223. The quantity of thoroughly slaked lime paste that a barrel of ordinary lump lime will yield varies from about 2.6 to 2.75 barrels. The voids in sharp and clean sand are about 1/3 of its bulk. Probably the best plastering mortar is obtained by using sufficient lime paste to fill the voids in the sand 1 1/2 times; that is, the lime paste should be 1/3+1/6 = 1/2 the volume of the sand, or 1 part of lime paste to 2 of sand. Measured in barrels, the lime (unslaked) and sand should be in the ratio of 1 to about 5 1/2. If lime is sold by weight, the same result will be had by using 2 1/2 barrels of sand to 100 pounds of dry lime. For 100 square yards of plastering, laid 3/8 inch thick, it will require, at the above proportions, about 2 1/2 barrels of lime and 14 barrels of sand. For the best class of work, the quantity of hair used should be 11/2 bushels to 1 barrel of lime for the first, or scratch, coat, and 1/2 bushel to 1 barrel for the second, or brown, coat. Generally, however, plastering mortar contains less lime and hair than above given, although these proportions are not too much for first-class work.

It is a difficult matter to regulate the proportion and uniform mixing of common plaster, unless the operations are constantly watched. The ordinary method of mixing is to add as much sand to the slaked lime as the mixer thinks it will stand, and it is hard for the architect to determine whether or not there is too much sand; but considerable experience will enable one to judge the quality of the mortar by its appearance, or by trying it with a trowel when ready for use.

224. For scratch and brown coat on wood lath, with 3/4-inch grounds, the following quantities are generally allowed for 100 square yards: 1,400 to 1,500 laths; 10 pounds 3-penny lathing nails; 2 1/2 barrels, or 500 pounds, of lime; 45 cubic feet, or 15 barrels, of sand; and 4 bushels of hair. For the best quality of white coat, the estimate is 90 pounds of lime to 50 pounds of plaster of Paris, and 50 pounds of marble dust. The same proportions can be used for large or small quantities.