BUILDING STONE Qualities for Building choice of stone for use in important structures is always a matter of moment, and frequently involves considerable difficulty, on account of the wide variation in the characteristics of the stones commonly used. Stones belong ing to the same classes frequently differ greatly in their physical properties and much care needs to be exercised in securing materials of proper strength and endurance.
The qualities which are of importance in the selection of stone for structural uses are strength, durability, appearance, and cost. The relative importance of these in any particular structure depends upon the location of the structure and the purpose for which it is intended. For ordinary masonry, the most important quality of the stone is usually its durability. The element of strength is commonly of minor consequence, except in portions of a structure where the conditions are such as to bring severe stresses upon the masonry. A pleasing appearance is always desirable, but any stone possessing proper structural qualities may usually he so employed as to produce a good effect, where the purpose of the structure is not distinctly artistic. In architectural and monumental work, the appearance of the stone may be of first importance, while the strength of the stone, or its cost, is of less consequence. Stone for such uses, when in exposed situations, must possess durability in order to preserve the beauty of the structure and prevent disfigure ment or discoloration of its surfaces.
The cost of the stone is always a matter of importance, commonly limiting the choice, and frequently being the determining factor in selection of stone. The cost of stone depends mainly upon the ease with which it may be quarried and worked, and the distance and means of transportation to the place where it is to be used. The equipment of a quarry for handling and working stone often determines its availability for a particular use. The kind of finish to be given the surfaces, and the suitability of the material to the proposed treatment are also important in their effects upon cost.
A good building stone should be dense and uniform in structure, and should have no seams or crow-foots filled with material which may disintegrate and form cracks upon exposure. The fracture should be clean and sharp, and the surface free from earthy appear ance.
Hardness and toughness are important properties in a stone which is to be subjected to wear or abrasion of any kind. Stones lacking in toughness and easily abraded have sometimes been seri ously defaced by dust and sand particles carried by strong winds. The hardness of the stone depends both upon the hardness of the minerals of which it is composed and upon the firmness with which they are bound together. Toughness depends upon the resistance to separation of the mineral grains. Rocks of hard material may be lacking in toughness and easily worked when weakly cemented.
40. CIassification of Building Stones.—All rocks are aggregations of various mineral constituents, more or less firmly held together. Geologically, as to their mode of occurrence, they are divided into three groups, as follows: (1) Igneous Rocks.—Those which have been forced up in a molten condition from unknown depths and subsequently cooled. When the molten rock has cooled and solidified below the surface of the ground it is known as plutonic, when above ground as volcanic.
(2) Sedimentary or Stratified Rocks.—Rocks formed by being deposited as sediment in layers, and consequently showing bedding lines and stratifications. Limestones and sandstones belong in this class.
(3) Metamorphic Rocks, formed by subjecting igneous or strati fied rocks to great heat or pressure or to both.
Building stones may also be classified according to their chemical and physical properties into three groups: Crystalline, siliceous rocks, including granites, gneisses, traps, etc. Calcareous rocks, including limestones and marbles.