Finding Local Clays, Preparing Clay, and Slip-Casting
( Originally Published 1937 )
MAKING POTTERY IS one of the most fascinating pursuits for the countryman. There is adventure in seeking clays; in creating with them, and in developing glazes. There is suspense in the firing; and thrill in opening the kiln afterward; and pride and satisfaction in the useful and beautiful products. Vases, baking dishes, soup bowls, pitchers, plates, beer mugs, tiles, book-ends and candlesticks are among the subjects possible for the amateur. It can become a family project, for pottery making enlists the enthusiasm of young and old alike. I have seen three year olds and grandmothers participating in this art with equal enjoyment. You can buy pottery clays ready for use. But it is fun to seek local clays or to develop a clay body of your own.
In most states of the union a published geological survey is obtainable which offers many hints to the seeker after local clays. Without this, look by the banks of streams, railway or road cuttings, cellar and other excavations. Clay as dug is usually too wet to be plastic when worked up, so a small quantity should be spread out on a board to dry a little, then kneaded to a plastic state. If a strip rolled out the size of a lead pencil, can be coiled around the finger and uncoiled again without. cracking, the clay will be found reasonably workable for pottery. Many local clays, however, will not do this and need to be enriched by the additions of ball clay or bentonite.
1. Spread the clay out and expose it. to the weather. The frost, sun and wind have great value in opening up a clay.
2. Allow it to dry; break it up by pounding and put it through a coarse sieve (1/4" mesh). Take care not to breathe the clay dust, as it is harmful to the lungs.
3. Shake it into hot water, at the same time adding any other necessary ingredients. Let soak over night or longer. Then mix thoroughly to a thick creamy slip. Do not be afraid to put your hands into this slip and feel it for lumps.
4. With a fibre sink brush, rub it through a sieve (16 to 40 wires to the inch, as required) made by fastening the bronze wire screen, or silk bolting cloth, to a wooden frame.
5. Dry this slip until it is of a plastic consistency, by hanging it in flour or sugar bags, in a warm place open to air currents; leaving it exposed to the air in wooden boxes or tubs; or pouring it into plaster drying basins. The last is the quickest method.
6. Knead the clay until it is soft, smooth and free from lumps. Then "wedge" it: take a fair-sized lump of clay, pass it over a taut wire so that it is cut in half, then dash first one half and then the other forcibly down on the wedging bench. Gather up this mass of clay, knock it into a lump and repeat 'the process again and again until the clay, being cut through with the wire, shows no air holes. Take care to have your clay sufficiently moist.
7. Put the "wedged" lumps of clay in a damp storage cupboard, made of a box, tub, garbage can or crock. Pile the clay on a board supported on a couple of bricks. Fill water to the tops of the bricks. Then wet heavy cloths and lay them over the clay with the ends hanging into the water. The container should be covered and the water replenished frequently. The clay improves greatly with age; short clays may become quite workable after seasoning two or three months.
The foregoing is the ideal routine in preparing clay. Eliminate one or more of these steps when conditions warrant it. Some clays, for example, are so dean that it is unnecessary to screen them. It is sometimes possible to eliminate wedging, where work is not fired highly, or is unimportant. Where careful work is to be done and much time spent on it, all steps should be followed. Where the firing approaches the vitrifying point of the clay (i.e., the point at which it begins to become dense, non-porous, and glassy) it is absolutely necessary to have the clay perfectly mixed in the slip stage, and perfectly wedged.
Preparing Clays Without Washing
Where it is impracticable or unnecessary to wash clay (i.e., make slip of it and strain it) or where it is desirable to add grog, flint or other grit to ball clays or local clays in the plastic state, then follow this course:
1. Throw the dry lumps of clay into water, hot water if possible. Allow this to soak a day or so and siphon off the clear water. Then let it dry until it is plastic.
2. Weigh out the correct amount of grit to go with the clay, and throw it into water. When it has settled siphon off the clear water and allow it to dry somewhat.
3. Spread part of the damp grit on a clean floor (and see that the whole floor is carefully swept so no dirt will get into the clay)- Now lay your plastic clay on the grit about four or six inches thick and spread more grit on your clay. Repeat until the pile is about one and a half feet high.
4. If in winter wear rough high-cut boots, if in summer use bare feet and go ahead and tread the clay down, piling it up again when it spreads out until it is perfectly mixed, without any lumps. Carefully scrape it off the floor, store it away and wash the floor clean again.
This treading of clay should be a social gathering of the young craftsman. There should be music—a fiddle, a guitar or an accordion. There should be two armed with wooden shovels to rush in and pile the clay up when it has been jumped down by the treading partners.
It is best to tread clay on a warm evening in summer. After a day of some gentle occupation such as designing, modeling or painting, after the quiet physical inactivity of the day's work it is good to jump, to dance to music, to feel the blood pounding through one's arteries, to get hot, sweaty, and dirty with clay up to one's knees and elbows. Then to race swiftly across the fields to wash and splash in the warm river under the benevolent radiance of the full moon—then singing on the bank and softly stealing home again.
PROPORTIONING THE INGREDIENTS
A craftsman developing a pottery body should have a knowledge of the properties of the various ingredients. They are classified into plastics, including ball-clays and china clays, and non-plastics, including ground flint (more properly ground quartz), ground feldspar, limestone, talc, etc. Drying shrink-age in a body is increased by the plastics and de-creased by the non-plastics.
Ball clays approach the theoretical formula for ideal clay Al203, 2SiO2, 2H20, but containing fusible impurities, they vitrify at lower temperatures than china clays. Ball-clays are very plastic.
China clays or kaolins are the nearest to the formula above. They are used to give whiteness to a pottery body and to raise its firing point. Kaolin is the chief plastic ingredient in porcelain, but because its plasticity is much inferior to that of ball-clays, a porcelain body is nearly always short and difficult to handle on the wheel.
Flint (SiO2) is used to raise the firing point of a body, to open it up where it is too "fat" or "tight," whiten it, and increase its coefficient of expansion. The addition of flint to a body, while actually de-creasing its plasticity, may make it easier to work by "opening it up." Where a glaze cracks or "crazes" because it shrinks more than the body on which it is applied, the addition of flint to the body will make the body shrink more (this refers to the shrinkage that takes place on cooling, not the drying or firing shrinkage), and the glaze will fit it better.
Feldspar (K20, 2Al203, 6SiO2) like flint, opens up a body in the raw clay. It increases the firing shrinkage, because it melts and runs down between the particles of flint and kaolin cementing them together as a glassy flux. This vitrifying effect is complete only at high temperatures.
Whiting (CaCO3) is ground marble or limestone and is frequently used up to about 10% to whiten a body and make it dense. It lowers the firing point of the body.
Talc is used to toughen bodies and lower their firing points. It may tend, however, to shorten the firing range.
Bentonite is a fine colloidal clay which swells up to several times its own volume when added to water. About 7% of the weight of water will make a very stiff jelly. It is useful to add to clays that are "short" to make them more plastic.
Sodium silicate and sodium carbonate are added in minute quantities to the water with which slip is made for casting modeled figures and hollow ware in plaster moulds. They make the water suspend a larger proportion of clay than it would otherwise. Although they are sometimes used up to 1/10th of one per cent of the weight of the clay, smaller amounts are usually enough. A dozen drops of waterglass to a gallon of slip is sufficient for most work. If used to excess these deflocculents have just the opposite effect and cause the slip to become very stiff.
In working out your recipe for a clay body, build up the proportions according to the qualities you require. For a starting point you might use something like this, which is a recipe I worked out four years ago and which has proved quite satisfactory in use. Its slight tendency to shortness is largely overcome by seasoning for a few weeks. Bentonite helps this also. The color is a pale buff. May be fired up to about Cone 2 or 3 depending on thickness of pieces. Thin pieces vitrify at this temperature.
50 lbs. Hanover Clay (United Clay Mines Corp., Trenton, N. J.)
Another good body, devised to make use of a local blue clay. This body fires a light salmon red, between coes 08 and 03.
Sift the bentonite together with the flint, and add them to the slip after the clays have been thoroughly broken up into a smooth cream with the water.
50 lbs. Hanover clay
If you have no bentonite and wish to use this slip for casting purposes, add two tablespoonfuls of waterglass to the water before soaking the clay in it.
Make plaster moulds of the pots or the figures, in two or more pieces, according to the shape. De-sign the pieces to draw off the figure without pulling the clay. Make the moulds about 2" thick. Put no waterglass, soap or grease in the mould, and have it clean, and dry. Tie it up and pour the slip into it carefully, to avoid bubbles, keeping the neck of the mould filled over the top for about 10 0 minutes. Then test the thickness of the cast by scraping against the edge. If it is thick enough, pour out the liquid clay in the middle and leave it propped on battens to drain, until the cast is firm. Removing the mould requires a good deal of care and practice. Note in which direction the mould should be drawn from the cast, and which piece should be removed first, before you tie the mould to fill it.
Touch up the seam after the cast has stiffened up, and if you wish for an even color (almost an impossibility in dark colored clays), model over the whole surface lightly, with a soft cloth.