Pottery, for Artists Craftsmen & Teachers Part 2

With great care and much taste pieces can be pinched and welded together into delightful forms, best shown in the refinements of the Cha Noya pottery of j.a.pan. This pinching must be done carefully and above all sympathetically. To those that know or feel the possibilities of the clay it will respond readily. Much more taste and judgement is required in the making of a really satisfactory pinched shape, than is needed in forming one with coils. Square, polyhedral, or irregular shapes other than circular may be _stuck up_ or pieced together in the following manner: Upon a table nail two strips of wood one quarter of an inch thick, ten or twelve inches apart. Sprinkle between the strips with flint or fine sand and batter out some soft clay thereon. Sc.r.a.pe the surplus clay off with a straight-edge and then roll the clay between the strips flat with a rolling pin. Upon this thin slab mark out the sides, base, top, etc., of the shape to be built. Run a thin knife round each shape, but do not cut quite down to the table. Cut along the inside edge of each strip to allow the whole slab to contract evenly and allow it to toughen slightly.

Carefully remove and reverse the slab and separate the pieces, being careful not to distort the shapes in the process.

The shape must be stuck up before the pieces become brittle or too stiff to bend. Roughen each opposing edge and moisten with slip, that is, clay mixed with water to the consistency of thick cream.

Press the edges firmly together and weld well each joint with soft clay.

(Fig. 5.) This operation should be most thorough, as any weak joint will inevitably open when fired. Lids should be cut slightly larger than the shapes they have to fit and sandpapered true when dry. Sharp edges and angles should be smoothed with the fingers before the shape gets hard.

[Ill.u.s.tration: FIG. 5--"STICKING UP" FLAT-SIDED POT.]

The chief pitfall to avoid in this process is a hard wooden look. The ductile plastic qualities of the clay should be remembered and such additions as feet or handles should emphasize this important point.

Finally, the careful sympathetic craftsman, with infinite patience, by utilizing all three methods, can build vessels of almost any shape,--square, round, elliptical, banded, strapped, bossed, fluted, and embellished with handles, spouts, and feet. Ancient and mediaeval pottery is rich in such forms.

[Ill.u.s.tration: TOOLS FOR CLAY WORK.]

CHAPTER IV

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.

[Ill.u.s.tration: FIG. 6--MANDREL, ROLLED IN PAPER.]

[Ill.u.s.tration: FIG. 7--"ONE PIECE" MOULD FOR BOWL--METHOD OF RELEASING ORIGINAL.]

The shape correctly drawn, a mandrel, a steel tapering to a point, the b.u.t.t 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 sh.e.l.l 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.

[Ill.u.s.tration: FIG. 8--A TWO-PIECE MOULD.]

[Ill.u.s.tration: 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 sh.e.l.l 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.

[Ill.u.s.tration: TURNING TOOL.]

[Ill.u.s.tration: FIG. 10--CHISEL POINTS.]

[Ill.u.s.tration: 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 quant.i.ty of shapes, cast their blocks, and turn them on a hired lathe, preferably run by power.

[Ill.u.s.tration: 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.

[Ill.u.s.tration: 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 a.s.sembled, 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 pa.s.sed 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 a.s.sembled, 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.

[Ill.u.s.tration: 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 sc.r.a.ped 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 gla.s.s 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.

[Ill.u.s.tration: FIG. 15--MOULD READY FOR SLIP.]

[Ill.u.s.tration: 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, sc.r.a.pe 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 sc.r.a.ped 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.

[Ill.u.s.tration: FIG. 17--SHAPE READY FOR Tr.i.m.m.i.n.g.]

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 repet.i.tion.

PRESSING