This section is from the book "Modern Buildings, Their Planning, Construction And Equipment Vol1", by G. A. T. Middleton. Also available from Amazon: Modern Buildings.
Each course should be numbered consecutively by roman numerals, and the several stones in each course by arabic figures. These numbers should be marked on the stones when received from the quarry, and before sending to the banker, and the same numbers should be retained throughout the work. Care should be taken to distinguish between face and angle or quoin stones. Sills, plinths, strings, cornices, copings, etc., etc. are separately marked and kept distinct, as are also the moulded jambs, mullions, heads, transoms, and tracery stones of windows, doors, etc., together with hoods, panels, arches, niches, and all enrichments.
Plinths, plain or moulded, are dressed to the scantling lengths, the rake of splay set with a template from the details, and worked to the face required.
For angle stones the material is worked square to the extreme length, and the splay or mould worked back on the two faces and at right angles, to form the mitre (see sketch, Fig. 224). In moulded plinths the same method applies, except as regards the template, which is cut to the form desired.
Strings are treated much in the same way. The rough faces are first splayed off to the rake of the moulding, and the finished mould droved and rubbed to the template. Copings require, in addition to the above, a special bevel cut to the rake or weather desired. These bevels are cut to the angle desired from the horizontal plane, drafts being run at each end of the stone, and the rake checked by the straight edge. The intervening rough stuff is knocked off and the whole brought down to the rake, forming a plane surface on the line of the drafts first sunk.
In all cases of copings, sills, and salient stones a groove or throating should be worked where convenient, on the under and projecting part, to throw off the water and prevent its entry into the adjacent joint. In many examples of mediaeval work the copings have an embattled outline, formed by breaking the continuous line of the coping and sinking the intermediate stones to a lower plane. This serves to break up a long line of coping, which might otherwise give a sense of monotony.
In corbels, springers, kneelers, and apex stones the beds should be horizontal in the main wall; but where they abut on and form part of the coping, as in gables, that portion of the stone which oversails and is coincident with the coping should have its beds or joints at right angles with the rake of the gable (see Fig. 225).
In cornices the stones should be carried through the entire thickness of the wall, especially where salient stones project to any great extent. Means of tying them down to ensure stability should also be employed, such as metal cramps run in with lead, interties, copper or gun-metal bolts, and other devices, and the stones joggled one to the other (see Fig. 226).
Sills are composed of one or more stones forming the base of a window opening, frequently projecting beyond the general face of the wall, and weathered, - that is, sloped outwards to throw off wet. In other words, the weathering should be brought to a lower surface than the plane of the square block out of which the sill is worked, leaving a narrow tabling as a seat for the wood sill of the window. This tabling should have a groove for the reception of a galvanised iron tongue. Where mullions are employed a seating is left square with the tabling before described, and the section of the mullion worked down to meet the weathering. The same applies to the junctions of the sills with the quoins, and the sides of the openings forming the jambs.
Heads, or lintels, are heavy stones bridging the opening over doors, windows, entrances, etc., and serving to transfer the superincumbent weight of the wall to the jambs. They are usually squared and finished with a splay, chamfer, or moulding, as the case may be, with mitred returns worked to the mould and within the stone. Cut templates have to be employed with the mould worked on in reverse, as it cannot be scribed at the ends and carried through as in string or plinth mouldings.
Lintels are sometimes cut with a curved, segmental, or other soffit, or with segments of arches inscribed within the face, ogees, or other ornamentation (see Fig. 227).
There are many forms that can be applied to the flat lintel, but the above will suffice as examples.
There are various methods of connecting stones together in addition to the adhesion provided by the use of mortar and cement. These are designed with a view to prevent shifting, or to provide stability where strain is anticipated; or else to form an additional protection from the entrance of rain water.
Where stones are bedded together and no particular strain is anticipated (it being presumed that it is not convenient to use one scantling) a plain vertical joint with two V slots from head to base is made, the opening thus left being filled with cement and pebbles, or with a slate dowel run in with cement.
For raking walls, such as towers, spires, etc., joints are sometimes fashioned in the form of a rebate (see •A, Fig. 228), the joint being horizontal. Another form, known as tabling (shown at B), is sometimes employed, but it is more expensive, as it involves considerably more work, and there may also be liability to the splitting off of the projection should undue strain be thrust upon it.
In gables a rebate is sometimes formed at the joints of the raking stones to keep the water out, as at C. It is a good joint, and serves its purpose well, but is expensive.
For the seats of sills, where the mullions or jamb stones rest, it is customary to insert a dowel about 1 inch square, usually of slate run in with cement. This effectually prevents sliding. Dowels are also used to connect thresholds with the stone immediately above in the same manner, and iron or slate dowels are inserted in thresholds, with 1 or 2-inch projections, to secure the wood frames of doors.
Where the bases of heavy iron columns or girders rest on stone templates, lewis bolts are inserted, being sunk from 4 to 6 inches in a dovetailed opening in the stone. The lewis is dropped in, and lead is run in to secure it.
Joggle joints are used to a great extent in landings, etc., where the widths are too great to admit of the convenient use of one single stone slab. The joint is formed as shown at A in Fig. 229, the joggle being cut to slightly taper, and run in with cement. Sometimes a longitudinal slate dowel is inserted, as at B, run in with cement as before. This is not so effectual as the joggle. A rebated joint is also sometimes employed, as at C.
Joggle joints are occasionally used in lintels, the projecting portion being rounded off and engaging with a groove of contrary section and equal size in the adjacent stone, as at D.
Secret joints are sometimes employed in Classic architecture for portiere, entablatures, and lintels. Outwardly the joint is vertical, but the centre of the stone is cut in the form of a flattened wedge, with a corresponding cavity in the adjacent stone into which it engages, as at E. It is a good joint where no very severe strain is anticipated, and when metal cramps are undesirable. These are often used in gables, copings, entablatures, etc., and are generally of copper, galvanised iron, or gun-metal. Slate cramps cut to a double dovetail are cheaper and very effective (see F, Fig. 229).
A very elaborate form of jointing was used in the construction of the Eddystone lighthouse, of such a. nature that the structure partakes more of the nature of one solid stone than a collection of small parts. The joints were formed of a combination of rebates and dovetails ingeniously formed to engage with one another not only horizontally but vertically. The work was carefully tested and the complete sets of each course laid out, fitted and numbered before being sent out to the site. A sketch of one of the stones will be sufficient to illustrate the general principles guiding the design of the structure (see Fig. 230).
In spires and domes a frequent method of counteracting the thrust of the superincumbent weight is that of cutting a deep groove in the base or adjacent course, and inserting one or more stout chains or steel bands carefully run in with lead. Stonework in spires and domes should gradually diminish in thickness as the courses near the apex. In square towers with spires the angles are frequently spanned with squinch arches. These serve to throw the thrust at the angles more towards the main surface of the walls (see Fig. 231).