It is impossible to state arbitrary, set, hard-and-fast proportions for the mixing of plastering for either exterior or interior work. The different makes of lime and grades of sand, alone, vary sufficiently to make any such statements exceedingly inadvisable; while the pur pose and conditions under which the plaster is to be used, frequently occasion considerable changes in its proportions.
The lime is slaked in a mortar-bed, a box of boards about 4 feet wide and 7 feet long, and a foot to eighteen inches high, set in some convenient location with its bottom about levelwith the top of a second box placed at one end, and about two feet lower in grade. Both mortar and lime-slaking beds should have tight bottoms and strong sides, well braced to resist the pressure that will come upon them when they are full. A quantity of sand already screened should also be near at hand. Poorly screened sand later causes extra trouble and work. Gravel in the mortar delays workmen while plastering and floating, and much good plaster material will be lost in hurriedly throwing or picking out these gravel stones in the rush of applying the mortar on the wall.
The barrel lime is emptied into the upper box, and water is poured on while a workman breaks up the lumps and works the mass back and forth in various directions with a hoe. The thorough work ing of the material at this stage is necessary to ensure its complete slak ing. The tendency of the careless workman is to hoe back and forth in the center of the bed without any regard as to whether he is stirring up the mortar that is down on the bottom boards,or whether the corners are drawn into the mixture and worked as evenly as the remainder of the box. If the paste is not thoroughly mul evenly worked to an equal consistency throughout, if the water is not conducted to every particle of lime, or if the other ingredients are mixed in before the paste is evenly prepared, the lime will be apt to blister and slake out unevenly, causing trouble after it is upon the wall. If the corners, for instance, are imperfectly mixed, lumps of clear lime will afterward appear. Many of these lumps will pass unnoticed under the hoe of the work man tempering the mortar, and will not be found until they are flat tened out under the wall trowel of the plasterer.
If too much water is used in slaking the lime—especially if a too great amount is added at once—the pile is chilled and forms into lumps that slake too tardily. If too little water is added, the lime is left so dry (burns, as the plasterers call it) that many small particles entirely fail to slake through lack of sufficient moisture. When too much water drowns the lime in the first place, it becomes so •thoroughly chilled that a considerable portion of its strength is lost; and the proc ess of slaking is, by the very excess of water, much retarded. The process is also slowed up if very cold water is added, although the water soon becomes heated from the reaction of the lime. At the start, just enough water should be put on to initiate the slaking process. After this, as the slaking proceeds, more water should be added as needed, taking care to keep the lime thoroughly moist at all times. A very active and quick slaking lime should be covered with water from the very beginning, to guard against the possibility of burning. If the lime once burns, it will afterward be impossible, by any amount of working, to get out all the fine lumps that are then caused. Rich lime will afterwards work cool, is little likely to crack, and bears troweling when being finished, without the surface peeling off, blister ing, or staining.
If lumps of unslaked lime escape through the screen when the lime is run off, and get mixed into the mortar, it becomes very difficult to eradicate them afterward. It is not possible for the plasterer to get these lumps out of the mortar when working it on the wall; and the results of their afterwards slaking out will continue to appear long after the house is finished. If they occur in the first coat, at various times after the work is completed—frequently extending throughout the entire first year—these lime lumps will suddenly blow or expand, forcing out the surface plastering outside them and making a large blister or lump, generally about an inch in diameter, which, if upon the ceiling, almost invariably falls off. If this unslaked lime gets into the final coat, much the same result occurs, although the particles are of necessity smaller in size. Instead of being large, the resulting holes are then comparatively small, running generally about the size of the head of a pin, and the entire surface of the plastering is fre quently pitted, the particles thrown off appearing about the room in the shape of a white dust.