All these destroying agencies are resisted in different degrees by the various kinds of build ing stones. Granite, being composed largely of hard quartz, is but slightly acted upon by the acids present in the atmosphere. Its density also prevents the absorption of much water, and so it is not greatly affected by small changes of temperature. On the other hand, intense heat causes granite to disintegrate rapidly. On ac count of its hardness and strength, this stone may be used in all places where friction and pressure are expected; and by the stonecutter's art it becomes one of the most beautiful stones as well.
A sandstone may or may not be durable. This stone yields to destroying agents, principally on account of the disintegration of the material that cements the particles of sand together. Under the best conditions this cementing material is silica, and the sand is quartz; and when this composition is well consolidated, the resulting stone is comparable with granite in ability to resist the action of the air, and is more durable in the case of fire. On the other hand, sandstones are often cemented by material containing sol uble matter, such as clay and iron. A notable example of such stone is the Portland brown sandstone used in the construction of many handsome residences in New York. Added to the tendency to disintegrate through chemical action, is the fact that most sandstones are por ous, and are consequently much affected by frost.
The breaking stresses per square inch in stone beams subjected to bending, crushing, and shearing, are given in Tables II and III. These values should be divided by four or more, in de termining the proper sizes to be used in building.
Stone in the mass, as that under foundations, seldom fails by compression under any loads in use in engineering or architectural construction. The values given in tables of compressive strength of stone, are usually derived by break ing cubes measuring two or four inches on a side. When somewhat larger cubes are tested, the breaking load per square inch is greater, and the natural inference is that the unit-strength in creases with the cross-section. Nothing, how
ever, is definitely known of the compressive strength of large blocks of stone, as no testing machines are capable of breaking such speci mens. When it is remembered that the weight of all the iron of a railroad track from New York to Chicago could be safely borne by a block of granite twenty feet square, it may cause wonder that stone structures ever fail. In nearly every case of such failure, a close examination reveals other than direct compression. Occasionally an eccentric loading may bring an enormous weight upon a small area, as on the sharp edge of a block, and cause failure by compression; but faulty foundation bed, flexure, or disintegration through the action of natural or artificial, chem ical or physical agencies, nearly always are re sponsible for failures that occur in stonework.
Table III gives the strength per square inch of cubes of stone of various kinds. The results are of value, as strength usually indicates dens ity and ability to resist frosts and acids of the air. It is of course understood that the values in such tables are not to take the place of tests made for a particular stone, when it is possible to have this done. As the table covers quite a range of quarries, it gives a good indication of what may be expected of stone of different kinds found anywhere.
The strength of stone masonry differs ma terially from that of the stone of which it is formed. The strength of the mortar and the thickness of the joints used are large factors in determining what a structure will support. For the reason mentioned in a paragraph above, no tests have been made on full-sized masonry structures; and so the strength of small cubes of stone and that of the mortar are to be taken as indicating the probable relative strength of the built-up masonry rather than the absolute strength.
Beauty and Cost of Stone. Beauty is a rela tive term; and a comparison of stones in this re spect must usually be a matter of personal pref erence. Fitness in the choice of stone and of its finish, is the main thing to be sought. A tunnel portal of polished marble in the mountains would be as lacking in beauty as a rough stone interior might be in a city building.
The cost of stone is subject to so many changes and to so many local conditions, that nothing more than a very general statement can be made concerning it.
A large part of the total cost of stone as used in building, is the expense incurred in cutting the stone. Within the past few years, the fol lowing prices for sawing stone have been quoted: Sandstone, 8 to 10 cents; limestone, 15 to 18 cents; marble and granite, 25 to 35 cents—all per square foot. It costs nearly $5.00 to quarry a cubic yard of granite, split to three dimensions. If split in random sizes, the cost is perhaps $4.00. Granite broken out by very heavy blasts, to be afterwards used in this form or to be split into regular shapes, is sometimes quarried as cheaply as 50 cents or less per cubic yard. Bedford lime stone costs from $1.50 up per cubic foot, ready for the wall.