172. Method of Finishing

This also has a great deal to do with the durability of a stone. As a rule, the less jar from heavy-pounding that the surface is subjected to, the more durable will be the surface, for the reason that the constant impact of the blows tends to destroy the adhesive or cohesive power of the grains, and thus renders the stone more susceptible to atmospheric influences. This applies particularly to granites and limestones. Only granites and the hardest sandstones should be pene or bush hammered; all others, if dressed, should be cut with a chisel. Sandstones may afterward be finished with a crandal, if desired. For granites a rock-face surface would probably prove most durable, since the crystalline facets thus exposed are best fitted to shed moisture and the natural adhesion of the grains has not been disturbed. For all other stones, however, a smoothly sawn, rubbed or polished surface seems best adapted to a variable climate.

173. Strength

Whenever a stone is to be used for foundations, piers, lintels, bearing stones, etc., its strength should be considered, and if it has not been demonstrated by practical use under similar circumstances, cubes of the stone about 6 inches on a side should be carefully tested for the crushing strength. If the stone has all the appearance of a first-class stone of its kind, its strength may be assumed to be equal to the average strength of stones of that kind. The safe working strength for piers, etc., should not exceed one-tenth of the crushing strength. Tables giving the crushing strength of many well-known stones and the safe working strength for stone masonry are given in the appendix.

The method in which a stone is quarried sometimes has much to do with its strength. If the stone is quarried by means of explosives the stone may contain minute cracks, which cannot be discovered until the stone receives its load, when their presence is unpleasantly manifested. Such an occurrence could only take place in some stone like lava or conglomerates. The cracking and splitting of stones in buildings is often due more to imperfect setting than to lack of strength in the stone. Any stone that will meet the requirements for durability will have sufficient strength for all purposes, except in the positions mentioned above.

Hardness. - For many purposes the hardness of a stone must be considered, as when it is to be used for steps, door sills, paving, etc.

Granite, quartzite, or siliceous sandstone, and bluestone are the best stones for this purpose.

Cheapness. - This often has more to do with the choice of a building stone by the owner than the architect could wish. The cost of the stone when cut depends not only upon the cost of the rough stone delivered at the site, but also upon the ease with which the stone may be worked; whether the stone is to be smooth or rock face; plain or moulded; and also to some extent upon its weight. One stone may be cheaper than another in the rough, but the extra labor of cutting may make it the most expensive when put in the wall. The heavier a stone is the greater will be the cost of setting and transportation.