Hard stones, also, generally wear smooth, and
become polished, which makes them unsuitable for some purposes.
Brittleness is a defect which frequently accompanies hardness,
particularly in coarse-grained stones; it prevents them from being
worked to a true surface, and from receiving a smooth edge at the
angles. Workmen call those hard stones which can only be sawn into slabs
by the grit saw, and those soft which can be separated by a common saw.
Expansion of Stone by Heat.--Rocks are expanded by heat and contracted
by cooling. Variation in temperature thus causes some building stones
to alternately expand and contract, and this prevents the joints of
masonry from remaining close and tight. In the United States with an
annual thermometric range of more than 90 deg. Fah., this difficulty
led to some experiments on the amount of expansion and contraction in
different kinds of building stones. It was found that in fine-grained
granite the rate of expansion was .000004825 for every degree Fah., of
increment of heat; in white crystalline marble it was .000005668; and
in red sandstone .000009532, or about twice as much as in granite. In
Western America, where the climate is remarkably dry and clear, the
thermometer often gives a range of more than 80 deg. in twenty-four
hours. This great difference of temperature produces a strain so
great that it causes rocks to crack or peel off in skins or irregular
pieces, or in some cases, it disintegrates them into sand.
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