It has been found by repeated experiments that a barrel of Rockland lump lime, thoroughly slaked, will yield on an average 2.72 barrels of lime paste. Some limes will yield more and others less, the average of four Eastern limes tested being 2.62 barrels of paste. It has also been demonstrated by repeated experiments that the average sum of voids in sharp, clean, silicious bank or pit sand, taken from different locations and thoroughly screened, is .349 of its bulk. It was also shown that the best mortar is obtained by mixing with the sand such an amount of lime paste as will be from 45 to 50 per cent, greater than the amount needed to fill the voids of the sand, which practically requires a proportion of 1 part lime paste to 2 of sand. This is the proportion usually specified on Government work.
As it is difficult to measure the lime paste, it would perhaps be better to specify that only 5½ barrels of screened sand should be used to one cask of lime. Where lime is sold by weight about the same proportions will be obtained by specifying 2½ barrels of sand to 100 pounds of dry lime.
* Henry Longcope, in the Brickbuilder.
Mixed in the above proportions it will require about 2½ casks, or 500 pounds, of lime and 14 barrels (42 cubic feet) of sand to cover 100 yards of lath work, 3/8 inch thick over the lath.
The proportion of hair to lime should be for first-class work, 1½ bushels of hair to one cask, or 200 pounds, of lime for the scratch coat, and ½ bushel of hair to one cask of lime for the brown coat. This is considerably more, however, than will be found in most plaster.
The proportion of lime given above is none too rich for first-class plaster, either for the brown or scratch coat, but it is seldom, if ever, that brown mortar is made as rich as this, and much first-coat work is inferior to it.
In fact, it is almost impossible to regulate the proportion and uniform mixing of common plaster. Where lime is sold by the cask it can be done by mixing one cask of lime at a time and measuring the sand, but where lime is sold by weight it would be necessary to keep scales on the ground for weighing the lime; and in either case it would be necessary to have an inspector to watch the making of the mortar.
In practice the lime is slaked and as much sand mixed with it as the mortar mixer thinks best or the plaster will stand, and it is almost impossible for the architect to tell whether or not there is too much sand. It seldom happens that there is too little sand.
After considerable experience with mortar, one can tell something about its quality by its appearance when wet up, or by trying it with a trowel, but practically the architect and owner is in most cases at the mercy of the contractor, and about the best that can be done, when using common plaster, is to insist on the best materials, mixing in the hair after the lime is cool and giving the contract to an honest and intelligent plasterer.