The characteristics of a stone in this respect will depend in some cases upon its hardness, but will also be influenced by the soundness of its texture; by its freedom from flaws, shakes, vents; etc.; and also by its natural cleavage and other peculiarities.
A soft stone of even grain and without distinct beds would naturally be selected for carved work, while a hard stone in thin layers, easily separated, would be well adapted for building good and economical rubble masonry. (See Part I.)
The hardness of stone is often of importance, especially if it is to be subjected to a considerable amount of wear and friction, as in pavements. It is, moreover, important when the stone is to be used for quoins, dressings, and other positions where it is required to preserve a sharp angle or "arris." Hardness combined with toughness is also essential in good road metalling, which should not, however, be liable to splinter or to grind readily into dust.
It does not follow because a stone is hard that it will weather well; many hard stones are more liable to atmospheric influence than those of a softer texture, whose chemical composition is of a more durable nature.
Stone used for work exposed to the action of water should be hard running or dripping water soon wears away the surface. The blocks of stone in marine works are subject to serious injury not only from the impact of the waves themselves, but from the sand and stones thrown against them by the force of the sea.
It is generally laid down that the compression to which a stone should be subjected in a structure should not exceed 1/10 of the crushing weight as found by experiment.
Practically, however, the compression that comes upon a stone in any ordinary building is never sufficient to cause any danger of crushing.
By a reference to page 81 it will be seen that these stresses would be safely borne even by the softer descriptions of stone.
The weakest sandstones that exist will bear a compression of 120 tons per foot, while the resistance of ordinary building stones ranges from 140 to 500 tons per square foot, and in the case of granites and traps rises as high as 700 or 800 tons per square foot.
It is possible, however, in some forms of arches, in retaining walls, and in other structures, that a considerable pressure may be concentrated upon certain points, which are liable to be crushed.
The weight of a stone for building has occasionally to be considered.
In marine engineering works it is often advisable to use heavy stones to resist the force of the sea.
A light stone would be best adapted for arches, while heavy stones would add to the stability of retaining walls.
The appearance of stone is often a matter of importance, especially in the face work of conspicuous buildings.
In order that the appearance may be preserved, a good weathering stone should of course be selected, free from flaws, clayholes, etc.