Of course, if the chemical analysis shows 90 per cent. or more of silica it is reasonably safe to assume that the material contains free silica. That is the case with most of the silica sand that is used in sand blast machines, also quartz, flint, quartzite, and glass sand. But granite frequently contains less than 70 per cent. although it has been proven by means of a complicated and delicate petrographical procedure that about one- third of the silica in granite is present as quartz which is undoubtedly free, crystalline silica.
In view of these facts generally known to mineralogists and in view of the chemical results for percentage total silica it seems reasonably certain that the following materials named in Table I contain free, crystalline silica, using the term “free” in its strict interpretation, namely – all granite dust samples, all the sand blast samples, construction sand, powdered quartzite, all the flint samples, quartz, and glass sand.
On the other hand the following samples probably do 3not contain free, crystalline silica, although the writer would hesitate to say with certainty until the term free silica is defined, namely – all of the clay samples, pumice, slate, kieselguhr, glass abrasives.
