s 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.

As soon as the piece begins to detach itself from the mould the air enters
the apparatus, and the pressure gauge connected with the air pump begins
to lower. It is then necessary, without a moment's loss of time, to remove
the screw press, the disk, and the upper part of the mould itself, in
order to facilitate as much as possible the contraction of the piece.
Finally, an hour or an hour and a half later, it is necessary to remove
the lower part of the mould, this being done in supporting the entire
affair by the middle. The piece and what remains of the mould are, in
reality, suspended in the air. All these preparations are designed to
prevent cracking.

_Moulding by Vacuum._--The operation by vacuum follows the same phases as
those just described. It is well, in order to have a very even surface,
not to form a vacuum until about three hours after the paste has been made
to ascend. Without such a precaution the imperfections in the mould will
be shown on the surface of the object by undulations that are
irremediable.

The first flange or vein must be preserved, and it is cut off at the
moment the piece is detached.

Moulding by vacuum, aside from the advantages noted above, permits of
giving the pieces a greater thickness than is obtained in the pressure
process. According to Mr. Renard, when it is desired to exceed one inch at
the base of the piece (the maximum thickness usually obtained), the
operatio