REQUISITES FOR GOOD BUILDING STONE.
The requisites for good building stone are durability, strength, cheapness, and beauty.
Durability. The durability of stone is a subject upon which there is very little reliable knowledge. The durability will depend upon the chemical composition, physical structure, and the position in which the stone is placed in the work. The same stone will vary greatly in its durability according to the nature and extent of the atmospheric influences to which it is subjected.
The sulphur acids, carbonic acid, hydrochloric acid, and traces of nitric acid, in the smoky air of cities and towns, and the carbonic acid in the atmosphere ultimately decompose any stone of which either carbonate of lime or carbonate of magnesia forms a consid erable part.
Wind has a considerable effect upon the durability of stone. High winds blow sharp particles against the face of the stone and thus grind it away. Moreover, it forces the rain into the pores of the stone, and may thus cause a considerable depth to be subject to the effects of acids and frost.
In winter water penetrates porous stones, freezes, expands, and disintegrates the surface, leaving a fresh surface to be similarly acted upon.
Strength is generally an indispensable attribute, especially under compression and cross-strain.
Cheapness is influenced by the ease with which the stone can be quarried and worked into the various forms required. Cheapness is also affected by abundance, facility of transportation, and prox imity to the place of use.
Appearance. The requirement of beauty is that it should have a pleasing appearance. For this purpose all varieties contain ing much iron should be rejected, as they are liable to disfigurement from rust-stains caused by the oxidation of the iron under the influ ence of the atmosphere.