This section is from the book "A Treatise On Architecture And Building Construction Vol2: Masonry. Carpentry. Joinery", by The Colliery Engineer Co. Also available from Amazon: A Treatise On Architecture And Building Construction.
113. The effects of the weather on the exposed edges of the joints in masonry usually cause the mortar to crumble and fall out. For this reason, it is customary to refill the joints, to a depth of from 1/2 inch to 1 inch, with specially prepared mortar. This operation is called pointing. It is generally done when the walls are completed, but, if the season is too far advanced, it should be deferred until spring. Under no circumstances should it be done in freezing weather, nor in extremely hot weather, as then the mortar will dry too rapidly.
Portland cement mortar, made of equal parts of sand and cement, and such coloring matter as may be desired, mixed with just enough water to give a mealy consistency, makes the most durable mortar for pointing.
114. Before applying the pointing mortar, the joint should be raked out to a depth of about 1 inch, cleaned with a stiff brush, and well moistened, so that the fresh mortar will adhere better to the stone. A pointing trowel is used for applying the mortar, which is thoroughly pressed in, and given a smooth surface by a tool called a jointer; at (a), Fig. 64, a concave-edged jointer is shown, and at (b), a convex-edged jointer. It will be seen that the jointer gives either a raised or a sunk joint, as may be desired. The latter is the more durable joint, but the first makes the best looking work.
The different forms of pointing are shown in Fig. 65, in which a indicates the concave joint; b, the convex; and c, the convex projecting beyond the face of the stone.