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.
Mitre Joint is the line or plane of union bisecting the angle of junction of blocks, mouldings, etc, when meeting at any angle less than two right angles. In masonry, mitres are scarcely ever permitted between planes, and are never allowed at quoins or angles excepting in mere decorative features, and the mouldings of these are usually either housed or returned on the same block, with an occasional exception, as, for instance, when window sills are returned and mitred to strings.
The remarks in the Bricklayers' Section under this head are more or less pertinent to this trade also. Besides these and others to be found under Bed Joint and Flushed Joint of this section, it may be observed that for ashlar the mortar must be carefully prepared so as to be quite free from grit, 3/16 in. being a good average thickness, whilst for ashlar facing, or bastard ashlar, ft in. is sometimes deemed almost too thick. The face of the joint is lipped with plasterer's putty, or, as described under Fine Joint, for a distance back varying from £ in. to 2 ½ in. ; but some prefer using a fine variety of mortar coloured to match the stonework. In the present day it is usual not to have projecting joints, but to leave the mortar within the surface of the work. For rubble masonry the mortar should be of excellent quality, owing to its forming so large a proportion of the walling. Where it is compulsory to use so much, poor indeed must be the work that is bolstered up by an impoverished quality, for obviously within reasonable bounds the worse the mortar the less of it the better.
Whenever a hole is cut in masonry to receive the corkings or tenons of iron columns, stanchions, standards, door-posts, or ends of balusters, railings, etc, the resulting junction is correctly described as a mortise joint.
Formerly this term held for a projection left in one stone to fill up a vacuity made for it in another, but it is now not common.
Notch or Notched Joint is one formed by sinking or cutting indentations in slabs, flags, landings, or blocks to make them fit into their allotted places. Slabs or front hearths are notched for chimney-pieces. Paving is notched to shape where necessary. Ashlar may be notched to receive arched flooring, transoms, etc, as in Fig. 17. Stones in rubble walling are notched as occasion requires with fair ends against ashlar quoins.
An air-space or open joint is often produced, amongst other causes, by part of the foundation yielding - which usually manifests itself by a crack or "thread " - or by projections being left on the beds beyond the drafts. Open joints exposed to the action of waves of translation may cause much havoc. In arch work, another form of danger arising from a similar source has been indicated under Arch Joint. There are but two ways of curing this evil. If the opening is slight, and only traverses the work like a thread and does not widen, it may be fresh pointed and concealed, but where the crack ominously gapes and gradually extends, all half measures are useless, and the work must be pulled down and the foundation rendered firm and unyielding.