; bleached or refined oil makes the lead break up readily and excellent for stippling purposes and for flatting, being shorter and of stiffer consistency. White lead ground in boiled oil is specified only for admixture with red lead for use on ironwork and for mixing with varnishes that do not take kindly to raw or refined oil, but the practice has been pretty well abandoned.
The lead mixing and grinding rooms should be kept at a normal temperature during the cold season, and white lead in oil should not be mixed, ground or stored in rooms where the temperature is less than 50 degrees F. at any time, 60 degrees F. being preferable. The reason for this caution is that white lead mixed, ground and stored at lower temperatures will to a certain extent show a gritty appearance that will not disappear even when used in the warmer season, or when strained through the finest sieve. It is also of advantage to have the oil in which white lead is to be ground of a normal temperature at not below 55 or 60 degrees F. during the winter season, so as to be limpid enough to mix and grind well. During the hot season, of course, the temperature of the oil is above the degrees named, according to where it is stored. But one of the principal precautions in white lead grinding is to avoid oil foots, and therefore it is essential to store the oil in tanks so arranged that it can be drawn without disturbing the sediment that collects in the bottom of storage tanks. By having the supply pipes above the bottoms of tanks and outlets arranged in the bottoms the sediments or foots can be drawn off from time to time, thus avoiding the source of many complaints that often are thought to be unfounded because not traced to their source. Dry white lead mixes best with oil when it has been made what is termed bone dry in the usual manner on steam jacketed drying pans of copper, or when filter pressed and then placed on wooden trays in drying ovens or kilns, and is then permitted to cool down and absorb the natural moisture of the air, which, however, should not exceed 25/100 of 1 per cent. When white lead in the so-called dry state exceeds one-half of 1 per cent. of moisture, it will not mix well with oil, heating up too much in the mixer and will not pass smoothly through the mill, and the output will be lessened to a great extent. Very much depends upon the care of the workmen in mixing the lead and oil, because by improper manipulation the material may be overheated in the mixer, and by choking up the mixers by overcharging the apparatus at times much power is wasted. White lead may be spoiled in mixing as well as in subsequent grinding through overheating. This may be determined by its having a decided yellow cast while hot, and though it will be a pure white again on cooling its opacity is destroyed.