The Clay FAQ—Water-based Potter’s Clay and its use in Sculpture

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.

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 which resists drying out. Called WED (Waterbased Extended Drying) clay, it is a potter’s clay with agents added to retard the drying cycle, affording additional time to work.

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 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 sufficient plasticity to be used as-is. If it bends smoothly, and doesn’t seem to contain rocks or sand, then it might work for making sculpture without further refinement. Clay can also, of course, be purchased from a ceramic supply outlet. Find a local one, as shipping costs are expensive for bulk materials like this. In general, clay used for sculpting has ground-up fired clay, called “grog” added to it for stiffness and heat-shock resistance. While this makes the clay grainier, it also is easier to sculpt and fire. But some sculptors prefer the smoothness of porcelain, despite its relative lack of plasticity and quirkiness in firing.

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 letting them dry out thoroughly (surprisingly, it takes longer to recycle semi-dry clay) breaking them up with a hammer if neccessary, then 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 plaster slab, which sucks out the water until the clay can be kneaded or “wedged”. When it is moist, it can be shaped with one’s fingers or blunt modeling tools, it retains impressions well, and it releases well from plaster molds. Although clay is generally considered non-toxic, clay dust contains large amounts of free silica, which is very hazardous to inhale, causing a disease called “silicosis”. It is essential to avoid breathing this as much as possible, and to keep the workspace clean, without accumulations of dry clay underfoot. Also, glaze ingredients can be even more highly toxic, containing minerals like barium, lead, ground glass, cobalt and even uranium. Wearing an approved respirator when dealing with dry ingredients is essential.

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. If a smooth surface is desired, natural sponges are used damp, which easily round over slight convexities. 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 (for smoothing out seam lines etc on dry greenware, try one of those fine rectangular green nylon scouring pads used in washing up.) It is easy to make ones own tools for impressing or modeling clay, and natural objects like leaves, pinecones, and shells can be used as well.

Plaster and clay have a unique interaction with one another, the hardness and absorbancy of the plaster complementing the softness and stickness of the clay. As well as being the only material good for slipcasting into, plaster is particularly suited for pressing into clay, since its ability to suck water from the clay prevents it from sticking. It is very easy to make plaster molds from clay originals if the clay is fresh and isn’t allowed to dry out. Potters’ plaster (often called #1 pottery plaster) is best for working with clay; it is extra absorbant, and has a nice setting curve. But be sure to avoid getting tiny bits of plaster in the clay one intends to fire. The plaster will calcine in the firing and revert to its pre-crystallized state. When it recrystalizes by drawing water from the air, it expands slightly—enough to excavate a characteristic cone-shaped crater from the side of a piece of fired work months after it has emerged from the kiln. (Look for the tiny white dot at the bottom of the hole.) Press-molds were used by Pre-Colombian cultures to produce sculptural ware, but in this case their molds were made from once-fired or “bisque” clay, which works similarly—either clay is simply pressed into a one-sided mold with ones fingers and removed, or stamps are used to add texture to a flat slab, which is then peeled up (cloth or plastic can be a useful temporary reinforcement) and used in various ways. Sections of a piece can be made by draping slabs over convex “hump” molds or by allowing a clay slab to “slump,” through a hole cut into plywood for small spans, or supported like a hammock for large ones.

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. Sections can be added together much more easily now, because the clay will support itself without collapsing from its own weight. Edges to be joined must be “scored”-many tiny parallel and then crossing cuts made along the edge, and then coated with “slip”, in this case consisting of the same clay, mixed with water to a thick cream consistency. Then the sections can be carefully pressed together and cleaned up with a sponge.

When a piece is thoroughly dry—but not before if you don’t wish it to explode—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.

If one wishes to cast multiple pieces in clay and fire them, a multi-part mold is made in plaster, enclosing an empty volume. 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 difference between slip and clay is the presence of a “deflocculant.” This is a chemical agent that is added to clay powder, along with water, which reduces the plasticity of the clay so that it becomes pourable with the addition of less water than would otherwise be the case, and reduces the tendency to form lumps. It does this by reducing the attraction that the tiny flat plates have for one another.

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 without getting too thick to pour or forming lumps. Try for a consistency like heavy cream. This makes a slip that will dry on the inner surface of the mold quickly, and will release without warping, cracking or sticking. Do a small-scale test before mixing a large batch, in order to get the proportions right. Some clays won’t deflocculate properly at all, and this is good to find out. One can buy dry clay formulated especially to work as a casting slip. For small projects, it is often easiest to buy liquid slip already mixed at a ceramic supply.

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 by cutting 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.

If you want to try to fire in thicker sections than potters recommend, some people report success in mixing various materials into their clay bodies. These range from grog and perlite (a white volcanic material found in garden supplies) to combustible materials like sawdust, flour, chopped straw (used for bricks), and paper pulp. This last addition has caught on considerably, as it affords some ability to join pieces when bone-dry, and additional tensile strength in the green body. To make it, leave some newspapers in a bucket of water overnight, (toilet paper works even quicker) then stir it up with the paintmixer bit in a portable drill. Wedge the resulting glop into your clay. Some ceramics suppliers sell it pre-mixed (look for the bags with the green mold).

Your clay body may also need to be adjusted if the piece is intended for outdoor use in any but the sunniest of places. The freeze-thaw cycle that slowly dilapidates the hardest rocks does the same thing to ceramics, and will destroy some vulnerable bodies in a single season. High-fired ware is generally less susceptible to freeze-thaw damage than low-fired, since it’s vitrified, and resists absorbing water, which expands as it freezes. Test a fired sample by putting it in a freezer overnight. Then boil some water, and plunge the frozen piece in If it doesn’t crack, this is a good sign. If it did, you might consider adding talc, or some other tempering agent. To figure out how much of something like this to add to your clay, you do what potters call a “line-blend”. So you’d take, in this case, a big ball of clay and divide it into ten equal little balls. Then you weigh out what seems like too little, and what seem like just too much of the material you plan to add. These become number one and number ten. Then weigh out eight quantities which smoothly gradate between the first and the second, mix them in, scratch the ID number in the surface, fire, and test. This is done in glaze formulation as well, to determine the effects of additives.

The key to making armatures and interior supports 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). Be aware though, that pushing the drying process, especially at the outset, will encourage cracking. 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 also 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 directly to the bisque surface, which should be rinsed clean and free of dust, or over an “underglaze” composed of metal oxide colorants in a clay binder. These are available in pans like watercolors, tinted to approximate the final color. Also, one can get underglaze chalks and pencils. After the glaze firing, it is possible to use “overglazes”, also called “lusters” and “china paints”, to achieve various effects from suble color changes to metallic accents. These are fired to a much lower temperature than the glaze itself.

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 any exposed clay, while metal oxides in the glaze can reduce to the metallic state for irridescent and metallic effects. Pit firing, where the ware is literally buried in a pit, in the middle of a fire, also blackens the surface, while preserving the effects of burnishing.

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 bisque-ware, 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.

by Andrew Werby

Grateful acknowledgement to JD Kromkowski and Bernice Davies for their contributions to this FAQ.