To prepare a bed for the stone use plenty of mortar, heaping it somewhat in the center and with 2 in. or 3 in. in depth at the edges. If the bed of the stone is concave it may be necessary to build a portion of the mortar bed to some height; this may be done by using spalls in connection with the mortar. In any case the aim is that the center or concave part of the stone shall first make contact with the mortar, thus insuring that all air shall be expelled as the stone settles and the mortar spreads. The operation may require mortar of different consistency for different stones. Obviously a stone heavy in proportion to the area of its bed should have a stiff mortar, while for a lighter stone it will be sufficient to turn on the hose as the mortar is being spread. When the stone is down in place and the hooks are off, a bar should be introduced under one edge or corner and the stone floated back and forth a few times. This floating tests the adequacy of the bed, brings the stone to a better and more intimate contact with it and flushes out the superfluous mortar. A slight application of force to the bar will be sufficient to effect this if the stone is properly bedded. No amount of force will float the stone if it has sunk too far in its bed so as to rest at any point upon the masonry or any projecting point underneath, and the fact will at once be apparent. Even the location of the "hard spot" is shown, i.e., the point about which the stone may rotate without otherwise moving. The stone should be picked up at once and the hard spot be removed or a thicker bed be prepared. When the stone is finally set the superfluous mortar which has been forced out around the edges may be shoveled up for use elsewhere, but strictly subject to the following precautions:
Conditions must be such that the remaining mortar cannot fall away from any part of the bottom of the stone just set, these conditions being consistency of mortar and thickness of bed joint. The mortar which has been squeezed out around the edge of a large stone, piles up and creates what may be called a head (in the hydraulic sense) acting against the escape of more mortar and holding up that mortar to contact with the bottom of the stone. If any of this mortar is removed, the mortar remaining under the stone may not have enough support to prevent its falling away from the stone for some distance in from the edge. The remedy may be: first, for a thin bed joint, leave enough mortar; second, for a thicker joint, produce the effect of two or more thin joints by driving in thin spalls as underpinning. Clearly if the bed joint is quite thin or is thus made quite thin, very little mortar relatively is required to hold the bed up in place. Fig. 19 illustrates the several conditions.
Fig. 19. Showing the bedding of a stone in mortar.
If a proper bed joint is not preserved after it is made the result is no better than if it had never been made in the first place. This whole matter is one that requires most watchful attention. Carelessness in this respect is the cause of more imperfect work than any other detail of masonry construction.
In the past it has been common to specify that many if not all of the stones after having been bedded once shall be picked up in order to demonstrate that the bed had been properly formed and the stone properly bedded. Such a specification is absolutely useless except for the purpose of occasionally enlightening and satisfying some person who has not had enough experience to have acquired an inspector's eye. An inspector who knows his business and attends to it need never be in doubt as to whether a stone is properly bedded. If the stone is picked up it is almost invariably necessary to do some work on the bed before lowering it again, even if the first bedding was perfect. Such work is to shovel to the center the mortar that has been flushed out around the edges. If this is not done the stone on being lowered the second time is often liable to disarrange, sink deeper into or find a hard spot in the bed; if this work is done it restores the conditions and adds little or nothing to the knowledge attending the first bedding of the stone.
Much attention is paid to the bond, that is to say, the arrangement of the large stones, the aim being not only to introduce as many stones as possible but to avoid continuity of joints in any direction. This result, as applied to joints in any vertical plane, is attained with very little effort. It requires, however, some watchfulness to see that stones properly break joint across the horizontal planes or natural layers in which the construction progresses. As soon as some area has been covered with large stone the vertical joints are filled with spalls and mortar, hand laid up by masons with trowels. As regards soundness and cleanliness the spalls, of course, should be subject to the same careful inspection as the large stone. They should be of various sizes, say from the size of a brick up to what one man can readily handle; smaller ones are not economical as they require about as many motions and as much time to handle, and it is cheaper to use mortar. The mortar used with spalls should be thinner than for large stone, and this is accomplished sufficiently by occasionally turning the hose over that portion of the work, the mixing or tempering being done with the trowel incidental to and coincident with the spreading and handling of the mortar. The process of filling the vertical joints with spalls and mortar necessitates the introduction of frequent vertical breaks or joints, limiting the area filled at one time or by a particular mason.
For economy of labor these vertical stops should be built at places of narrowest vertical joint between large stone. Some care should be used that the exposed faces should not be left rough but be smoothed and pointed up so as to be comparable with the adjacent rock face. Obviously these vertical stop-offs should be arranged and constructed on the same idea that the large stone are set, i.e., to avoid any continuity of joint. As the filling approaches the height of the large stones it is well to stop off below their elevation, say at the level of the lowest of them and below the highest. This method economizes time and mortar and furthers the desired breaks in horizontal planes.
On stopping work, as for the night, all exposed mortar joints or surfaces should be smoothed off with the trowel, in effect pointed. This compacts the surface mortar so that when set it is more dense. The smooth surface not only affords less or no lodging place for dirt but is much more readily cleaned if dirty; also it is a better surface to build on or against as air is less liable to be trapped. Loose, detached or thin splotches or excrescences of mortar often dry out in a short time. Consequently they never acquire a set and amount to nothing but so much sand which is difficult to remove satisfactorily on a rough surface. All this, to say nothing of appearance, is another test of the quality of a mason. The work of the good mason will present a neat, cleaned up and pointed up appearance.