In plastering mortar where hair is required, a still further loss of strength would result, as the hair would be so rotted or eaten by its long exposure to the action of the wet lime as to be almost or quite worthless. The hair cannot well be mixed evenly, except at the time when the mortar is first run off, while it is in a very thin paste. If, after a lime-and-sand mixture had been standing for some months, it were attempted to bring it to a sufficiently fluid state to receive the hair properly, by wetting it down a second time, a considerable propor tion—varying from a quarter up to almost a half—of its strength would be sacrificed.
Bearing these facts in mind—once certain that the lime is slaked— it would appear better that not more than a week should elapse before the use of this mortar; and a less time than that is, under many circum stances, undoubtedly desirable. It is evident that no more lime-and sand mortar should be mixed at one time than can be used within a few days at the most. The length of time that mortar should be allowed to stand, is determined more or less by the dryness or moisture of the atmosphere. The dryer the atmosphere, the shorter the time, as the setting of the mortar is, in part, a chemical result of the drying out, or evaporation, of the water of crystallization, as it is called.
It has already been said that limes made in different parts of the country vary extensively in their chemical composition and properties. A knowledge of the chemical composition of lime mortars and the individual peculiarities of the lime locally used, is necessary before applying or attempting to utilize the principles here set forth. In the eastern part of the United States, the limes frequently contain from a third to a half of carbonate of magnesia; and the mortar in which such limes are employed sets very readily.
To sum up, the lime should be slaked as evenly and thoroughly as possible. It should be run off from the slaking bed through a fine sieve into the mortar-bed It should lie there no longer than is abso lutely necessary; and if it could be possible to add the hair and sand while the original mixture is sufficiently moist to take up and work the entire amount of the latter material to be added, the resulting mixture would undoubtedly be that much the stronger and more durable.
Mixing the Mortar. 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 excep
tionally 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 propor tion, 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 mix ture becomes much harder to work when the tempering is partly com pleted, 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 The hair may be mixed in either before the adding of the sand or when but a very small pro portion 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.