1. In modern practice, plastering occurs in many departments of
architecture. It is more particularly applied to the ceilings and
interior walls of buildings, and also in rough-casting on their
exterior.
2. In plastering the interior parts of buildings, three coatings of
mortar are commonly applied in succession. The mortar for the _first
coat_ is composed of about twelve parts of sand, six of lime, and
three of hair, with a sufficient quantity of water to bring it to the
proper consistence; that for the _second coat_ contains a less
proportion of lime and hair; and that for the _third coat_ is composed
exclusively of lime and water.
3. The mortar is applied directly to the solid wall, or to thin strips
of wood called _laths_, which have been fastened with small nails to
the joists, and other parts of the frame of the building. The tools
with which the plasterer applies the mortar are _trowels_ of
different sizes and shapes, and the _hawk_. The latter instrument is
a board about a foot square, with a short handle projecting at right
angles from the bottom.
4. In all well-finished rooms, cornices are run at the junction of the
wall and ceiling. The materials of these cornices are lime, water, and
plaster. The lime and water are first incorporated, and the plaster is
added with an additional quantity of water, as it may be needed for
immediate application. The composition is applied in a semifluid
state, but the plaster causes it _to set_, or to become solid
immediately. In the mean time, the workman applies to it, in a
progressive manner, the edge of a solid piece of wood, in which an
exact profile of the proposed cornice has been cut.
5. Ornaments of irregular shape are cast in moulds of wax or plaster
of Paris, and these are formed on models of the proposed figures in
clay. Such ornaments were formerly the productions of manual
operations performed by ingenious men called _ornamental plasterers_.
The casts are all made of the purest plaster; and, after having been
polished, they are fastened to the proper place with the same
substance saturated with water.
6. The branch of this business called _rough-casting_, consists in
applying mortar to the exterior walls of houses. The mode in which the
work is performed varies but little from that adopted in plastering
the walls of apartments. It, however, requires only two coats of the
cement; and, when these have been applied, the surface is marked off
in imitation of masonry. It is likewise sometimes colored, that it may
resemble marble or some other stone.
7. The cement is commonly made of _sharp sand_ and lime; but sometimes
a kind of argillaceous stone, calcined in kilns and afterwards reduced
to powder by mechanical means, makes a part of the composition. The
qualities of this material were first discovered by a Mr. Parker, who
obtained letters patent for this application of it, in England, in
1796; hence it has been called _Parker's cement_.