The quartz, sand or flint from which silex or silica is produced, differs according to location of the mines, but the best method is to heat the mineral at very high temperature and then plunge into cold water, which process makes it so brittle that it will break down more readily. For use in fillers or paint the floated article is the only proper one to use. To test the material for purity, weigh out, say, 10 grains of the suspected article, boil it in hydrochloric acid in a test tube, filter off the silica, wash to free it from the acid, dry and reweigh it. The loss or difference in weight represents the lime which is present in the sample. For fineness and color test as suggested for gypsum or barytes. Silica is a great absorber of oil in grinding, as it requires 30 to 32 pounds of oil for grinding 68 to 70 pounds of the fine floated silica to a medium paste. Its specific gravity averages 2.45 and its dry weight per gallon is about 7 pounds. When ground in oil and permitted to stand about it has the tendency to separate from the oil and set hard to a greater degree than gypsum, but when mixed and ground with pigments of low specific gravity, it holds pretty well in suspension, especially in a vehicle of good body.
One of the most popular paste wood fillers ever made in this country was composed of silex and was patented, but the patent has been extinct for many years. Starch was used as a means to hold the silex in suspension in the vehicle for a time, but later on additional patent claims described the suspensor as flour middlings. At the present day the use of starch or flour is very little in vogue, the manufacturers depending upon the vehicle for holding the silica or silex in suspension.