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.
The bed upon which the stones are laid should be level, and cleared of dust or refuse, and well moistened with water. Upon this the bed of mortar is spread evenly. Wooden wedges of the thickness of the joint are then laid on the face of the bed, and the stone carefully lowered upon the wedges, to be moved into exact position by the aid of a "pinch bar." In using a bar or rollers in handling cut stone, it will be necessary to protect the edges of the stone by bagging or other "softening." When the stone is in its final position, the wedges may be removed and the stone settled into place and leveled In-striking with a wooden mallet. In the case of heavy stones where there would be danger of the weight of the stone squeezing the mortar out of the joint, the wooden wedges are allowed to remain until the mortar has set. The bed of mortar should be kept back an inch or so from the face of the stone, so that the stone shall not bear on its outer edge. This will save raking out the mortar when the wall is to be pointed, and will prevent any danger of the splitting off of "spalls" on the face, which might occur on account of the mortar on the face of the joint becoming hard sooner than the inside, when the unequal settlement would bring pressure on the edge of the stone.
Fit;. 136. Iron Ties.
Fig. 1ST. Ashlar Masonry.
The same defect may occur if the bed of the stone is cut hollow or slack, as in Fig. 139, when the settlement of the mortar will bring the whole pressure upon the front edge of the stone with the same result. For this reason, care should be taken that the bed joints are square and true. Door and window sills should be bedded only under their ends, as the natural settlement will cause them to break if bedded under the opening. (Fig. 140.) Stone work in damp situations should be set in cement mortar, but lime may be used if the situation is dry. Limestone and marble, and some sandstones, are often badly stained by the use of cement mortar, and inquiry in respect to this should always be made before using an unfamiliar stone.
In case of danger from this source, Lafarge cement, a cement made of lime, plaster of Paris, and marble dust, may be used, which should be plastered over the back of the stone as well, if cement must be used in the backing.