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
operation is as follows: The piece is moulded normally, and it is
supported by a vacuum; but, at the moment at which, under ordinary
circumstances, it would be detached, the paste is made to ascend a second
time, when the first layer (already thick and dry) acts as a sort of
supplementary mould, and permits of increasing the thickness by about 2/5