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Q: Is pottery clay any good for sculpture?
A: There are various products that are called clay of one sort or another. The most basic is water-based or potter's clay. This is what has been used for millennia for making ceramic objects, and it has been the primary material in which sculptors have traditionally worked out their ideas. It is formed by the natural decomposition of rock into microscopic flat plates, which trap water between them, causing its characteristic "plasticity". It is often deposited by rivers in deep layers, from which it is extracted for use. Some clay is ready to use as dug, that found in other locations may need refining or mixing with other components to be useful.
If you find some clay; try this
test: roll it between your hands to form a cylinder approximately
half an inch in diameter and four inches long. Bend this double.
If the clay cracks, it is either too dry or it lacks
Potter's clay has various advantages
and disadvantages for sculpture. It is relatively inexpensive,
or free if you find a place to dig it up. It can be easily sculpted
with a minimum of tools, and can be smoothed with water, and
cleaning it off one's hands and equipment is easy. Scraps and
any unfired pieces can be reclaimed by soaking them in a bucket
of water until they soften, breaking up lumps with ones fingers
until a more or less even consistency is attained. The resulting
glop is then placed on a thick
Clay is the most malleable of materials. It is easily shaped by hand, and much beautiful work has been done without using any tools at all, just fingers. Forms can be built up from coils or rolled-out slabs, or the potter's wheel can be used to quickly produce a hollow unit for subsequent assembly. A wide range of tools can be used to shape it as well, from wooden sticks or metal shapes to loop-ended tools, "ribs" (rigid or flexible flat shapes), and numerous other implements, such as are commonly found in kitchens. It is easy to make ones own tools for impressing or modeling clay as well. Plaster is particularly suited for pressing into clay, since its absorbancy prevents it from sticking, but once-fired clay itself can also be used this way. If a smooth surface is desired, natural sponges are used damp, which easily round over slight convexities.
As it dries to the "leather-hard" stage, clay can be carved cleanly with knives and other sharp tools, and can be burnished until shiny with a rock or a metal tool. When it is thoroughly dry, it can be "fired" in a "kiln", which is an oven capable of temperatures from 1000 to 2000 degrees Fahrenheit. At the lower end of this temperature range, the "chemical water" is driven off, and the piece is called "bisque". At this point it is still porous, but will not dissolve if placed in water. At the higher end of the range, the piece will "vitrify", becoming denser and less porous. This point is different with earthenware and stoneware clays. Earthenware clays should be bisque-fired at a lower temperature than stoneware, and should never be taken to stoneware temperatures, as they will liquify. While stoneware is more durable, some sculptors like the wider range of colors available in the earthenware range, and the utility bills are lower as well.
Some sculptors use clay as a
permanent medium, firing their work in a kiln. Others use it
as an intermediary step, making a clay model from which a mold
is taken. The mold can be used to make sculpture in plaster,
concrete, bronze, or other materials. For this application a
clay has been developed
If one wishes to cast multiple pieces in clay and fire them, a multi-part mold is made in plaster. Then a clay "slip" is poured in, left to build up a layer in the mold cavity, and poured out, leaving a hollow shell which is left to dry some more, then removed. The resultant forms can be assembled to make quite complex composite objects, like the famous figurines of Meissen and Dresden.
Slip is clay in a liquid form.
Clay, as I mentioned, is composed of microscopic plates, which
cling to each other's flat surfaces, giving it plasticity- the
quality that makes it hang together as it is being worked.
The deflocculants most commonly
used are sodium silicate (waterglass) and sodium carbonate (soda
ash), often in combination. Just a little is needed: one third
of one percent of the weight of the dry ingredients. Using too
much will accelerate the deterioration of your plaster molds.
Mix the deflocculant into the water first, then add the clay.
The water will weigh between a third and a half as much as the
clay that goes into it. Mix dry clay powder into the water/deflocculant
until it won't accept any more
Another indirect method using a clay original is the "waste mold" technique. One makes a sculpture in clay, covers it in plaster, digs out the clay, and then one pours in more plaster. Use a couple of applications of liquid dish detergent as a release, allowing it to dry between coats, and put some colorant in the plaster one pours in. This will help when (destructively) removing the outer plaster shell, so one can see where the mold stops and the casting begins. Originals made in water-based clay can be molded with 2-component synthetic rubber compounds, but it helps to seal the surface first with shellac followed up with wax mold release.
If one wishes to fire a piece of clay sculpture of any size, it must be hollow, or it will tend to explode in the kiln. This will also occur if there is any residual moisture left in the clay. I find it's easier to build a clay form hollow in the first place than to attempt to hollow it out afterwards, but both methods will work. It is possible to build an interior support system- using clay slabs joined together and weakened sufficiently bycutting out voids in them- so that they will crack before the skin of clay that covers them. Another approach would be to make a combustible support structure using excelsior or another somewhat flexible material- what you don't want to do is trap anything rigid, that will not move when the clay shrinks, since this will cause cracking.
The key to making armatures for pottery figures, etc, is accounting for the shrinkage of the clay during hardening. For small pieces, a method I've used is to build a "skeleton" in heavy aluminum wire, then wrapping newspaper around the wire somewhat loosely, so the clay can contract without cracking. When the figure is set to leather-hard, cut the piece longitudinally to remove the armature, then score and use slip to rejoin the pieces. Alternatively, one can build the skeleton out of a light wood like pine (don't use anything that makes toxic fumes when burned) and leave it in through the firing process, when it burns out. These supports may also be removed before firing if this is practical. For larger pieces, it is best to work in sections, because having further to shrink, the piece will tear itself apart trying to contract against an unyielding internal structure.
As far as the drying process is concerned, one can accelerate this by keeping the piece in a well-heated and ventilated space for a week or so before firing. During the winter, especially in damp places, a thick clay piece will have problems drying thoroughly without some kind of help (like ventilation and heat). One must, of course, make sure the air inside a hollow sculpture is vented to the outside, since a totally enclosed volume of air will expand when heated and crack the piece. It helps to leave the kiln "candling" (at the lowest sustainable heat) for a couple of days before turning it up, to get rid of trace moisture; this helps the pieces inside to fire without exploding.
Once fired to bisque, the clay
objects can receive surface treatments of various kinds. It is
possible to use "engobe" (colored clay slip) before
firing, but most other techniques work better on once-fired pieces.
Potters have worked out numerous formulae for glazes, which are
combinations of ground-up minerals applied to the bisqueware
by dipping, brushing or spraying and which form a glass coating
on the surface of the clay when fired to the correct temperature.
These may be bought ready-mixed at ceramic supply outlets, or
they can be concocted from scratch. In either case, it is advisable
to test the glaze on a sample of the clay it will be used with
before using it on a large amount of work. Glaze can be applied
A thick glaze can tend to obscure
fine details in sculptural objects, but it also can hide minor
surface eccentricities. If the detail, especially fine negative
texture, is important, then the glaze is best applied thin. If
it is to be accentuated, then stains or underglazes can be applied
and then sponged off the high spots before being covered by the
glaze. Areas of the surface can be protected with a "wax
resist", a waxy solution that, once dry, resists the further
application of glaze. Various firing techniques will also influence
the final effect. Raku, where red-hot objects are removed from
a kiln (frontloader only) and confined in a sealed space with
combustible material to create a reducing atmosphere, blackens
If the ceramic objects in question are not to be used for food, don't need to sit outdoors, and archivalness is not a concern, just about any paint can be used on them. Any of these constraints limit ones options. On bisqueware, the main problem is the absorbancy of the clay body, which will suck the color and solvents out of the brush and leave craters on the surface, also impairing the ability for the paint to form a film. Try sealing the surface with progressively less dilute coats of (waterbase) gesso or use shellac. Once the surface has been sealed, one can paint with just about any paint - I've seen oil paints used very effectively. Alternatively, to darken the surface somewhat and give a little shine, try rubbing on some shoepolish, then rubbing off the excess. If the piece must survive outdoors; mix some cement colors with some portland cement and water, then apply as a wash to your (unprimed) bisque surface. Any waxes or sealers will have to be renewed periodically if the piece is left outdoors.
Some clay resources follow:
Andrew Werby, born in 1952, started
making ceramic sculpture while in elementary school. At the University
of California at Berkeley he started becoming involved in the
technique of lost-wax bronze casting, which Peter Voulkos and
others were reviving as a skill for artists to master directly,