TESTING STONE 6. Of the above four qualities, only two—durability and strength—are susceptible of laboratory testing, and even for these qualities the best known laboratory tests are not conclusive. The deterioration and partial failure of the masonry in some of the best known cathedrals of Europe, which commanded the best available talent in their construction, are startling illustrations of the imprac ticability of determining from laboratory tests the effect on stone of long-continued stress, combined perhaps with other destructive influences. Although the best technical advice was obtained in selecting the stone for the Parliament House in London, and the stone selected was undoubtedly subjected to the best known tests, it was apparently impossible to foresee the effect of the London atmos phere, which is now so seriously affecting the stone. Several of the tests to be described below should be considered as being negative tests. If the stones fail under these tests, they are probably inferior; if they do not fail, they are perhaps safe, but there is no certainty. A long experience, based on a of the characteristics of stones which have proven successful, is of far greater value than a dependence on the results of laboratory tests. The tests attempt to simulate the actual destructive agencies as far as possible, but since a great deal of stonework which was apparently satisfactory when constructed and for a few years after, has failed for a variety of reasons, attempts are made to use accelerated tests, which are supposed by thehr concentration to affect the stone in a few minutes or hours as much as the milder causes acting through a long period of years.
7. Absorption. It is generally said that stones having the least absorption are the best. The absorptive power is measured by first drying the stone for many hours in an oven, weighing it, then soaking it for, say, 24 hours, and again weighing it. The increase in the weight of the soaked stone (due to the weight of water ab sorbed), divided by the weight of the dry stone, equals the ratio of absorption. The granites will absorb as an average value a weight of water equal to about of the weight of the stone. For sand stone the ratio is about The test for absorption has but little value except to indicate a closeness of grain (or the lack of it), which probably indicates some thing about the strength of the stone, as well as its liability to some kinds of disintegration.
S. Test for Frost. The only real test is to wash, dry, and weigh test specimens, very carefully; then soak ,them in water, and expose them to intensely cold and intensely warm temperatures alternately. Finally wash, dry, and weigh them. If the freezing has resulted in breaking off small pieces, or possibly in fracturing the stone, the loss in weight or the breakage will give a measure of the effect of cold winters. However, as such low temperatures cannot be produced artificially except at considerable expense, and as a sufficient degree of cold is ordinarily unobtainable when desired, such a test is usually impracticable.
An attempt to simulate such an effect by boiling the specimen in a concentrated solution of sulphate of soda and observing the subse quent disintegration of the stone, if any, is known as Brard's test. Although this method is much used for lack of a better, its value is doubtful and perhaps deceptive, since the effect is largely chemical rather than mechanical. The destructive effect on the stone is usually greater than that of freezing, and might result in condemning a really good stone.
9. Chemical Test. The most difficult and uncertain matter to determine is the probable effect of the acids in the atmosphere.
These acids, dissolved in rain water, soak into the stone and combine with any earthy matter in the stone, which then leaches out, leaving small cavities. This not only results in a partial disintegration of the stone, but also facilitates destruction by freezing. If the stone specimen, after being carefully washed, is soaked for several (lays in a one per cent solution of sulphuric and hydrochloric acid, the liquid being frequently shaken, the water will become somewhat muddy if there is an appreciable amount of earthy matter in the stone. Such an effect is supposed to indicate the probable action of a vitiated atmosphere. Of course it should be remembered that such a con sideration is important only for a structure in a crowded city where the atmosphere is vitiated by poisonous gases discharged from fac tories and from all chimneys.