This section is from the book "A Treatise On Architecture And Building Construction Vol2: Masonry. Carpentry. Joinery", by The Colliery Engineer Co. Also available from Amazon: A Treatise On Architecture And Building Construction.
44. The stonework which enters into the construction of buildings may be divided into three classes: rubble, ashlar, and trimmings. Before describing these, however, a few general observations, applying to all classes of stone masonry, are necessary.
45. Whatever may be the quality of mortar used, the wall should contain as much stone and as little mortar as possible, as the former is the stronger material. In rough walling, if the stones are pressed together until the more prominent angles on their faces come almost into contact, the interstices being filled with mortar, there results better work than if a thick, yielding mass of mortar is allowed to remain in the joints. Absolute contact is not advisable in stonework, any more than in brickwork, as the shrinking of the mortar in drying may leave the stones bearing only on the projecting angles.
The joints in stonework vary in thickness from 3/16 inch to 1/2 inch; a 1/4-inch joint is probably the best for ordinary work, while a 1/2-inch joint should be used for rock-faced work only.
46. Stone being of a brittle nature, the longer pieces in a wall must be properly supported and well bedded, in order to prevent their breaking, and it is best to avoid extremely long lengths. There is a certain medium which should be observed; and while in stone, as well as in brick walls, a compact mass, as little broken as possible, is most desirable, yet the mason will often find it better to break a very long stone into two or more shorter ones, even though by so doing he makes additional joints.
47. All bed joints in stonework should be full and square to the face, and in no case made as shown at a, Fig. 16. If the joints are hollow, the least settlement will throw the whole pressure upon the edges of the stone, and cause spalls to break off, which action not only spoils the appearance, but also endangers the stability of the walls. Stone cutters very often work the joints hollow, and leave the back of them slack, as shown at a, Fig. 17, as it requires less work than to dress them properly. If the back of the stone is thus left slack and underpinned, it is liable to break in the middle. It is also necessary that the abutting surfaces should not be convex, so as to rock on each other and cause instability.
Rusticated joints, as shown in Figs. 14 and 15, are often used in the basement and first story of many tall buildings, to lessen the liability of spalling in the lower courses of stone.
48. Stonework should be laid either with cement, or with cement-and-lime mortar, for damp places; while for dry situations, lime mortar may be used. Cement is always preferable, however, for general use.
In work that is to be pointed, no mortar should be placed within an inch of the front edges of the stone, as this saves raking out the joints preparatory to pointing. Sometimes slips of wood, just the thickness of the joint, are set on the edges of the lower course; in setting the stone, the superfluous mortar is pressed out and the stone rests on the wooden slips, which are removed when the mortar is hard.
Portland and Rosendale cements discolor most limestones and marbles, and some sandstones. By exercising care, the mortar may be kept from the face of the stone, and the joints may be pointed afterward, with mortar that will not stain the stone. A cement made of plaster of Paris, lime, and marble dust, called Lafarge cement, is sometimes used for setting marble and limestone; it is claimed that this will not cause discoloration.