CHAPTER VII
HOW TO MAKE A PLASTER MOULD
In building pieces of pottery whose walls have a decided flare, a plaster mould will be found most useful. Its sides support the coils of clay and enable the potter to form his piece much more rapidly and surely than he could by eye, or even with the cardboard outline.
It should, however, be used with judgment, rather as an aid in beginning a piece which is afterward finished by eye than as a mould in which pieces are duplicated. Turning out many pieces exactly alike savours of commercialism and does not develop the potter’s individuality. Several bowl-shaped moulds, varied in form and flare, if used as suggested, will be a great assistance to the potter. They are made quite easily, as follows:
_A Bowl-shaped Mould of Plaster_
=Materials required=:
6 or 8 pounds of clay, well mixed,
A bowl, or mould in bowl shape,
A dishpan full of plaster of paris,
A dishpan of water,
A bag of ground flint,
A piece of oilcloth 8 inches wide by 1¼ yards long,
2 clothes pins,
A stout cord or rope,
A wooden wedge,
A few drops of olive oil,
The wooden modelling tools,
The rectangular tool of sheet steel,
A large hoe-shaped tool,
A large kitchen spoon.
Having chosen the bowl that you wish to copy, dust it with a cotton cloth in which ground flint is tied, and roll out a piece of clay two or more inches larger in diameter than the bottom of the bowl, and about an inch thick. Press this carefully into the bottom, making sure that it touches everywhere. Now roll out several other pieces of the same thickness, large enough to reach from the upper edge of the bottom piece to the rim of the bowl, and four or five inches wide. The upper edge of the bottom piece having been cut even and wet with slip, these pieces are pressed down firmly and joined to it with the nail stroke before mentioned. Great care is also taken to press these pieces against the sides of the bowl. When the bowl is lined in this way with a coating of clay an inch or more in thickness at every point close against its walls, the clay is smoothed evenly on top, on a line with the rim of the bowl (using the rectangular tool of sheet steel) and set outside to harden.
When somewhat firm, the bowl is filled in with clay until only a space the size of a man’s hand is left in the middle. It is then set away, this time over night, to harden.
The next day the clay will have dried and shrunk sufficiently to enable the potter to slip it out of the mould. Any cracks or hollows that may be found on the outside are wet with slip and filled in with clay of the consistency of the piece. After the clay is quite dry, the form of the mould should be made perfectly true by hand (using the oval sheet-steel tools) or on the wheel. If the latter method is chosen, place the clay mould bottom side up on the wheel, centre it (according to the directions in Chapter III), and, taking a large hoe-shaped tool, hold it firmly, bracing the hand still better with a stick laid across from a plaster mould, or other convenient object, on the table, to one’s lap. Having set the wheel in motion, hold the dull point of the tool so that it just touches the wall of the mould, near the wheel. Move the point up very slightly with each revolution of the wheel—this will trim and perfect the sides. The bottom is made even in the same way. Now wash the wheel outside of the mould carefully and oil it with olive oil. Take a piece of heavy oilcloth, about eight inches wide and long enough to reach around the wheel, overlapping about a foot. Placing it with the right side in, draw it tightly and fasten with clothes pins (see Fig. 33). Next tie a rope or stout cord around the oilcloth, about on a level with the wheel, and, to make it more secure, wedge it with a piece of wood. Roll strips of clay about the diameter of a lead pencil, and stop the cracks where the oilcloth overlaps, also between the oilcloth and the wheel, very carefully, so as not to touch the clay mould. Be careful, from now on, not to move the wheel until the mould is made. Now mix your plaster of paris, as follows: Have an empty basin or dishpan, large enough to hold the quantity necessary for the mould—you will learn to judge this pretty well by eye. Put in as much water as you will need, and sift gradually into it, by the handfuls, the dry plaster, pressing out all lumps; in this way the water will reach every particle. When there is a small island of plaster, about an inch above the surface of the water, there will be enough. Let the plaster get thoroughly saturated by the water, as it will in a few minutes; then mix with the hands or a large spoon until it is the consistency of thick cream. Pour it gradually around and over the clay mould, not all in one place, until it is about an inch and a half or two inches above the bottom of it. Let it set for an hour or more until it seems perfectly hard. The oilcloth is then taken off, and with the dull point of the hoe-shaped tool the bottom is trimmed true on the wheel, in the same way as the wheel-made pottery is finished. The sides should also be smoothed and made even with the straight-edged sheet-steel tool. It then looks like a great frosted cake. When the plaster is hard and set, the mould is taken from the wheel and reversed, so that the clay may be removed. To do this, dig out the inside of the clay mould with a large sheet-steel tool, taking care not to come near the plaster, which would be injured by the steel. The shell of clay remaining can be easily lifted out with the fingers.
One who does not own a wheel can make a mould by setting the clay bowl, bottom up, in the centre of a small shallow bread or dish pan, which must then be well oiled on its inner surface. The plaster of paris is mixed and poured around and over the clay mould, as already described.
If one uses a great deal of clay, plaster basins may be made for drying out the superfluous water from the clay after it has been mixed. These are moulded in the form shown in Fig. 34, the straight-sided circular cavity in the centre being about three inches deep, and the whole slab perhaps sixteen inches square. The basin part, like the bowl-shaped mould just described, is first formed in solid clay, and the mould is made in the same manner.
Plaster slabs, both round and square, to hold the pieces of pottery while they are being made and dried, may also be moulded.
_Rose-Bowl Started in a Mould_
=Materials required=:
A bowl-shaped mould of plaster, 3½ inches in diameter at the bottom and 9½ inches at the top,
About 3½ pounds of clay,
A bag of ground flint,
The wooden modelling tools,
The oval tools of sheet steel,
The sharp-pointed steel tool,
A bowl of slip,
A small sponge,
A rolling-pin.
A charming rose-bowl may be built up in the plaster mould described in this chapter. About three and a half pounds of well-worked clay will be needed, and the usual tools.
A small lump of clay is first patted out with the hands on a table or board, then rolled smooth with a rolling-pin until it is three-eighths of an inch thick and about six inches across. This is laid into the bottom of the mould, which has previously been dusted with ground flint tied in a cotton cloth. The clay is pressed lightly, but carefully, against the bottom and sides, and then made even at its upper edge with a wooden tool. Strokes of the wooden modelling tool, cutting this upper edge criss-cross, and a touch of slip, prepare it for the first roll of clay, which is made and attached as described in Chapter II. These coils need not be as thick as those used in making the first pieces. As the worker gains experience, he can make the walls of his pieces much lighter than at first, and still keep them strong. Subsequent coils are joined in the same way, taking care to press each against the wall of the mould, as well as upon the coil beneath. When the sides of the bowl are covered, a coil is attached above the edge of the mould. This should be almost vertical, instead of flaring, and a second coil (which is joined after the first has stiffened out-of-doors for twenty minutes) is brought in slightly toward the centre. The bowl is then left over night, when it will be quite dry and have shrunk sufficiently to slip easily out of the mould. It is turned bottom up on a table, and the cracks between the coils are wet with slip and carefully filled in with clay of the consistency of the bowl. After it has been set away to harden for a few hours, it is made smooth and even with the oval tools of sheet steel as described in previous chapters. In trimming the walls to an even thickness, they may be made comparatively thin—a little less than a quarter of an inch. Next the bottom is finished and the potter’s mark made. The edge, after it has been bevelled by eye, is perfected on the ground-glass slab.
The piece is now ready for its decoration. This is the design shown in Fig. 35. The circumference of the top of the bowl is divided into fifths, and marks are made in pencil half an inch below the rim. Just below each of these marks the design is drawn, placing it so that if a line were drawn straight down from the pencil mark, one of the oval figures would be on each side of it. The centre of the design is pierced, as indicated in Fig. 35, by the method described in Chapter IV., and a deep incised line surrounds it.
A pale green mat-glaze (see Chapter V.) completes the bowl.
The Making of a Tile