By CHAS. LAUTH.
The operation of moulding presents numerous advantages over other methods
of shaping porcelain, for by this process we avoid irregularities of form,
twisting, and visible seams, and can manufacture thin pieces, as well as
pieces of large dimensions, of a purity of form that it is impossible to
obtain otherwise.
The method of moulding small objects has been described with sufficient
detail in technical works, but such is not the case with regard to large
ones, and for this reason it will be of interest to quote some practical
observations from a note that has been sent me by Mr. Constantine Renard,
who, for several years, has had the superintendence of the moulding rooms
of the Sevres works.
The process of moulding consists in pouring porcelain paste, thinned with
water, into very dry plaster moulds. This mixture gradually hardens
against the porous sides with which it is in contact, and, when the
thickness of the hardened layer is judged sufficient, the mould is emptied
by inverting it. The excess of the liquid paste is thus eliminated, while
the thicker parts remain adherent to the plaster. Shortly afterward, the
absorption of the water continuing, the paste so shrinks in drying as to
allow the object to detach itself from the mould. As may be seen, nothing
is simpler when it concerns pieces of small dimensions; but the same is
not the case when we have to mould a large one. In this case we cannot get
rid of the liquid paste by turning the mould upside down, because of the
latter's size, and, on another hand, it is necessary to take special
precautions against the subsidence of the paste. Recourse is therefore had
to another method. In the first place, an aperture is formed in the lower
part of the mould through which the liquid may flow at the desired moment.
Afterward, in order to prevent the solidified but still slightly soft
paste from settling under its own weight at this moment, it is supported
by directing a current of compressed air into the mould, or, through
atmospheric pressure, by forming a vacuum in the metallic jacket in which
the mould is inclosed.
The history and description of these processes have been several times
given, and I shall therefore not dwell upon them, but shall at once
proceed to make known the new points that Mr. Renard has communicated to
me.
The first point to which it is well to direct the manufacturer's attention
is the preparation of the plaster moulds. When it concerns an object of
large dimensions, of a vase a yard in height, for example, the moulder is
obliged to cut the form or core horizontally into three parts, each of
which is moulded separately. To this effect, it is placed upon a core
frame and surrounded with a cylinder of sheet zinc. The workman pours the
plaster into the space between the latter and the core, and, while doing
so, must stir the mass very rapidly with a stick, so that at the moment
the plaster sets, it shall be as homogeneous as possible. In spite of such
precautions, it is impossible to prevent the densest parts of the plaster
from depositing first, through the action of gravity. These will naturally
precipitate upon the table or upon the slanting sides of the core, and the
mould will therefore present great inequalities as regards porosity. Since
this defect exists in each of the pieces that have been prepared in
succession, it will be seen that when they come to be superposed for the
moulding of the piece, the mould as a whole will be formed of zones of
different porosities, which will absorb water from the paste unequally.
Farther along we shall see the inconveniences that result from this, and
the manner of avoiding them.
[Illustration: FIG. 1]
The mould, when finished, is dried in a stove. Under such circumstances it
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
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.
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
of an inch. The piece is held, as at first, by vacuum, and the paste is
introduced again until the desired thickness is obtained.
Whatever be the care taken, accidents are frequent in both processes. They
are due, in general, to the irregular contraction of the pieces, caused by
a want of homogeneousness in the plaster of the moulds. In fact, as the
absorption of the water does not proceed regularly over the entire surface
of the piece, zones of dry paste are found in contact with others that are
still soft, and hence the formation of folds, and finally the cracking and
breaking of the piece. The joints of the moulds are also a cause of
frequent loss, on account of the marks that they leave, and that injure
the beauty of the form as well as the purity of the profile.
Mr. Renard has devised a remedy for all such inconveniences. He takes
unglazed muslin, cuts it into strips, and, before beginning operations,
fixes it with a little liquid paste to the interior of the mould. This
light fabric in no wise prevents the absorption of the water, and so the
operation goes on as usual; but, at the moment of contraction, the piece
of porcelain being, so to speak, supported by the muslin, comes put of the
mould more easily and with extreme regularity. Under such circumstances
all trace of the joint disappears, the imperfections in the mould are
unattended with danger, and the largest pieces are moulded with entire
safety. In a word, we have here a very important improvement in the
process of moulding. The use of muslin is to be recommended, not only in
the manufacture of vases, but also in the difficult preparation of large
porcelain plates. It is likewise advantageous in the moulding of certain
pieces of sculpture that are not very delicate, and, finally, it is very
useful when we have to do with a damaged mould, which, instead of being
repaired with plaster, can be fixed with well ground wet sand covered with
a strip of muslin.
_Drying of the Moulded Pieces._--When the moulded pieces become of a
proper consistency in the mould, they are exposed to the air and then
taken to the drying room. But, as with plaster, the surface of the paste
dries very quickly, and this inconvenience (which amounts to nothing in
pieces that are to be polished) is very great in pieces that carry
ornaments in relief, since the finishing of these is much more difficult,
the hardened paste works badly, and frequently flakes off. In order to
remedy this inconvenience, it suffices to dust the places to be preserved
with powdered dry paste.--_Revue Industrielle._
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