Building stone may sometimes be discolored by the oxidation of pyrite or other iron compounds in its surface. This may or may not be an injury to the appearance of the structure. Pyrite is apt to cause rusty- blotches which are objectionable, although when evenly and finely distributed through sandstone the result is some tunes enhances its appearance. Sandstones in which iron oxide is the cementing medium are often changed in color by oxidation. Siliceous sandstones are not affected in this manner.
Seasoning of Slone.—All stone is improved by being allowed to stand and dry out before being used in construction, the evapo ration of the quarry water being accompanied by hardening of the stone, and the formation of a crust upon the surface. In most eases this indurating effect is comparatively small, but some soft lime stones and sandstones, which are easily cut and weak when first quarried, soon acquire considerable hardness and strength, the sup position being that the quarry water contains a small amount of cementing material which is deposited in the pores of the stone upon the evaporation of the water. For this reason the cutting of the stone should be done before the seasoning has taken place, in order that the surface skin may not be broken. This is particularly the case where elaborate dressing or carving is to be done.
Tests for Durability.—Observations of the stone where it has been used in construction or where it has been long exposed in the quany is the best means of determining the probable durability of a stone. Stone frequently varies considerably in character in dif ferent parts of the same quarry, and this trust be taken into account in the selection.
There are no standard tests for durability. A number of tests have been proposed and sometimes applied for cotnparisons of stones, but there is no standard to which they may be referred.
Absorption Tests. Tests to determine the amount of water absorbed by stone are sometimes made. A stone absorbing little water is less likely to he injured by frost or atmospheric gases than one absorbing water freely; in making this test, it is usual to dry the stone at 100° C. until it ceases to lose weight, then soak the stone for twenty-four hours in water and weigh again.
Weight of water absorbed X 100 The percentage of absorption = Weight of dry stone The method recommended for brick (see Section 59) may also be employed for stone, although there is no standard for comparison of the results.
Frost Tests.—Tests of the effect upon a stone sample of repeatedly freezing and thawing it, while saturated, have sometimes been made. About twenty repetitions are usual, and the loss of weight or the loss in compressive strength of samples is measured. This test requires considerable time and a means of producing low tem peratures. The differences obtained are usually very small, and not easy to evaluate.
Another test intended to simulate the effects of freezing is known as the Brard test, which consists in boiling the specimen in a con centrated solution of sulphate of soda, then exposing it to the air, and observing the effects as the salt crystallizes in the pores of the stone. This is much more severe than the ordinary freezing test, and may be partly due to chemical action.
Acid Test.—Samples of the stone are sometimes immersed in weak solutions of hydrochloric and sulphuric acid, to determine the presence of soluble material, by noting the loss of weight after several clays. Exposure to an atmosphere of carbonic acid, or oxygen, is sometimes employed and changes of color observed. None of these tests has been definitely formulated and standardized.