This section is from the book "Cyclopedia Of Architecture, Carpentry, And Building", by James C. et al. Also available from Amazon: Cyclopedia Of Architecture, Carpentry And Building.
The amount of sand to be mixed in with the lime paste is a variable quantity, depending upon the sand itself, upon the quality and thickness of the lime paste, and also upon the nature of the work for which the mortar is intended. With exceptionally rich limes, sand to the amount of about two times the bulk of the lime measuring the slaked lime in the form of a rather firm paste - may be added. As will be seen, this is a most uncertain proportion, for a great deal depends upon the firmness of the lime paste alone. Allowing for variation in size of the lumps of lime and their closer or looser packing together, it may perhaps be better to say that the sand should bear a relation to the lime, before it is slaked, of from three to four and one-half times its bulk.
The richer the lime and the finer the particles of sand, the more of the latter should be employed, although the finer sand does not make as hard or as good mortar as the coarser variety. If both are clean and sharp, the finer and coarser varieties of sand may be mixed together with good results. Most laborers are apt to stop adding sand, merely because the mortar mixture becomes hard to work when the paste becomes too thick. This is poor policy, inasmuch as the mixture becomes much harder to work when the tempering is partly completed, a day or two later.
The fineness of the sand is an important factor. A rather coarse as well as sharp sand is considered best, as the amount and capacity of the voids left in such a mixture would be of such size as, without any doubt, would provide space to contain lime sufficient to cement this granular mass very firmly together. The close pressure and contact of the sand particles would also lessen the possibility of settlement or shrinkage, with accompanying map-cracks. The hair may be mixed in either before the adding of the sand or when but a very small proportion of the latter has been worked into the lime mixture. The hair is generally mixed with the mortar by means of an iron rake. It should be thoroughly mixed, and enough should be used to make it impossible to find any small sections of the mortar in which the hair cannot be seen. This will require from one and one-half to two bushels of hair to a cask of lime.
If the mortar is to be used as a first coat on stone, brick, or similar surfaces, it will carry more sand, and hair is not considered so essential, a half-bushel to the barrel of lime being generally ample. If too little sand is used, the plaster is liable to dry too quickly when setting, and, after it is dry, will crumble very easily, showing up too white, or ashy gray, in appearance. If too much sand has been used, the plastering is liable to fall off, and will crumble when rubbed between the fingers.
Mortar for a second coat on lath may be of about this same consistency of mixture. For the final cost (the putty coat or hard finish) but very little sand was used. The harder the finish, the less the amount of sand. For this coat, the sand is mixed at the time when the putty is run off. For hard finish, when marble dust, brick dust, or anything of that sort is added, it is generally mixed together on the mortar-board immediately before applying. Stucco, or plaster of Paris, is never mixed with putty until immediately before using, on account of its rapid setting, which occurs in a few moments after mixing. When once set before being applied, it becomes useless. No more water than is necessary should be added, either in the mixing of the mortar at first or in its subsequent tempering, as over-much wetting of the lime deprives it of a considerable proportion of its strength, and also retards the setting process by giving that much more moisture that is necessary to be disposed of by evaporation or crystallization.
A bushel of lime is standardized to weigh 80 pounds; 200 pounds is allowed to the barrel; a bushel contains about one and one-quarter cubic feet. A barrel of sand is supposed to contain 3 cubic feet of sand, and a bushel of sand weighs about 120 pounds, and wet mortar 130 or 132 pounds. When hard, mortar is figured to weigh about 110 pounds to the cubic foot.
To summarize - one barrel of lime, 200 pounds, will take about a cubic yard of sand. In most localities a load of sand is supposed to contain twenty-seven cubic feet, or a cubic yard; but it is frequently less than this, extending down to two-thirds of the amount. To the barrel of lime should also be used about two barrels of water and - as we have seen - upwards of two bushels of hair for a first coat. Hair comes in paper bags weighing generally something under eight pounds and containing enough hair to beat up into a measured bushel. This amount of material, when the lime has been slaked and the whole mixed together, will amount to 35 or 40 yards (about 5 barrels) of mortar; and the amount should cover about 40 square yards of lathed area, requiring about 600 laths to surface.
The final skim coat is mixed roughly to the following proportions:
A cask of lime to a half-tub of water, which should take up about a barrel of the hard, clean sand used in the surface coat. Generally the plasterer uses a larger barrel or hogshead for water, than the cask in which the lime is delivered. Also, in some localities, the lime will run somewhat more than 200 pounds to the barrel, Maine lime from Rockland being supposed to average 220 pounds. Rockland lime is considered in the East good lime for scratch and brown coats, but many masons prefer Jacob's lime for the finish coat.
It should be remembered that the bulk of the completed mortar mixture does not equal the total combined bulk of its various ingredients, but is less than the aggregate bulk by about one-quarter.
Interior plastering is now applied either in two or in three coatings. Three coats are always necessary on metal or wire lath, the first coat being required to stiffen the body of the material sufficiently to allow thorough working of the remaining coats. Even upon wood laths, three coats make a better job of plastering than two. Extra strength and body are obtained by the addition of the extra coat, provided time be allowed to dry out each of the coats thoroughly before the next coating is added. It has now, nevertheless, become the general custom to employ but two coats on the less expensive grades of residence work.
The plaster mortar is applied to the walls with a hand trowel of steel, about four and one-half inches wide by twelve inches long, having a wooden handle that is parallel with the back of the blade. After the mortar is put on and roughly smoothed out with the steel trowel, the darby, a long wooden trowel, about four inches wide and three feet in length, is taken by the workman and used - with a scouring motion -to level the plaster surface and work it to an even thickness and uniform density. The flat part of the darby is generally of hard pine, a half-inch or slightly more in thickness.