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-
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 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.
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.
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
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