The lap or bond given to the stone varies, according to the nature of the work, from once to once and a half the depth of the course; and it should under no circumstances be allowed to be less than from 4 to 6 inches, according to the size of the stones.
The best bond for ashlar consists of headers and stretchers 3 alternately on the same course - an arrangement similar to Flemish bond in brickwork (see Fig. 50). It is however seldom executed in this way on account of the expense of so many headers.
In setting ashlar, the stone should first be placed in position dry, to see if it will fit, the upper surface of the last course should then be thoroughly cleaned off and wetted: on this a bed of mortar is evenly laid, with a strip of lime putty about 3/4 inch wide along the front edge. The block, with its bed joint well cleaned and wetted, is then laid evenly in its place, and settled by striking it with a mallet.
2 Mr. Kirkaldy has proved by experiment that the effect desired is not produced, and that the practice is a bad one.
3 These are, properly speaking, bricklayers' terms, but may be, and often are, used for convenience with regard to masonry. Sc. Inbonds and Qutbonds.
Ashlar walling is described as "coursed" or "random." Coursed Ashlar walls consist of blocks of the same height throughout each course. This is the most usual form in which ashlar is built, but it is the most expensive, as great waste of material and labour is occasioned by reducing all the stones to the same height.
Fig. 122. Ashlar Walling with chamfered and rusticated Quoins and Plinth.
Fig. 122 shows coursed ashlar walling, with chamfered and rusticated quoins and plinth, and Fig. 123 ashlar with rebated joints and moulded quoins.
Random Ashlar walls are built with rectangular blocks of all sizes and dimensions. This is a cheaper kind of work, as it enables a larger proportion of the stone as quarried to be used without waste in reducing to fixed sizes; but it is generally considered inferior in appearance to coursed work, and is very seldom adopted.