174. Fire Resistance

The ability of a stone to withstand the action of fire is often of much consequence, especially when it is exposed to fire risks on all sides, as is the case with most business blocks. Of the different kinds of stone used for building the compact, fine-grain sandstones withstand the action of fire the best; limestones and marbles suffer the worst (becoming calcined under an intense heat) and granites are intermediate. The best sandstones generally come out uninjured, except for the discoloration caused by smoke. Granites do not collapse, but the face of the stone generally splits off and flies to pieces, often with explosive violence.

175. New Stones

If, in selecting a building stone, it is deemed advisable to use a stone from, a new quarry, and the weathering qualities of the stone have not been tested by actual use in buildings, the architect should insist upon a chemical and microscopic test of the stone by an expert to see if there is anything in the composition or structure of the stone that would render it unsuited for building purposes, and if the report is favorable, and the stone meets the tests described in the following sections, he may then use it with a free conscience.

An architect cannot be too careful about using a new stone, or one that has not been used under similar circumstances, and whenever he is obliged to use such a stone he should take pains to obtain as much information in regard to it, from all practical sources, as possible.

The writer has known of a case in which a stone, which had for a long time been used for making ashlar, was used in the piers under a seven-story building, and the piers commenced to crack under only about one-one-hundredth part of the breaking strength of the stone as given in a published report of tests on the strength of stone, and it cost nearly $200,000 to repair the damage and substitute other stone. The failure of the stone (which was a lava stone) was supposed to be due to fine cracks produced in blasting out the stone from the quarry.

It will not always do, either, to rely upon the past reputation of a stone for durability, as the quality of building stones from the same quarry often differ.