The method of applying these plasters does not differ materially from that already described for lime mortar, except that the second (corresponding to the brown) coat is put on directly after the first coat, and is finished with the darby instead of with the float. Being of the nature of cement, or plaster of Paris, these mortars set instead of drying, and but little water should be used in working them. Only as much material should be mixed as can be applied in one and a half hours, and material that has commenced to set should never be remixed.
Only clean water should be used, and the tools and mortar box should be kept perfectly clean and the box cleaned out after each mixing.
When using the hard plasters on wood laths, the laths should be thoroughly dampened, or expanded, before the plaster is spread, so that they will not swell after the plaster has commenced to set. Brick, stone and tile work should also be well sprinkled before applying, these mortars.
Most of the manufacturers of hard plasters recommend that when their plaster is to be used the laths be spaced only from 1/3 to ¼ inch apart, and that ¾-inch grounds be used, claiming that a less quantity of their material is required than of ordinary lime mortar.
A gentleman who has had much experience with cement plasters, however, says that "More failures are made in using hard plaster by using too thin coats, too weak keys and too weak material (when sold unmixed with sand) than from any other cause.
"To do a good job of hard plastering it is necessary to use a sufficient amount of cement to give it tensile strength, a good wide key, and a good thick coat of plaster. Where it is spread very thin it is sure to crack and give an unsatisfactory wall."
For lath work a better wall will be obtained, although at a little more expense, by putting on 7/8-inch grounds and having a 3/8-inch key.
Sand finish is generally made by mixing sand with the same plaster as is used for the brown coat.
Full directions for applying the various grades of these plasters are furnished by the manufacturers, and architects should see that these instructions are carefully and faithfully followed, as when improperly applied these plasters are inferior to the ordinary lime mortar.
The principal advantages gained by the use of these plasters are: uniformity in strength and quality, greater hardness and tenacity, freedom from pitting, less weight and moisture in the building, saving in time required for making and drying the plaster, minimum danger from frost and greater resistance to fire and water.
Frost does not harm these mortars after they have commenced to set or the chemical action has taken place. When used in freezing weather they must not be allowed to freeze during the first thirty-six hours after applying; after that time frost will do no harm.
Those plasters which are already mixed with sand and fibre also have the additional advantage of thorough and uniform mixing of the materials and absolute correctness of proportion. This latter advantage is perhaps most appreciated by the architect, as it prevents all chance of using a poor quality of sand, or too much of it, and saves him a great deal of labor in the superintendence.
The benefit to the owner in using these plasters consists in securing much more substantial walls than is possible with the ordinary hand-made mortar, less risk from fire and less expense for repairs.
The slight additional expense of using them is hardly to be considered in comparison with the benefits obtained, and it is probable that these plasters will in a short time become generally adopted, they being already extensively used in the largest and most costly buildings.
For business buildings the saving in the time required in drying the plastering will more than pay for the additional expense.
On account of their greater density these mortars will not harbor vermin nor absorb noxious gases or disease germs, and are therefore especially desirable for hospitals, schools, etc. Heat, air and moisture will not pass through them as through lime plaster.
A wall of hard plaster, wood or metal lath is also much more resonant than one of lime mortar, and for this reason, and also on account of their greater strength, these mortars should be especially valuable for plastering churches, opera houses and public halls.