often happens that there forms upon the surface of the plaster a hard
crust which, although it is of no importance as regards the outside of the
mould, is prejudicial to the interior because it considerably diminishes
its absorbing power. This trouble may be avoided by coating the surfaces
that it is necessary to preserve with clear liquid paste; but Mr. Renard
advises that the mould be closed hermetically, so that the interior shall
be kept from contact with warm air. In this way it is possible to prevent
the plaster from hardening, as a result of too quick a desiccation. I now
come to the operation of moulding. In the very first place, it is
necessary to examine whether it is well to adopt the arrangement by
pressure of air or by vacuum. The form of the objects will determine the
choice. A very open piece, like a bowl, must be moulded by vacuum, on
account of the difficulty of holding the closing disk in place if it be of
very large dimensions. The same is the case with large vases of wood form.
On the contrary, an elongated piece tapering from above is more easily
moulded by pressure of the air, as are also ovoid vessels 16 to 20 inches
in height. In any case it must not be forgotten that the operation by
vacuum should be preferred every time the form of the objects is adapted
to it, because this process permits of following and directing the drying,
while with pressure it is impossible to see anything when once the
apparatus is closed.

[Illustration: FIG. 2.]

_Moulding by Pressure of the Air._--The plaster mould having been put in
place upon the mould board, and the liquid paste having been long and
thoroughly stirred in order to make it homogeneous, and get rid of the air
bubbles, we open the cock that puts the paste reservoir in communication
with the lower part of the mould, care having been taken beforehand to
pour a few pints of water into the bottom of the mould. The paste in