The Functions of Sand as used in plaster are (1) the production of regular shrinkage and the prevention of excessive shrinkage, otherwise cracking is the result; (2) to form channels for the crystallization.
Sand should be clean, sharp, and hard. The size of the grains does not influence the strength of the mortar, but the finer the plaster, the finer the sand must be. Fine sand is best for hydraulic lime and coarse for fat limes, coarse stuff and Portland cement for floating. Uniformity of size is not desirable.
The Proportion of Sand to Lime will vary considerably, according to circumstances, and is difficult to determine. One part of lime to two parts of sand is a usual mixture.
Sand is Cheaper than Lime, and it must be remembered that this is an inducement to use too large a proportion of sand in order to cheapen the plaster.
Sand is Obtained from rivers, pits, or the sea. Sea sand, or from tidal rivers, should be avoided, as the salt never dries, and will come out on the surface sooner or later, discoloring the wall papers, paint, etc., and keeping the walls damp.
River Sand is often used, but it is not to be recommended, because the sharpness of the grains is worn off by the action of the running water. It is easily obtained, however, and the light color of much river sand causes it to be used in internal work with the white cements.
Pit Sand is best. It sometimes contains loam or clay, which should be carefully washed out.
All Sand for High-Class Plastering is best washed.