All stones should be set in a full bed of mortar, and any stone too large to be easily lifted by one man should be set with a derrick.

In some localities slips of wood are prepared of the thickness desired for the joints and laid on the top of the stone below, so that when the stone is set the mortar squeezes out until the stone rests on the slips of wood. After the mortar has set or hardened the slips are withdrawn. The bed of mortar should always be kept back an inch or more from the edge of the stone. This will prevent the stone bearing just on the outer edge, and save raking out the mortar preparatory to pointing. In damp places stonework should be set in cement, or lime and cement mortar; in dry situations it may be set in lime mortar.

Most of the granular limestones and marbles, and some sandstones, are stained by either Portland or Rosendale cement, and when using any of these stones for the first time the architect should ascertain their liability to stain. The mortar for bedding the stone can always be kept from the face of the stone by exercising a little care, and the joints afterward pointed with some material that does not stain. Stone masons are often very careless in setting stonework, and do not bed the stones evenly, so that when the weight comes upon them they crack.

208 Setting Stonework 100129

Fig. 119.

Marble and limestone are sometimes set in a cement made of lime, plaster of Paris and marble dust, and called Lafarge cement. When such cement is used for setting, and other cements for the backing, the back of the stone ashlar should be plastered with the former cement. Window and door sills should only be bedded at their ends when set and no mortar put under the middle of the sill, otherwise the settlement of the walls will break the sill.

Protecting. - The carpenter's specifications should contain a clause providing for the boxing of all mouldings, sills and ornamental work with rough pine to prevent the stone being damaged during the construction of the building. It is said that hemlock stains the stone, and should therefore never be used for this purpose.