For rubble walls, single scaffolds, resting partly on the walls, similar to those used for brickwork (q.v.), are em ployed; for ashlar and other gauged stonework (see below) self supporting scaffolds are used with a second set of standards and ledgers erected close to the wall, the whole standing entirely inde pendent. The reason for the use of this double scaffold is that otherwise holes for the putlogs to rest in would have to be left in the wall, and obviously in an ashlar stone wall it would be im possible properly to make these good on the removal of the scaf fold (see further SCAFFOLD).
Stone freshly quarried is full of sap, and thus admits of being easily worked. On being exposed to the air the sap dries out, and the stone becomes much harder in consequence. For this reason, and because carriage charges are lessened by the smaller bulk of the worked stone as compared with the rough block, the stone for a building is often specified to be quarry worked. In the erection of St. Paul's cathedral, Sir Christopher Wren required that the stone after being quarried should be exposed for three years on the sea-beach before use.
All beds and joints should be truly worked and per fectly level. If the surface be convex it will give rise to wide un sightly joints; if concave the weight thrown on the stone will rest on the edges and probably cause them to "flush" or break off and disfigure the work. Large stones are placed in position with the aid of hoisting appliances and should be tried in position before being finally set. Great care should be taken to avoid frac turing or chipping the stone in the process of handling, as it is impossible to make good such damage. All stratified stones—and this includes by far the largest proportion of building stones— when set in a level position should be laid on their natural bed, i.e., with their laminae horizontal. The greatest strength of a stone is obtained when the laminae lie at right angles to the pres sure placed upon it. In the case of arches these layers should be parallel with the centre line of the voussoirs and at right angles to the face of the arch. For cornices (except the corner-stones) and work of a like nature, the stone is set with the laminae on edge and perpendicular to the face of the work. With many stones it is easy to determine the bed by moistening with water, when the laminae will become apparent. Some stones, however, it is
impossible to read in this way, and it is therefore advisable to have them marked in the quarry.
For the shafts of columns especially it is necessary to have the layers horizontally placed, and a stone should be selected from a quarry with a bed of the required depth.
Of whatever quality the stone may be of which a wall is built, it should consist as much of stone and as little of mortar as pos sible. Only fine mortar is admissible if we are to obtain as thin joints as possible. The joints should be well raked out and pointed in Portland cement mortar. This applies only to some sandstones, as marbles and many limestones are stained by the use of Portland cement. For these a special cement must be employed, composed of plaster of Paris, lime and marble or stone-dust.
Bond is of not less importance in stone walling than in brick work. In ashlar-work the work is bonded uniformly, the joints being kept perpendicularly one over the other ; but in rubble work, instead of making the joints recur one over the other in alternate courses they should be carefully made to lock, so as to give the strength of two or three courses or layers between a joint in one course and the joint that next occurs vertically above it in another course. In the through or transverse bonding of a wall a good proportion of header stones running about two-thirds of the distance through the width of the wall should be provided to bind the whole structure together. The use of through stones, i.e., stones running through the whole thickness of the wall from front to back, is not to be recommended. Such stones are liable to fracture and convey damp to the internal face.
As with brickwork so in masonry great care must be exercised to prevent the different parts of a building settling unequally. When two portions of a building differing considerably in height come together, it is usual to employ a slip or housed joint instead of bonding the walls into each other. This arrangement allows the heavier work to settle to a greater extent than the low portion without causing any defect in the stones.
The footings of stone walls should consist of large stones of even thickness proportionate to their length; if possible they should be the full breadth in one piece. Each course should be well bedded and levelled.