density.  The  cementitious  material  becomes  over- extended  since  it  must  coat  a  relatively  larger  overall aggregate  surface.  An  excess  of  coarse  aggregate adversely affects workability-the mix becomes harsh working and difficult to apply. Plaster shrinkage during drying can be caused by an excess  of  either  fine  or  coarse  aggregate.  You  can minimize this problem by properly proportioning the raw material, and using good, sharp, properly size- graded sand. Generally, any sand retained on a No. 4 sieve is too coarse to use in plaster. Only a small percentage of the material (about 5 percent) should pass the No. 200 sieve. Vermiculite Vermiculite is a micaceous mineral (that is, each particle  is  laminated  or  made  up  of  adjoining  layers). When  vermiculite  particles  are  exposed  to  intense  heat, steam forms between the layers, forcing them apart. Each  particle  increases  from  6  to  20  times  in  volume. The expanded material is soft and pliable with a color varying between silver and gold. For  ordinary  plasterwork  vermiculite  is  used  only with gypsum plaster; therefore, its use is generally restricted to interior applications. For acoustical plaster, vermiculite is combined with a special acoustical binder. The approximate dry weight of a cubic foot of 1:2 gypsum-vermiculite plaster is 50 to 55 pounds. The dry weight of a cubic foot of comparable sand plaster is 104 to 120 pounds. Perlite Raw perlite is a volcanic glass that, when flash- roasted,  expands  to  form  irregularly  shaped  frothy particles containing innumerable minute air cells. The mass is 4 to 20 times the volume of the raw partlicles. The color of expanded perlite ranges from pearly white to grayish white. Perlite is used with calcined gypsum or portland cement for interior plastering. It is also used with special binders for acoustical plaster. The approximate dry weight of a cubic foot of 1:2 gypsum-perlite plaster is 50 to 55 pounds, or about half the weight or a cubic foot of sand plaster. Wood Fiber and Pumice as wood fiber and pumice, are also used. Wood fiber may be  added  to  neat  gypsum  plaster,  at  the  time  of manufacture, to improve its working qualities. Pumice is a naturally formed volcanic glass similar to perlite, but heavier (28 to 32 pounds per cubic foot versus 7.5 to 15 pounds for perlite). The weight differential gives perlite an economic advantage and limits the use of pumice to localities near where it is produced. WATER In plaster, mixing water performs two functions. First,  it  transforms  the  dry  ingredients  into  a  plastic, workable mass. Second, it combines with the binder to induce hardening. As with concrete, there is a maximum quantity  of  water  per  unit  of  binder  required  for complete hydration; an excess over this amount reduces the  plaster  strength. In all plaster mixing, though, more water is added than is necessary for complete hydration of the binder. The excess is necessary to bring the mix to workable consistency. The amount to be added for workability depends on several factors: the characteristics and age of the binder, application method, drying conditions, and the tendency of the base to absorb water. A porous masonry base, for example, draws a good deal of water out of a plaster mix. If this reduces the water content of the mix below the maximum required for hydration, incomplete curing will result. As a general rule, only the amount of water required to attain workability is added to a mix. The water should be potable and contain no dissolved chemicals that might accelerate or retard the set.  Never use water previously used to wash plastering tools for mixing plaster. It may contain particles of set plaster that may accelerate setting. Also avoid stagnant water; it may contain organic material that can retard setting and possibly  cause  staining. PLASTER BASE INSTALLATION LEARNING OBJECTIVE: Upon completing this section, you should be able to associate the names and purposes of each type of lath used as a plaster base. You should also be able to describe  the  procedures  used  in  plastering, including   estimating   materials   and   the procedures  for  mixing  and  applying  plaster bases. Although sand, vermiculite, and perlite makeup the For plastering, there must be a continuous surface great majority of plaster aggregate, other materials, such to which the plaster can be applied and to which it will 7-3