This section is from the book "Cyclopedia Of Architecture, Carpentry, And Building", by James C. et al. Also available from Amazon: Cyclopedia Of Architecture, Carpentry And Building.
1. Build the masonry, as far as possible, in a series of courses, perpendicular, or as nearly so as possible, to the direction of the pressure which they have to bear, and by breaking joints, avoid all long continuous joints parallel to that pressure.
Fig. 147. Built-up Cornice.
Fig. 148, Construction of Arch.
Fig. 149. Solid Skewbacks.
3. Lay all stones which consist of layers in such a manner that the principal pressure which they have to bear shall act in a direction perpendicular, or as nearly so as possible, to the direction of the layers. This is called laying the stone on its natural bed, and is of primary importance for strength and durability.
6. The rougher the stones, the better the mortar should be. The principal object of the mortar is to equalize the pressure; and the more nearly the stones are dressed to closely fitting surfaces, the less important is the mortar. Not infrequently, this rule is exactly reversed; i.e., the finer the dressing, the better the quality of the mortar used.
All projecting courses, such as sills and lintels, should be covered with boards, bagging etc., as the work progresses, to protect them from injury and mortar stains. When setting cut stone, a pailful of clean water should be kept at hand, and when any fresh mortar comes in contact with the face of the work it should be immediately washed off.
General Inspection, The superintendence of cut stonework requires constant attention, as it is expensive and annoying to be obliged to remove a stone after it has been set in the wall. There are also many devices by which defective stock and workmanship may be concealed, which can only be avoided by vigilance and the exercise of considerable knowledge of practices.
Fig. 150. Flat Arch of Stone.
Fig. 151. Wooden Centering for Arch.
Granite often develops local defects, such as seams, knots, or brown stains known as sap. The latter will be apparent upon examination, but seams are to be detected by striking the stone with a hammer, when a flawless stone will ring clearly.
Sandstones will often be found with small holes, called sand holes, and of an uneven color; there will also be local discolorations which are sometimes developed by cutting, and do not appear until so much expense has been put upon the stone that the contractor is often tempted to pass it as perfect if he can do so. Considerable firmness may be necessary to obtain work in exact conformity with the specifications, but this should be insisted upon at all times.' Another defect which requires the exercise of great judgment is the matter of patched stones. Expensive stones are often marred by the cutting or handling, and are then so skilfully patched by the use of melted shellac and stone dust, that they may be overlooked until set, when they often cannot be replaced without considerable delay. In this case, if the owner agrees, they may be allowed to remain, but the superintendent should see that the patching is properly done. Where a moulding in this case has been knocked off and simply stuck on again with shellac, the superintendent should insist that a square block should be dovetailed in and the moulding re-cut.
In the dressing of the stone, care should be taken to see that the work is as finely cut as called for, and mouldings should be cut according to details and matched together perfectly. Ashlar work should be cut with full bed joints without hollows or thin edges (Fig. 152), and should be cut so as to lie on its natural bed. Anchors should be freely used, and the bonding of ashlar and backing carefully watched. The setting of columns, arch stones, and moulded work should be especially noted to see that the mortar is kept well back from the edges to prevent their splitting off, and the tying of continuous courses to each other at the ends should be insisted upon. Projecting courses must be bevelled off' on the top to shed water and should also have a drip cut on the under side, so that the water will drop off and not run down the side of the building. (Fig. 153.) The bed which lies in the wall, however, must not be bevelled, but must be cut level to maintain the full bearing of the stones in the wall.
Fig. 152. Improper Stone Cutting.
When the exterior masonry of the building is done, the whole of the stonework must be washed down and pointed. This should never be done in freezing weather, and if done in extremely hot weather there will be danger of the mortar drying too quickly. For ordinary stonework, Portland cement mixed with an equal quantity of sand, and enough water to make a stiff mortar, makes a good pointing mortar; but for limestone, marble, or any stone which will be stained by cement, lime mortar, Lafarge, or other non-staining cement must be used. If the joints are not already clean, they must be raked out to the depth of an inch and moistened, and the pointing mortar applied with a small pointing trowel, and then rubbed in smooth with a jointing tool. Either a concave, flush, or projecting joint may be made, but the concave joint is more durable than the others. The washing down of the stonework will be done at the same time as the pointing, and should be done with dilute muriatic acid, using a stiff scrubbing brush. Wire brushes are sometimes used to clean down marble and granite work. The trade of pointing is in some localities made a distinct branch of building trade, and in others the work is done by the mason.
When possible, a professional "pointer" should be obtained, as he will usually have better appliances, ready for immediate use, than will the mason, whose use for them will naturally be occasional.