It is only proposed, in this chapter, to draw attention to some of the chief points requiring attention with regard to building stones generally, after which a short account will be given of the nature and characteristics of the chief building stones in general, as against local use. Granite and other building materials, not mentioned elsewhere, will be found treated of in Chapter XXIII (Miscellaneous Materials. Asphalte And Granite).
It will be patent to the student that, whereas hard-and-fast rules can be laid down for the guidance of bricklayers, to enable them to make good, solid, well-bonded work with a material of regular and standard size, no such rules can be made for the use of a material, such as stone, which is of no certain fixed size, and is subject to the varying influence of innumerable matters of both great and small importance. Therefore a great deal must necessarily be left to the judgment of the mason, who should thoroughly understand the principles, which he must endeavour, in more or less degree, to carry out, as far as possible, according to circumstances. He should never, under any circumstances, allow one joint to be exactly over another; but he can make the joint, in any position, over another solid stone, whether it be half or three-quarter bond. All he has to do - subject to judgment and discretion - is to break the joints; and, in addition to this, he must take care that every stone, excepting projections and weatherings, is laid on its natural bed - i.e., on the bed on which it lay when in the quarry, before it was taken from the solid mass. This bed - except in very close, hard, uniform, and even-grained stones - can easily be distinguished, as all formation and stratification, shells, changes of colour, and nature in the stone, grit or other, are practically laid horizontally on the stone. Therefore such marks, indications, and peculiarities are generally found in lines approximately parallel with the natural or quarry-bed of the stone.
On the oiher hand, all projections, weatherings, and especially copings, should be joint bedded; that is, the natural or quarry-bed must be placed vertically and at right angles to the horizon or bed of the course, which is necessitated by the fact that wet and frost will penetrate the bed of a stone, and lift it up and sometimes off altogether. Therefore it is best to tie the beds in, as it were, together at the joints by neighbouring stones, so that they cannot get away.
The mason should also take great care that the surfaces of all beds and joints are at true right angles, with the face of the stone or the direction of the superincumbent pressure of the weight above, so that the pressure on the stones is equally distributed over the whole area of the stone - uneven-ness tending to cause the angles to split off the stone, from the influence of gravity, making it take its proper bed and bearing under the pressure (as fig. 198), or sometimes the stone cracks (as in fig. 199).
Bonders and their uses will be dealt with later on, in connection with the subject of walling.
There are two classes of building stone in general use, called, from their respective composition, sandstones and limestones. Sandstone is composed of pure, clean sand, cemented together by carbonate of lime or of magnesia and other binding substances, while limestone is little more than pure carbonate of lime. The varieties of stone are of numerous colours, including red, blue, brown, white, and yellow, and their innumerable shades. The grain varies from fine to coarse, with beds which are oftentimes difficult to distinguish, some being close and some open> though their strength may be the same.