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.
54. When the outside facing of a wall is of cut stone, it is called ashlar, regardless of the manner in which the stone is finished. Ashlar is usually laid either in regular courses, with continuous horizontal joints, as shown in Figs.
23, 24, and 25, or in broken courses, without regard to continuity of the joints, as shown in Fig. 26. All ashlar should have straight and horizontal bed joints, and the vertical joints should be kept plumb. Failure to do this mars the effect of the work very materially.
55. Coursed ashlar is a class of stonework in which the pieces are uniform in size and the bed joints are continuous. When such stones can be obtained readily, this is not a very expensive kind of work. Pieces about 12 inches high, and from 8 to 24 inches long, are probably the best, both as regards cost and ease of handling. If this stone is cut from 30 to 36 inches in length, with the end joints plumb over one another, the cost is proportionately increased. Fig. 23 shows this kind of work; a indicates the 12" x36" ashlar, and b the backing, consisting of 12-inch rubble.
56. A good effect is produced by making the courses of two different heights, but cut in regular sizes, and having the vertical joints in alternate courses directly over one another. This class of work is shown in Fig. 24, in which a is a 14-inch course; b, a 6-inch course; and c, the backing. The latter may also be brick, as the ashlar can be well bonded into it. If the narrow band course b is cut with rock face, or in some different way than the wide courses a, the appearance of the work is further improved.
57. The stonework of many public and office buildings have rustic quoins and base or band courses, as shown in Fie:. 25, where a indicates the quoins, having a 1-inch bevel, or chamfer, at the joints; b, the plain, rubbed, or tooled stones forming the face of the wall; c, the rustic band course, having a 1 1/2-inch chamfer cut on it, so as to project beyond the quoins; and d, the stone or brick backing. This method of construction is very expensive, owing to the great amount of dressing required.
Broken Ashlar. It is often found that stones of. uniform size cannot be cheaply obtained, in which case, irregular sizes may be used, forming what is termed broken ashlar; by careful workmanship, a coursed effect may be produced with nearly continuous horizontal joints. Probably the great majority of stone buildings consists of this class of masonry. It generally takes a longer time to build broken ashlar than coursed work, and hence it is more costly, owing to the increased amount of labor required to fit and lay the different sizes of stone. This kind of ashlar, when properly executed, presents a pleasing appearance. It is generally laid up as rock-faced work, but in some cases, it is tooled or hammer-dressed. It should have no horizontal joints more than 4 feet long, and several sizes of stone should be used. Fig. 26 shows an ordinary broken ashlar wall, 2 feet thick, the sizes of stone used being 4, 6, 8, 10, 12, and 14 inches in height; a shows the quoins; b, the different sizes of ashlar; and c, the stone or brick backing.
Fig. 27 represents the same kind of a wall, using only three sizes of stone, 4, 8, and 12 inches in height; a shows an 8-inch quoin; b, a 12-inch stone; and c, the backing.