The mold described in the previous chapter is called, technically, a
"block mold" and is not, as a rule, used for making the clay ware. The
reason for this is that molds will wear out more or less rapidly and to
repeat the process of making new ones from the original form would be
tedious and expensive.
From the block mold a reverse is made, called a "case," and from this,
in turn, working molds are made in any required number. While it is
possible to use the block mold as a working mold, and, if only a few
pieces are required this is quite sufficient, yet, as it is often
necessary to have a number of molds, the student should understand how
to make a case.
A case may be defined as a mold from which a mold is made. If one can
imagine the visible half of the vase form as it appears in making the
mold, with the templates and ends cemented into one piece, one has a
conception of one half of a case. The problem is to make this with
permanent but movable ends so as to have a convenient form from which
half molds may be easily made.
[Illustration: Fig. 8. Offset plates. _F_, top plate, front view. _F'_,
side view. _G G'_, bottom plate.]
[Illustration: Fig. 9. Sectional view of mold ready for casing. _A_,
mold. _B B_, offset plates. _C C_, end plates. _N N_, natches.]
The ends are joined to the body by means of offsets and the first step
is the construction of these. One half of the block is taken and laid
upon its back, being supported by clay so that the face is level and
steady. An offset plate is now cut to fit each end. To make these a
piece of plaster is selected or made which is true and smooth on both
sides. The plates are cut of the same width as the mold and are beveled
at the upper edge so as to rise slightly from the mold face. The curve
at the end of the mold is cut out to fit and beveled in like manner.
Then two end plates are fitted. These should be about two inches higher
than the offset plates and are square at the top. Upon each of these two
or three clay natches are set, being placed low down near the face of
the mold. The mold and plates are well sized and bound together with
side walls just as in the making of the mold. Plaster is poured to a
height sufficient to well cover the natches and left to set hard. No
shaping is necessary. When well set the end plates and offset plates are
removed but the vase mold and the case are left attached together. The
other half of the mold is prepared and run in the same way, the same
offset plates and end plates being used with such slight refitting as
may be necessary. The work is now examined and all rough places and
scraps of adhering plaster are removed. The two halves of the case, the
half molds being still attached, are set up on end, back to back, being
separated by a thin piece of plaster or a strip of cardboard which
should extend two inches above the top. The top ends are now sized, the
natch holes having been smoothed off, a band of paper is tied around and
plaster poured on top to a depth of about one inch. When set the whole
is turned over and the operation is repeated on the other end. After the
final setting the ends are easily removed and by the insertion of a thin
knife driven by a light blow, the molds and case are separated. Each
half case is now laid on its back and the proper ends are fitted in
place. It only now needs the usual side walls to be tied on and molds
can be made with ease just as the original block mold was made.
[Illustration: Fig. 10. Mold and case in position. The top ends are
lifted to show fitting. The bottom ends are not shown.]
It now remains to make a case of the bottom mold. The bottom piece of
the block mold is taken and sized and with a strip of paper bound around
it, plaster is poured. The two are detached when set and the case is
finished. It consists of seven pieces; three are used in each half and
one for the bottom.
[Illustration: Fig. 11. Block of plaster with face of plate turned. _B_,
height of plaster to be poured. _C_, rubber belt.]
Thus equipped it is possible to make any number of working molds and if
the case should wear out or be damaged, a new one can always be made
from the block mold. The block mold itself, having been sized, is no
longer absorbent and cannot be used for making vases. The working molds
should be thoroughly dried before using and they will last longer.
Flat ware, such as plates and saucers, is made on, not in, a mold. The
diameter of the plate having been decided upon, a block of plaster three
inches wider is run. This is placed on the center of the wheel or jigger
and in it the face of the plate is turned. This must be sunk below the
level of the block and when finished, must appear as though the plate
itself were embedded in the plaster. One half of the thickness of the
edge is shown in such a way that there is no under cutting. Just outside
of this edge the plaster is turned so as to slope gently up to the level
of the block.
Without removing the block from the wheel the face of the plate is well
sized, a band of belting is arranged, of the same diameter as the edge
of the slope and plaster is poured to a depth of three inches. Out of
this the back of the mold is turned as shown in the illustration (Fig.
12).
[Illustration: Fig. 12. _A_, block of plaster. _B_, mold poured on face
of plate and turned.]
The top of this as it lies upside down is shaped with a straight, almost
upright slope which enables the mold to be set securely in the wheel
head. Around the exposed edge of the original block, three or four
natches are now bored or cut. They should be placed at irregular
distances so that there will be no doubt as to the putting together of
the sides of the case. If two circular pieces of plaster have to be set
together and held by natches there should always be either this
irregular spacing or some distinctive mark, because if this be not
provided for, two or three trials will always be made before the correct
fitting is found and these trials wear out the natches very quickly.
[Illustration: Fig. 13. _A_, bottom of case. _B_, Cavity for pouring
molds. _C_, top of case.]
The back of the plate mold and the edges of the block are now sized and
plaster is run to the level of the highest part of the mold but no
higher (Fig. 13). When this is set, the two halves of the case can be
separated and the mold taken out. Now when the halves of the case are
fitted together there will be a cavity the exact size of the mold. This
can be filled again and again with plaster, a new mold being formed each
time.
[Illustration: Fig. 14. Iron prong to fit wheel head.]
[Illustration: Fig. 15. _A_, plaster, with prong inserted. _B_, rubber
belt.]
In order to use these molds a special head must be provided for the
wheel. The regular head of the wheel should be detachable and in its
place an iron frame called a prong is fitted. This consists of a collar
either with a hollow cone or a screw to fit the shaft of the wheel, and
from this radiate four short arms. In order to use this a circular block
of plaster some two or three inches thick is poured on a table or slab
and just as this is setting, the prong, upside down, is pressed into it
just below the surface and held there until the plaster is hard enough
to support the weight of the iron. When hard, the whole is lifted and
the prong with the plaster attached is set in position on the wheel.
This now forms a rough plaster head and it must be turned true. In this
head a circular depression is to be turned which will exactly fit the
back of the plate molds. If the recess should wear larger as it will if
much used, a new head can easily be run. The same principle can be
applied to the making of molds for saucers.
[Illustration: Fig. 16. Wheel head with plate mold. The tool used is
shown in dotted outline.]
Cups and bowls are molded from the outside. A block of plaster about one
inch thicker than the height of the proposed cup is taken and centered
upon the wheel. Out of this the piece is to be turned, upside down,
leaving a ledge or platform, the outside diameter of which is the size
of the mold. The rubber belt is tied around this and the mold poured. If
for casting this will suffice, but if it is intended to make the cups
upon the wheel the outside of the mold must be turned to fit a
wheel-head which is hollowed to receive it. The making of the cups is
described in Chapter XI. A bowl is simply an enlarged cup.