Clays for Sculpture

Q: I want to try doing sculpture. What kind of clay should I use?

A: There are various products that are called clay of one sort or another. The most basic is water-based potter’s clay. This is what has been used for millennia for making ceramic objects. It is formed by the 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 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.

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 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. As it dries to the “leather-hard” stage, it can be carved cleanly with knives and other sharp tools, and can be burnished until shiny. When it is thoroughly dry, it can be “fired” in a “kiln”, which is an oven capable of temperatures from 1000° to 2000° 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. 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.

Although it is a basic and useful material, potter’s clay does have its disadvantages. It shrinks as it dries, so unyielding armatures buried inside will cause cracks if the piece dries out. To avoid this, sculptors keep clay models damp by draping them with moist cloth and plastic. When this clay is dry, it cannot be manipulated further. It cannot be fired in solid lumps very successfully; pieces must be hollow if they aren’t to explode in the kiln. The dust from potter’s clay also contains silica, which causes silicosis, a nasty lung disease, with prolonged exposure.

Aware of these disadvantages, sculptors during the Renaissance came up with a new product: oil-based, or “plastiline” clay. Replacing the water with oil, they created a material that doesn’t dry out or shrink. Although something made from this isn’t useful as a permanent object due to its softness, it makes an excellent model for molding, and changes can be made at any time. To work with plastiline, which comes in large lumps, strips are cut and warmed in the hands, then it is applied to the armature with the fingers. Loop tools and modeling sticks are used for shaping it. For more efficient warming, a hair-dryer or heat gun can be used carefully. Due to the large amounts of sulfur found in some formulations, plastiline may cause some mold rubber compounds to have problems in setting.

A new material has become available recently, called “polymer clay”, which is much like plastilene except that it can be baked to hardness in a home oven. This is available in various proprietary formulations, such as “Sculpy” which is very soft, and “Fimo” which is harder. Warming it gently will soften it. It comes pre-tinted in a wide range of colors, but it is expensive, so is mostly used for jewelry and miniature sculpture. Although the exact compositions of these are closely-held secrets, they are rumored to contain some toxic ingredients, so anyone using it one should wash his or her hands before eating or smoking.

There are various other clay like products available, some of which will air-dry to a more or less permanent object, like Binney and Smith’s “Model Magic”, “Celluclay”, or “Paperclay”. If anybody has experiences with these or any others they would like to share, I would be interested to compare notes.

by Andrew Werby

Also: Recipe for a sulfur-free version of plastiline.