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Tests of Stone

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TESTS OF STONE Many of the characteristics of stone are ap parent to the practiced eye of the workman. Others are easily seen with the aid of a magnify ing glass; while some are determined only after thorough tests by approved methods of physical, microscopical, and chemical examination.

Simple Tests.

Some of the simple methods of judging of the fitness of stone for a certain pur mosphere. These spots are unsightly, and do not show in new stone. Any change of color from that of the stone when newly split, should be noted. Sometimes the change is for the better, sometimes the reverse.

In some surfaces there will be noticed dis tinct seams or veins of different color and tex ture. The small magnifying glass and the knife blade will aid in determining whether these ir regularities are harder or softer than the other parts of the stone. If the appearance is of lit tle relative importance, these hard seams may not be an objection, while seams of disintegra tion would be. Again, small cavities, of, say, an inch or two in longest dimension, are blemishes; but if the glass and blade show that they are sur rounded by a hard, flinty substance, the stone is not of necessity made materially weaker by their presence. If, however, such cavities show the existence of a seam of softer material, it is a more serious matter, and the specifications will determine whether or not the stone is still ac ceptable.

If a thin slab of stone be struck with a ham mer, it should produce a clear, ringing sound, the resonance being closely proportional to its elasticity.

The color of stone is mainly dependent upon the amount and condition of the iron it contains. The bluish tints indicate that the oxidation of the iron is incomplete, and some further change may be expected before the red or rusty color is shown. Hence, in general, the reddish stones are the most durable.

Chemical Tests. Such tests can be made only by an expert in a well-equipped laboratory, and an interpretation of the results is important. The value of a stone as a building material de pends fully as much upon the physical charac teristics as upon the composition. Thus, granite and gneiss are identical chemically; but the lat ter stone is in layers which cause it to be split readily in one direction and with difficulty other wise. This characteristic is of course unfavor

able if the stone is to be worked, but gneiss is considered to be as durable as any stone. Com paring the granites and sandstones in Table I, it is seen that either may have the larger percent age of quartz, iron, alumina, lime, magnesia, or potash. Carbonic acid is usually present in sandstone in considerable quantities, and is fre quently found in minute cavities of the quartz of granite. For the same general locality, a sandstone containing 2 per cent of alumina is apt to be stronger than one having 10 per cent or more, and as a rule the strongest sandstones have a low percentage of this material. The spe cific gravity and the ability to absorb water fur nish no certain criterion of the strength of stones, although, in general, the stronger are the heavier and the less absorptive. A sandstone should not contain so much lime as to cause vio lent effervescence upon contact with muriatic acid.

Physical Tests of Building Stone. The usual physical tests are of tension, compression, shear, flexure, specific gravity, and absorption. In Tables II and III will be found the results of re cent experiments, made for the most part at the United States Arsenal at Watertown, Mass., and published as public documents entitled "Tests of Metals." Table IV, giving weights and ab sorption, is from other sources believed to be en tirely reliable. Some of the universities main tain extensive testing laboratories, and publish the results of work done, in bulletins, which are for free distribution.

Table III gives the results of tests of the compressive strength of cubical blocks of stone measuring 4 inches on a side unless otherwise stated. The shearing specimens were 4 inches by 6 inches and 12 inches long. The transverse tests in Table II were on specimens 4 inches by 6 inches and 24 inches long unless otherwise stated. The manner of loading for shearing and transverse tests, is shown in Figs. 1 and 2.

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