Durability, which depends chiefly upon chemical composition ; for a large proportion of lime will render the stone unfit to resist the acid atmosphere of towns - a stone that is not durable out of doors is said to "weather" badly. The durability is, however, to some extent influenced by its Physical Structure, thus marble is more durable than chalk, though chemically the same. Hardness (for quoins, eta), Facility for Working (for carvings, etc.), and Appearance, have sometimes to be considered. Classification of Building Stones may be taken as follows: - Granites and other igneous rocks. Sandstones.
Granite is composed of quartz, felspar, and mica. It is, as a rule, very durable and hard to work, and is used for heavy engineering structures and for massive buildings, also in the parts of ordinary buildings, such as steps, that undergo most wear.
Mica and some kinds of felspar are liable to decay, but quartz is always hard and durable ; therefore the more quartz a granite contains the better.
The best-known granites are found in Scotland and Cornwall.
They should be fine grained, hard, with a metallic ring, not friable at the edges ; tough, so as not to splinter when cut or holed; and non-absorptive. The best varieties come from Wales.
The best-known sandstones are as follows : -
These have a coarse grit, are very strong, can be obtained in large blocks of a light brownish-white colour, and are much used for heavy engineering work. The best-known quarries are Bramley Fall, Bradford, Scotgate Ash, etc. etc.
Mansfield Stone is found in Nottinghamshire in two colours, red and white, and is well adapted for ashlar work, columns, etc.
Craigleith Stone, found near Edinburgh, is the most durable sandstone in the country, and useful for any good masonry.
Limestones consist of grains of carbonate of lime cemented together by the same substance, or by the same mixed with silica.
They vary greatly in texture, being either granular, with grains varying much in size, or compact, not showing grains.
The principal varieties are : -
An even-grained, comparatively soft white stone ; some of it weathers badly. It is obtainable in large blocks, and much used for mouldings and carved work. There are several quarries, such as Box, Combe, Corsham, etc.
Several distinct kinds are found in the quarries. Roach and Whitbed Roach are full of shell casts, and not much used in ordinary buildings. Whitbed and Basebed, known also as "Bestbed," are most valuable white building stones, of even texture, and durable in most positions. Both descriptions present the same appearance, but Whitbed is harder to work and more durable than the other.
Kentish Rag is a hard, compact, non-absorbent gray stone, very difficult to work, and used chiefly for rubble. (See Part I.)
Caen Stone is found in Normandy, but much used in this country. It is of a cream colour, very soft when just quarried, easily worked and carved, but weathers badly.
Marble is a very dense, compact form of limestone that will take a polish ; some varieties are beautifully marked, and are used chiefly for decorative purposes.
The importance of placing stones in walls with their natural beds - in the layers in which they were geologically deposited horizontal - has been mentioned in Part I.; also that in cornices or over hanging work the natural bed should be vertical.