MOULDING, CASTING, AND PRESSING
"Our soundes is good, Our shapes is neat,
Its Davis cast us so compleat."
--From an old bell at Stoke Rivers.
The casting process, employed so extensively in commercial work, is in its
essence mechanical and therefore can never have the spontaneity or
character of thrown work. To-day when the thrower and turner with hand and
eye trained for good shapes is rare indeed, it is often the only method by
which the student can obtain large shapes of high finish for painting or
glazing in transparent colours.
In the process the shape is first designed and carefully drawn on paper,
allowance being made for the shrinkage of the finished pot which may be as
much as 1 in 6 with some kinds of slip. It should be refined in profile
with no returns that may bind or hold in the mould when drying, yet it is
obvious that simple shapes that can be built or thrown by the beginner are
not suitable for casting.
[Illustration: FIG. 6--MANDREL, ROLLED IN PAPER.]
[Illustration: FIG. 7--"ONE PIECE" MOULD FOR BOWL--METHOD OF RELEASING
ORIGINAL.]
The shape correctly drawn, a mandrel, a steel tapering to a point, the
butt wormed to screw on a lathe, and long enough to give some play at
either end, is rolled in stout paper, gummed at the edges, as in Fig. 6.
This is removed and allowed to set, thus giving a paper shell just fitting
the mandrel. Trim this square at the base and stick it upright by means
of a piece of clay on a well-oiled portion of a table. Around this as a
centre is fitted a roll of linoleum, oiled inside and secured with string,
with all its joints caulked with clay. The inside diameter of this
cylinder should be about 1/2" larger than the greatest diameter of the
drawn shape. Enough superfine plaster to fill this is now mixed. To do
this shake plaster by handfuls into a bowl of water until it appears to
refuse more, pour off any surplus water, and stir with a wooden ladle or
the hand, avoiding air bubbles.
[Illustration: FIG. 8--A TWO-PIECE MOULD.]
[Illustration: FIG. 9--POURING IN PLASTER.]
A little practice in casting plaster bats will give the experience
necessary for mixing plaster. When well mixed and a slight thickening is
perceptible, it is poured very carefully into the cylinder, the paper
shell being kept upright in the centre. (Fig. 9.) In about 20 minutes the
surface of the plaster will feel warm. It is now set and the linoleum is
removed. The plaster cylinder, when dry, is fitted on the mandrel, and
this screwed to the lathe head. Two or three chisels are now required.
(Fig. 10.) The rest on the lathe is clamped in a convenient position and a
cutting chisel held as shown (Fig. 11), cutting edge up. The cylinder is
revolved briskly as indicated by arrow, and the shape is roughly hollowed
out. Proceed gradually until the shape begins to emerge, taking care not
to cut too deeply. (Fig. 12.) Towards the finish use the calipers
frequently to check the measurements. At top and bottom about 1/2" waste
is left (Fig. 13 A), turned straight, the actual line of top and base
being slightly grooved in the plaster. The form may be pretty accurately
finished with the chisels and then nicely smoothed with sandpaper. For
this process the rotary movement is reversed.
[Illustration: TURNING TOOL.]
[Illustration: FIG. 10--CHISEL POINTS.]
[Illustration: FIG. 11--POSITION OF TOOL.]
The shape is now removed from the lathe and is ready for moulding.
With a kick wheel having a removable head the mandrel can be screwed on
and the block turned down in an upright position. The difficulty of
steadying the tool renders this method somewhat unsuitable for this
process, where perfect symmetry and a high finish are required. The
plaster should not be so dense for turning on the wheel or the kicking is
apt to become very arduous and this tends to dislocate the set of the
spindle. One of the best of ways is to draw a quantity of shapes, cast
their blocks, and turn them on a hired lathe, preferably run by power.
[Illustration: FIG. 12--PARTLY TURNED BLOCK ON LATHE.]
The finished shape is now well lathered with parting or stopping. This is
made by boiling 1 lb. soft soap, 1/4 lb. Russian tallow, and a small piece
of soda. Another stopping is made by dissolving 1 lb. soft soap in 1 qt.
water and stirring in 1/4 pt. paraffin oil. Several coatings may be
necessary to impart a waxy surface. When no longer absorbent, it is dried
and slightly polished with a bit of cotton waste. At the finish the form
should be clean, smooth, glossy, and non-absorbent.
With simple shapes, as in Figs. 7 and 8, a one-piece or a two-piece mould
can be made, and here the bottom waste is not necessary, but with any
return or foot a three-piece mould will be required. The waxed shape is
now divided perpendicularly exactly in half, by a pencil line. (A, Fig.
13.) It is then laid on its side and bedded in clay up to the pencil
lines, the clay being sloped slightly down from the marks. (F, Fig. 13.)
Box in now with well-soaped boards tight against top and bottom but
allowing about 1-1/2" at sides. Wipe the shape over with waste dipped in
olive oil but leave no surplus oil on the surface. Plaster well mixed as
before is poured in until about 1-1/2" to 2" above the greatest projection
of the shape, great care being taken to avoid or dislodge air bubbles. (B,
Fig. 13.)
When the plaster is set but still warm, the shape is removed and the side
of the plaster that rested on the clay trimmed flat and several joggles or
natches are made. (C, Fig. 13.) The shape is now replaced exactly as cast
and the new surfaces treated with parting and the whole slightly filmed
with oil as before. Great care must be used, for any oil on the actual
surface of the mould spoils the suction of the plaster at that spot. Box
in and then cast just as before. This gives two halves with waste top and
bottom.
[Illustration: FIG. 13--A: METHOD OF DIVIDING. B: MOULDING 1^{ST} HALF.
C: MOULDING 2^{ND} HALF. D: A METHOD OF SECURING BOARDS. E: MOULDING BASE.
F: MOULDING 1^{ST} HALF.]
The shape is now placed on the lathe and the bottom waste turned off, the
base of the shape being slightly hollowed. The creator having arrived so
far successfully may now unbend and scratch his mark on this new surface
before well waxing it.
Dowels are cut in the waste of the two halves as shown, the fresh parts
soaped, all fitted together and slightly oiled, then boxed in as in D and
E, Fig. 13. Plaster about 2" thick at the thinnest part is poured on and
the mould is complete. When set, the shape is removed and the three parts
trimmed on all the outside edges. (Fig. 14.) The three pieces are
assembled, firmly tied up to prevent warping, and thoroughly dried.
If preferred, the mould can be made cylindrical instead of square. This
will give a more even suction to the slip and may be worth the extra
trouble. For casting purposes a refractory clay containing a good
percentage of China clay, maturing at about 03-01 but remaining perfectly
white and porous, will be required.
Slip for casting is made thus: Clay, picked or broken into small pieces,
is thrown into a bucket of warm water well slubbed up by hand and passed
through a fine sieve (No. 80) with the aid of a stout brush and thinned
to the consistency of thick cream. This should be matured for some days,
frequently stirred, and again sieved before using. For very small or
fragile shapes, a finer sieve (120) is advisable. The mould, quite dry and
clean, is now slightly moistened with a scrupulously clean sponge and
water, the parts assembled, corded, and firmly wedged, leaving the top
free as in Fig. 15. The slip, thoroughly stirred, is poured in very gently
to avoid bubbles. With awkward moulds, a tube or funnel should be used to
prevent splashing. If the mould be placed on a whirler and turned to and
fro, it will prevent the heavier matter in the slip settling too quickly.
[Illustration: FIG. 14--THE THREE PARTS OF THE MOULD.]
As the slip sinks in the mould, the subsidence being due to the absorption
of the water by the plaster, the mould should be continually filled up.
After a few minutes the mouth is scraped free to test the deposit. When
this is thick enough, varying, of course, with the size of the shape, the
slip is poured out into another bucket. An unorthodox but often useful
trick for strengthening long necks is to slide a piece of glass over the
mouth and reverse the mould for a minute or so. The neck full of slip thus
allows a slight extra deposit on the part that most needs it when we come
to finishing off the lip.
[Illustration: FIG. 15--MOULD READY FOR SLIP.]
[Illustration: FIG. 16--FREEING TOP.]
Let the mould drain a little over the slip bucket and then reverse to dry
slowly. When the wet look has disappeared from the surface of the slip,
scrape the top free and run a knife around to prevent sticking as the
shape contracts. (Fig. 16.) In a few hours it will be dry enough to permit
of the sides being eased off and the shape left to dry on the base. (Fig.
17.) When tough enough to handle with safety, the waste and cast lines are
trimmed and finished off, any air bubbles or holes broken down and filled
with clay scraped from the waste or base. If this finishing is left until
the shape be dry, it is impossible to hide such defects. The greatest care
must be exercised in handling cast shapes, as they are exceptionally
fragile.
[Illustration: FIG. 17--SHAPE READY FOR TRIMMING.]
When quite dry, the whole form should be carefully gone over with a very
fine sandpaper. A superfine surface should be imparted by rubbing with the
hands. When using transparent glazes, as with under-glaze painting, it is
essential that all scratches be removed, and especially must all sharp
edges be eliminated on neck or shoulder, for the glaze running away from
these places imparts a hideously cheap look to what otherwise may be a
fine shape. All these points having received attention, the date is
scratched on the bottom of the shape and it is now ready to biscuit.
Generally speaking, it will be found that slip the consistency of cream is
right for casting, possibly thicker for big open shapes, and after the
right proportion is settled it is as well to test what it weighs to the
pint.
As will be readily seen, this process, whilst open to many objections,
lends itself to shapes that are refined and delicate and to those that
have flutings or raised ornament. Such decorations, or the spouts of jugs,
may be modelled in wax on the plaster shape before casting and appear in
reverse on the mould. Designs may be scratched on the mould or shape and
show as a delicate tracery beneath the glaze. All these things, however,
add to the difficulty of casting and should be approached by degrees and
with restraint. For in unskilled hands the process lends itself to
soulless and mechanical repetition.
PRESSING
Moulds having moderately wide mouths enable the potter to press his shapes
instead of cast them. This method in expert hands is even quicker than
casting and has the advantage of imparting a sturdier look to large
shapes. Pressing is also resorted to for those shapes to which it would be
difficult for the slip to obtain free access.
[Illustration: FIG. 18--ROLLING OUT CLAY.]
For the ordinary three-piece mould the procedure would be thus: The clay,
well wedged and quite plastic, is rolled out as described in Jigger and
Jolley work, to a suitable thickness. (Fig. 18.) Butter cloth or fine
linen will do instead of leather to roll the clay on. The insides of the
three parts of the mould are sponged and pieces of the thin rolled clay
roughly cut to fit them. These pieces are now fitted and well applied to
the three parts by dabbing with the damp sponge. A soft close-textured
sponge, or a soft felt dabber, is best for this operation. When closely
setting, the edges are trimmed and given a slight bevel. The top is cut
straight. Then the mould is assembled and firmly tied. Some of the waste
clay is rolled into thin _ropes_. With the bevelled edges slightly
moistened, these ropes are firmly wedged into the two side junctions and
round the base. Where the mouth is large enough for the insertion of the
hand this is not a difficult operation. If it be narrow, the two halves of
the mould may be tied up and the joints welded together before they are
assembled on the base. A coil of clay can then be placed on the edge of
the base just clear of the two sides which are now fitted over and tied
up. Then a stick sponge is used to join up the base to the sides. (Fig.
19.) After a little while the shape is fit to be removed and is finished
in the usual way.
Nothing can rival large thrown shapes for vigour or variety, but
unfortunately they are not always within reach of even the good craftsman.
Then this method offers the least objectionable substitute for them and in
clever hands is capable of many fine results.
The following method is used to mould handles or simple applied ornament.
Handles, feet, masks, etc., are usually pressed and stuck on the dry shape
with slip. To mould them some skill is necessary if the press is to be
quite accurate and free from twist or ugly seams.
[Illustration: FIG. 19--SECTION OF A MOULD. STICK-SPONGE WELDING BASE TO
SIDES.]
One way, when the handle or foot is symmetrical, is to cut the model
exactly in half. This must be done when the model is tough enough to
handle without bending or distorting it. One half is laid cut side down
upon a sheet of glass, and surrounded at a convenient distance with clay
walls. Plaster is now poured on to form one half of the mould, and
allowed to set. It is then removed and the smooth surface joggled and
claywashed (brushed over with claywater). The other half is then very
carefully applied to the half still embedded in the mould, the walls built
round and the other part of the mould cast. Then all is trimmed up and a
groove run round the form as shown. (Fig. 20.) For pressing, the form is
well filled with clay and the two halves of the mould strongly pressed
together. Any surplus clay will squeeze into the groove and when tough
enough to remove the whole is "fettled" and finished before drying and
sticking up.
[Illustration: FIG. 20--HALF OF MOULD FOR PRESSING HANDLE.]
With care and practice this method is possible: Build walls and pour in
enough plaster to form one half of the mould. Before it stiffens, very
carefully press in the handle or ornament just up to the halfway line and
allow to set. Joggle, claywash, and cast the second half. Finally, when
the object is of any size, clay walls may be used as described in the
chapter on Figurines.