25347 second, silicate of alumina. The free silica gives grain, refractoriness, porosity, and low shrinkage to the sand, while the silicate of alumina furnishes bond. Free silica would be useless without the silicate of alumina, as it would not hold together. Silicate of alumina would be worthless without free silica, as it would not have sufficient porosity and would have too great a shrinkage. A confusion exists in the use of the word "silica" in respect to sand, which I shall endeavor to

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... d by designating the silica existing in the free state -quartz silica.