There are several methods of giving additional strength to the joints of masonry, the most common of which will now be considered.
With regard to metal connections it may be said, once for all, that copper or bronze make the best, as they do not oxidise to any great extent. If iron is used, it should be well protected from air or moisture, and also painted or galvanised, or it will rust, increase in bulk, and split the stones. All metals are liable to do the same, more or less, by their expansion and contraction under intense heat and cold. Dowelled Joints are formed with slightly tapering pins, or "dowels," which fit into holes made in the stones opposite to one another (see Fig. 134).
They are sometimes placed vertically in a joint, as in Fig. 135, the upper part of which shows half the hole cut for the dowel, and the lower part shows the part of a dowel in position, or they may be double dovetail in plan, and placed horizontally, as in Fig. 136.
Fig. 134. Horizontal Dowel.
Fig. 135. Vertical Dowel.
Fig. 136. Dovetailed Dowel.
Fig. 137. Bed Plug.
Dowels are sometimes made to fit very loosely, and run with lead, cement, or brimstone, but accurate fitting is better.
A short vertical dowel in the centre of a stone is sometimes called a "bed plug" (see Fig. 137), and is useful for copings or heavy stones, built on an inclined ramp or gable, and for many other purposes.
Joggled Joints are similar to dowelled joints, except that the joggle or projection is a part of the stone instead of being detached like the dowel. To leave such a projection in working the stone would cause great labour and waste of material, and it is seldom done in practice.
The word "joggle" is often applied by masons to dowels, and to all sorts of joints in which any portion of one stone enters the other.
In these a prolonged joggle or tongue is worked upon one stone, and fits into a groove in the other. A similar joint is used in joinery, and shown in Fig. 297.
Fig. 138. Grooved and Tonqued Joint.
A modification of the joint, in which the groove and tongue, or joggles, are angular, is shown in Fig. 138.
A more economical joint is formed by cutting grooves in both the stones, and inserting a metal tongue.
Metal Cramps should be used as little as possible, for they are very liable by their rusting and expansion to destroy the work in which they are bedded; when used they should be placed in a channel cut in the upper surface of two stones, having dovetail-shaped sinkings at the ends, into which the turned - down and jagged extremities of the cramp may fit.
The channel should be deep enough to conceal the cramp, and is filled in with lead or cement to protect the latter from oxidation.
In old buildings of an important character the combined cramp and dowel shown in Fig. 140 has been adopted, but it would not be used in these days.
Fig. 139. Metal Cramp.
Fig. 140. Combined Cramp and Dowel.
Lead Plugs are formed by pouring molten lead into plug-holes (generally dovetail-shaped) formed in the stones, as shown in section Fig. 141. The holes slope downwards, in order that the lead may run at once into the ends and corners so as to fill them completely.
Rebated Joints between two stones are made by taking a check out of the end of each, so that they may overlap each other. They are exactly similar to those used in joiners' work, and shown in Fig. 293. An illustration of their use is given in Fig. 146.
Tabled Joints are those in which a wide projection is left on one stone, fitting into an indentation cut in the other. Joints formed like this are often said to be joggled. They involve a waste of material, and are used only for heavy work subject to concussion, such as the walls of lighthouses.
Special Joints of elaborate form, of dovetail form horizontally and sometimes vertically also, are used in lighthouses and in some other works intended to resist the action of the sea, but they cannot here be described.
Fig. 141. Lead Plug.