BUILDING STONE 12. Limestone. Carbonate of lime forms the principal in gredient of limestone. A pure limestone should consist only of carbonate of lime. However, none of our natural stones are chemi cally pure, but all contain a greater or less amount of foreign material. To these impurities are due the beautiful and variegated coloring which makes limestone valuable as a building material.
Limestone occurs in stratified beds, and ordinarily is regarded as originating as a chemical deposit. It effervesces freely when an acid is applied; its texture is destroyed by fire; the fire drives off its carbonic acid and water, and forms quicklime. Limestone varies greatly in its physical properties. Some limestones are very durable, hard, and strong, while others are very soft and easily broken.
There are two principal classes of limestone—granular and compact. In each of these classes are found both marble and ordinary building stone. The granular stone is generally best for building purposes, and the finer-grained stones are usually better for either marble or fine cut-stone. The coarse-grained varieties often dis integrate rapidly when exposed to the weather. All varieties work freely, and can be obtained in blocks of any desired dimensions.
13. Marble. When limestone is wholly crystalline and suitable for ornamental purposes, it is called marble; or, in other words, any limestone that can be polished is called marble. There are a great many varieties of marble, and they vary greatly in color and appear ance. Owing to the cost of polishing marble, it is used chiefly for ornamental purposes.
14. Dolomite. When the carbonate of magnesia occurring in limestone rises to about 45 per cent, the stone is then called dolo mite. It is usually whitish or yellowish in color, and is a crystalline granular aggregate. It is harder than the ordinary limestones, and also less soluble, being scarcely at all acted upon by dilute hydrochloric acid. There is no essential difference between limestone and dolo mite with respect to color and texture.
15. Sandstone. Sandstones are composed of grains of sand that have been cemented together through the aid of heat and pres sure, forming a solid rock. The cementing material usually is either silica, carbonate of lime, or an iron oxide. Upon the character of this cementing material is dependent, to a considerable extent, the color of the rock and its adaptability to architectural purposes. If silica alone is present, the rock is of a light color and frequently so hard that it can be worked only with great difficulty. Such stones are among the most durable of all rock, but their light color and poor working qualities are a drawback to their extensive use. Rocks in which carbonate of lime is the cementing material are frequently too soft, crumbling and disintegrating rapidly when exposed to the weather. For many reasons the rocks containing ferruginous cement (iron oxide) are preferable. They are neither too hard to work readily, nor liable to unfavorable alteration when exposed to at mospheric agencies. These rocks usually have a brown or reddish color.
Sandstones are of a great variety of colors, which, as has already been stated, is largely due to the iron contained in them. In texture, sandstones vary widely—from a stone of very fine grain, to one in which the individual grains are the size of a pea. Nearly all sand stones are more or less porous, and hence permeable to a certain extent by water and moisture. Sandstones absorb water most readily in the direction of their lamination or grain. The strength and hardness of sandstones vary between wide limits. Most of the varieties are easily worked, and split evenly. The formations of sandstone in the United States are very extensive. The crushing strength of sandstone varies widely, being from 2,500 pounds to 13,500 pounds per square inch, and .specimens have been obtained that require a load of 29,270 pounds per square inch to crush them.