ascending pushes this water ahead of it, and this slightly wets the
plaster and makes the paste rise regularly. When the mould is entirely
filled, the paste is still allowed to flow until it slightly exceeds the
upper level, and, spreading out over the entire thickness of the plaster,
forms a sort of thick flange. The absorption of the liquid begins almost
immediately, and, consequently, the level lowers. A new quantity of paste
is introduced, and we continue thus, in regulating its flow so as to keep
the mould always full. This operation is prolonged until the layer is
judged to be sufficiently thick, this depending upon the dimensions, form,
or construction of the vessel. The operation may take from one to five
hours.

The desired thickness having been obtained, it becomes a question of
allowing the paste to descend and at the same time to support the piece by
air pressure. The flange spoken of above is quickly cut, and the paste is
made to rise again for the last time, in order to form a new flange, but
one that this time will be extremely thin; then a perforated disk designed
for forming the top joint, and acting as a conduit for the air, is placed
upon the mould. This disk is fastened down with a screw press, and when
the apparatus is thus arranged the eduction cock is opened, and the air
pump maneuvered.

If the flange did not exist, the air would enter between the mould and the
piece at the first strokes of the piston, and the piece would be
inevitably broken. Its object, then, is to form a hermetical joint,
although it must at the same time present but a slight resistance, since,
as soon as the liquid paste has flowed out, the piece begins to shrink,
and it is necessary that at the first movement downward it shall be able
to disengage itself, since it would otherwise crack.