The proper mixing of lime mortar is nearly as important as the quality of the lime. The tendency to reduce the cost of building to the lowest possible point, and to shorten the time required for the various operations, has, with other influences, led to much neglect in the mixing of mortar, and it is safe to say that three-quarters of the lime plaster used at the present time is not properly mixed.
Where mortar is mixed by hand at the site of the building, the following method is probably the best that can be considered as practicable :
First the lime should be thoroughly slaked in a tight box, or, if the lime is not pure, so that a residue is left after slaking, it should be run off through a wire sieve into another box and allowed to stand for from twenty-four hours to seven days.
Second. After the lime has been slaked the required length of time the hair should be beat up and thoroughly incorporated with the lime paste with a hoe, and the proper amount of sand then added and the mixture thrown into a pile.
Third. After the mortar has stood in the pile not less than seven days, it should be wet up with water to the proper consistency in small quantities and immediately applied to the lathing or brickwork.
The ordinary method of mixing plastering mortar is to mix the hair and sand with the lime as soon as it is slaked, and then throw the mortar into a pile, the whole process occupying but one or two hours. The objection to this method is that the lime does not always get thoroughly slaked, and the hot lime and the steam caused by the slaking burn or rot the hair so as almost to destroy its function of strengthening the plaster. For all good work the architect should specify that the lime be slaked at least twenty-four hours before working in the hair.
For U. S. Government work the hair is not mixed in until the mortar is wet up for putting on, which is still better, but rather more expensive.
If the mortar is required in freezing weather it should be made under cover, and under no circumstances should the architect permit of the use of mortar that has been frozen.
The mixing of mortar in basements, although sometimes found necessary, is not desirable, as it introduces much moisture into the building. Mortar should never be made in the building when practicable to avoid it.
In New York City, Philadelphia, and possibly some other places, mortar, both for bricklaying and plastering, is now made by machinery in buildings specially arranged for the purpose, and delivered at the work in cart load lots in a wet and plastic condition, with the hair or fibre, and fresh water incorporated with the lime and sand, ready for use, without the addition of any other material or further manipulation whatever.
The advantages of having the mortar made in this way are that ample time is given the lime to slake, the hair and sand are not mixed with the lime until just before delivery, and the mixing is much more thoroughly and evenly done by machinery than is possible by hand.
Using mortar mixed at some other place than in the building permits of finishing the lower stories sooner than could otherwise be done, and also does away with the inconvenience of having a large pile of mortar stacked on the sidewalk or in the basement.
Machine-made mortar was used in the Corn Exchange, the Manhattan Life Building, the Home Life Insurance Building, and many other large buildings in New York.
The process of making the mortar in the Philadelphia plant is described as follows:
Into four slacking machines or revolving pans, about twelve bushels of lime are placed and enough water introduced to slack without burning. The pan is started and the lime is kept in motion by a mechanical arrangement consisting of three feet on a perpendicular shaft. When the slacking is complete a plug is removed, and the lime and water carried by a trough through three screens into a well; from this well it is pumped into vats located in the upper floors of the mixing house. Screening the lime eliminates all core or underburnt limestone, stones and other foreign matter so injurious to mortar, especially that used by plasterers.
When the lime and water is pumped into the vats it much resembles thick milk, which, after standing three weeks, assumes the consistency of soft cheese. Water is allowed to stand in these vats, which further aids in the slacking of any minute particles that have escaped through the sieves, and also to prevent the air from reaching it. (The lime used contains a considerable amount of magnesia, a pure carbonate not giving the setting qualities desirable.)
When mortar is to be made this lime paste is carried to the mixing pans, which are like those used in slacking, with the exception that they have two sets of feet; sharp, clean bar sand is also placed in the pans, and the machine thoroughly incorporates the lime and sand into a homogeneous mass, not a streak of lime and a streak of sand, but a material of uniform evenness. As a result of this care, I have tested brickquetts made of machine mortar and have obtained as great a tensile strain as 52 pounds to the square inch; in twenty-seven or twenty-eight days, out of three brickquetts broken, I secured 48, 52 and 50 pounds tensile strain. We never allow lime to air slack; neither do we mix the sand with the hot lime and allow it to stand. *
When mixing mortar by hand, the nearer the process approaches the above the better will be the quality of the plastering.