In commercial production the casting method is constantly used. It is a
means of making light and delicate pieces with ease and, of course, all
the pieces cast in the same mold are alike. This very fact, however, has
led to the method being disregarded by the studio worker who does not
wish to duplicate anything that he makes. If a single piece only is to
be made the work involved in molding is a waste of time and it is better
to strive for skill at the wheel, and yet there are occasions when a
knowledge of casting is of great value. In the preparation of trial
pieces there is no method better. To make these in sufficient number on
the wheel would be tedious except for the benefit of the practice
involved.
Directions for making molds have already been given and the slip which
will have been prepared in the process of clay making is ready for the
casting process. This slip should be thick, about the thickness of
buckwheat batter. To be accurate, a pint should weigh 26 ounces. For
small pieces or for vases with narrow necks it is advisable to use the
slip rather thinner. For large wares, on the other hand, or for open
bowls it may be slightly thicker. A few experiments will show the reason
for this. Two quart jugs are needed. They should be large of neck and
should deliver their contents freely and completely. Jugs with a deep
shoulder are not good as the slip hangs in the pouring. One of these
jugs is filled with slip which is to be poured carefully from one to the
other, allowing it to flow gently down the side. This is to break the
air bubbles which are nearly always found to be present and the pouring
should be repeated until the slip flows smooth and even.
The mold, being thoroughly dry, is tied around with twine, if in parts,
and wedged firmly so that it cannot leak. The slip is then carefully
poured so as not to touch the sides and the mold is filled until a small
mound of slip rises over the edge. This mound will at once begin to sink
as the water is drawn into the walls of the mold and slip must be added,
little by little, to make good the loss. A small quantity of clay will
now be found to have stiffened at the rim of the mold and if this be
carefully removed with a steel tool the thickness of the wall of the
vase will be seen. If not thick enough the mold must be continually
filled up until the necessary thickness is attained. The mold is then
carefully lifted, making sure that the bottom is held firmly, and the
slip is poured out. It should not be poured back into the casting-jug
but into another vessel.
The mold is now set upside down to drain. It should not be placed upon
the table but upon two sticks laid parallel so that the drip may hang
clear. Several molds may be filled in this way at one time and after
about twenty minutes the one first filled may be opened. The bottom is
gently detached and the upper part of the mold, consisting of two
halves, is laid upon the table on its side. A little gentle manipulation
will now suffice to lift the one half and the vase will be seen lying in
the other half as in a cradle. The clay is still very soft and must be
treated carefully. The half mold, with the contained vase, is taken in
the left hand and held nearly upright, the fingers below, the thumb on
the top. Now set the fingers of the right hand under the bottom of the
vase, rest the thumb lightly against the side and tilt the half mold
gently forward. If mold and clay are in good condition the vase will
fall forward to be supported on the fingers of the right hand and
steadied by the thumb. The half mold is now laid down and the vase taken
in both hands, set gently on a plaster bat and put aside to dry. It
often happens that the vase leaves the mold with reluctance. If the slip
be very new, or the mold either damp or hard or worn out there will be
some difficulty in effecting a separation. By allowing the work to stand
a while, however, and by slightly jarring the mold from time to time
with the ball of the thumb the piece can generally be removed without
damage.
In using a new mold it is customary to make what is called a "waste
filling." The mold is filled with slip and at once emptied. After
standing a few minutes it is forcibly opened and the thin layer of clay
inside is picked out with a ball of plastic clay pressed against it. A
tool should never be used as this will damage the face of the mold. If
the clay should stick obstinately a soft cloth used over the finger will
remove it. The reason for this waste filling is that it removes the scum
which occurs on all new molds.
Cast ware should not be touched until quite dry and then the spare at
the neck is carefully cut off, the seams scraped down and the whole
surface smoothed with fine sand paper and a soft cloth. Worn out linen
serves excellently for this purpose.
Cups and bowls, if molded, are made without spare at the top. In this
case great care must be taken to see that the edge is left clean and
smooth in the casting. The spare neck on a vase acts as a margin of
safety, as it is completely cut away in the finishing. If a piece has no
spare the edge must be left without blemish at the first.