The very general practice of plastering -walls and ceilings dates back not much more than a century ago. Previous to that time the walls and ceilings were either wainscoted, boarded, or covered with canvas or tapestries, or else left rough.
On account of its cheapness, its fireproof and deafening qualities, and its adaptability to decorative treatment, some kind of plastering will probably always be used for finishing the interior walls and ceilings of buildings.
There can be no question but that plaster made of a good quality of lime, thoroughly slaked and mixed in the proper manner, is very durable and also a valuable sanitary agent. Most of the lime plaster used at the present day, however, is very poorly and cheaply made, often of poor materials, and very much of it is far from durable.
The stones from which lime is made, and the method of preparing it for the market, are described in Sections 100 and 154.
Materials for making lime are found in nearly every State in the Union, but as no two quarries of stone are exactly alike, there is a great difference in the quality of limes from different stones. In some localities, also, lime is obtained from shells and marble.
The manner of working the lime also varies in different localities.
In New England and New York lime is generally put up in casks or barrels and sold by measure, but in many of the Western States it is sold loose, like coal, and by weight.
There are some limes which, while good enough for making ordinary mortar, are not suitable for making plaster; this is because all the particles of the lime do not immediately slake. Some of the particles, because they are over-burned or for some other reason, will not slake with the bulk of the lime, but continue to absorb moisture, and finally after a long period, extending sometimes over two years, they will slake or "pop" and cause a speck of plaster to fall off.
The author has seen walls and ceilings that were pitted all over from this cause.
It is therefore important that the architect, when building in a new locality, or upon commencing his practice, should make inquiries as to the slaking qualities of the lime at hand, and where more than one lime is available, which one is the best. In some localities four or five different qualities of lime, from as many different places, are found on the market, and in such cases the architect should be very careful to specify the particular lime which he considers best. [Limes are generally known by the name of the locality where they are quarried.] Even in the best limes some particles do not slake quite as quickly as others, and it is not generally safe to apply any plastering in which the lime has not been slaked from ten days to two weeks.
Sand, for plastering, should be angular, not too coarse nor too fine, and free from dust and all foreign substances. Methods of testing sand for foreign substances were described in Section 103.
To make the very best plaster, the sand should be screened, washed and dried; sand prepared in this way can sometimes be obtained in the larger cities, but in most work the sand is merely screened.
Of unprepared sand, river sand is generally the best, as it is less likely to contain impurities. Pit sand is very apt to contain clay.
Sea sand is less angular than other sands, and is also considered objectionable on account of the salt contained in it. It should never be used without thorough washing in fresh water. All sands require careful screening to take out the coarse particles, and sand for hard finish should be passed through a sieve.
Although the use of sand in mortar is principally to prevent shrinking and reduce the quantity of lime, it is also considered to have a valuable chemical function, causing the formation of a hard silicate of lime, which pervades and strengthens the plaster.
Hair and Fibre. - To make the coarse plaster hang together better, hair or fibre should be mixed with the mortar for the ground work.
Outside of a few of the large Eastern cities hair is almost entirely-used for this purpose. For several years Manilla fibre, chopped about 2 inches long, has been used instead of hair for ordinary mortar in New York City and vicinity. Most of the patent mortars contain either asbestos or Manilla fibre. Fibre is cleaner than hair, and is said to be less injured by the lime.
Most of the hair used by plasterers is taken from the hides of cattle, and is washed and dried and put up in paper bags, each bag being supposed to contain one bushel of hair after it is beat up.
The weight is generally given as 7 or 8 pounds, but it often falls much short of this.
If obtained from a local tannery, the hair should be thoroughly washed and separated before using.
Hair is generally described in the specifications as "best quality of clean, long cattle hair," but the plaster must take it as it comes in the bags.
Goat hair is used to some extent in the Eastern States. It is longer and of a better quality than cattle hair.