Shearing Strength

Seasoning.

All stones, and especially limestones and sand stones, when first quarried contain considerable quarry sap. When full of sap the stone works considerably easier under the tool than when well seasoned. This hardening by seasoning adds very much to the durability of the stone. If a stone freezes while full of quarry sap, it is nearly certain to crack; but if it is first allowed to season, it is not likely to be appreciably damaged by a single freezing. The cause of the hardening by seasoning has not been experimentally determined; but it is supposed to be due to the fact that the quarry sap holds in solution a small amount of calcareous or siliceous matter, and that in seasoning this material is drawn to the surface and is deposited in the pores of the stone by the evaporization of the sap. The matter in solution in the sap thus becomes an additional cement ing material and binds the grains more firmly together.

No determination has been made of either the quantity or the composition of the quarry sap; and the surprising thing is that an otherwise inappreciable amount of liquid can produce such a marked effect.

Physical Structure.

The physical properties which contribute to durability are hardness, toughness, homogeneity, contiguity of the grains, and the structure—whether crystalline or amorphous.

Although hardness (resistance to crushing) is often regarded as the most important element, yet resistance to weathering does not necessarily depend upon hardness alone, but upon hardness and the non-absorbent properties of the stone. A hard material of close and firm texture is, however, in those qualities at especially fitted to resist friction, and is therefore suitable for use in stoops, pave ments, and road metal, and to resist the wear of rain drops, dripping rair.-water, the blows of the waves, etc.

Porosity is an objectionable element.. An excessive porosity increases the layer of decomposition which is caused by the acids of the atmosphere and of the rain, and also deepens the penetration of frost and promotes its work of disintegration.

If the constituents of a rock differ greatly in hardness, texture, solubility, porosity, etc., the weathering is unequal, the surface is roughened, and the sensibility of the stone to the action of frost is increased.

The principle which obtains in applying an artificial cement, such as glue, in the thinnest film in order to secure the greatest binding force, finds its analogy in the building stones. The thinner the films of the natural cement and the closer the grains of the pre dominant minerals, the stronger and more durable the stone. One source of weakness in the once famous brown-stone of New York City lies in the separation of the rounded grains of quartz and feldspar by a superabundance of ocherous cement. Of course the further separation produced by fissure, looseness of lamination, empty cavities and geodes, and excess of mica tends to deteriorate still further a weak building stone, Experience has generally shown that a crystalline structure resists atmospheric attack better than an amorphous one, as has been abundantly illustrated in the buildings of New York City. The same fact is generally true also with the sedimentary rocks, a crystal line limestone or good marble resisting erosion better than earthy limestones. A stone that is compactly and finely granular will exfoliate more easily by freezing and thawing than one that is coarse grained. A stone that is laminar in structure absorbs moisture unequally and will be seriously affected by unequal expansion and contraction,—especially by freezing and thawing. Such a stone will gradually separate into sheets. A stone that has a granular texture, as contrasted with one that is crystalline or fibrous, will crumble sooner by frost and by chemical agents, because of the easy dislodgment of the individual grains.

The condition of the surface, whether rough or polished, influences the durability, the smoother surface being the better. The stone is more durable if the exposed surface is vertical than if inclined. The lamination of the stone should be horizontal.