This section is from the book "A Practical Treatise On The Joints Made And Used By Builders", by Wyvill J. Christy. Also available from Amazon: Practical treatise on the joints made and used by builders.
The remarks under Coarse Joint are here applicable, for coarse and thick joints are interchangeable terms. "What would be considered thick in facing, however, are not so in hearting or backing, and what were once thin in the former situation soon become thick through weak mortar and repeated pointing. The safe crushing strength of good mortar is certainly not less than 1 cwt. per square inch, and provided such were used, and the work built solid, and not carried up too fast (say not more than 2 or 3 feet per diem), properly distributed pressure would doubtless be obtained by combining thin facing with thick inside joints when laid as described in the succeeding paragraph. At all events, whether they are contrary to sound construction or not, having regard of course to the mortar of the present day, a goodly number of thick joints lie concealed in every structure.
Thin Joint is one ranging from 1/8 in. to ¼ in. thick, the minimum depending only upon there being no "brick and brick," but sufficient thickness to prevent touch, which is assumed in practice to be about 1/8 in. with the very best bricks, and ¼ in. with picked stocks uniform in size and regular in shape. There must always be a certain margin left for the imperfection of workmanship on the one hand, and for slight distortion of the bricks on the other. The thinner the joint the more necessary is it that the bricks should be soaked before laying, so that the moisture essential for setting may not be abstracted from the mortar. Less settlement occurs with thin than with thick joints, for the shrinkage of common mortar is proportional to its bulk, but in good work, where facing bricks are used, the difference is neutralised by the bricklayer knocking the bricks in the backing harder down than the others, whilst every course is flushed full and no snapped leaders allowed. In massive engineering structures, it is feasible to get ¼ in. bed joints throughout, with sound hard burnt stocks of good shape. The bricks, however, as already noticed, must be thoroughly soaked, and if not grouted, flushed solid with sufficiently thin mortar to penetrate into all vacuities. The action of the weather is less prejudicial to thin joints. Kiln bricks are usually made larger than clamp bricks or stocks to obtain thin joints in face work, since four courses of rough stocks, owing to their irregularity of shape, sometimes rise from 1½ in. to 2 in. more than the height of the bricks. Owing also to the unequal thickness of the commoner kinds of bricks, joints have sometimes to bo made here and there very much thinner than intended, otherwise there would be no means of keeping the thicker bricks down to the required gauge. Bricks with frogs or kicks can usually be laid with thinner joints than those without them, but to insure filling the frogs they must be laid uppermost.