During the Renaissance, painting media were developed which substituted oil for water as a vehicle for the pigment, resulting in an easily reworkable surface that allows for a high degree of control. Sculptors were likewise seeking ways to get around the limitations of ordinary water-based potter’s clay. While an excellent medium for quick terra-cotta sketches, it shrinks as it dries, and tends to develop cracks, especially if armatures are used. Once dry, it resists the addition of new pieces of clay. What was needed was a non-shrinking material that was soft enough to model but firm enough not to distort while a mold was being made, and which would permit addition to and removal of material for an extended period of time.
This need was filled by a similar means; the water in the clay being replaced by grease, oil, and wax. While sculptors in the past used various vegetable oils, animal greases, and beeswax; the modern sculptor can take advantage of petroleum-based products which are more stable, and less likely to go rancid. There are various brands of oil-based clay available currently. They are often called Plasticine, although this is apparently the registered trademark of the UK’s Bluebird Toy Co., which resents its generic use. Other variations one sees are “plastiline”, and “plastilina”. Some, like Roma Plastilina, contain sulfur, which interferes with the setting of some silicone rubbers. (If you wish to use high-sulfur clay as a model for a silicone mold, use G.I. 1000 from Silicones Incorporated—it doesn’t have this incompatibility problem.) It is usually a good idea in any case to give the model a sprayed-on coating of acrylic lacquer before molding, to provide a barrier and firm up the surface. I would advise testing any new clay material with the rubber one intends to use; as well assulfur, certain oils can also cause problems to sensitive rubber compounds.
Proprietary oil-clays come in a wide variety of hardnesses and compositions, and each sculptor has a favorite. Many like Chavant, which comes in a range of hardnesses, others prefer Kleen Klay, or HBX-2, made by the American Art Clay Company, (AMACO) in Indianapolis, Indiana. This is designed to soften rapidly in a microwave (about 60 seconds for a 2 pound block) Once heated and allowed to cool, it becomes extremely hard, to the point where it can be worked like wood. As well as using a microwave oven, one can warm pieces of oil-clay to a workable consistency using a hair-dryer or heat gun. An electric warming tray can be handy for keeping a quantity of clay soft—an electrical light dimmer wired into the circuit will allow fine adjustment of temperature.
Oil-based clay is used in much the same way as wax for building sculpture directly. It is usually advisable to start with an armature, which is an internal support structure used to keep extended parts from collapsing or falling off. On a small to medium scale, these can be built by firmly attaching heavy-gauge aluminum ground wire (available at electrical supply places for less than at art suppliers) to a plywood base, and applying the clay all around. For some extended forms, like standing figures, it can be helpful to have an exterior support as well, which holds the piece up from the top. One must be careful not to let the wire get too near the surface, since this can interfere with the sculpting. Supports can also be made with rigid plastic foam. This can take the place of a large quantity of expensive clay, if one is building large forms. The foam can be easily carved to shape, then it is poked all over with a sharp-pointed stick and coated with melted beeswax—this makes the clay adhere to the surface better.
Clay can be cut into workable pieces using a thin wire (some use guitar strings) fitted with toggles on the ends. (Use wooden sticks for these, or large washers). It is carved away using loop tools, which resist clogging and smearing while removing material. Old hacksaw blades, which can be bent somewhat, can be helpful in forming smooth curving forms, and piano wire held in a loop will define small concavities. After the form is defined, smooth wooden modeling tools and variously shaped metal ones can be used to put in the details.
If a very smooth surface is desired, it is possible to achieve this in oil-clay. One can use the plastic or rubber “ribs” sold by ceramic suppliers, or make ones own. There are new tools available that have a shaped piece of rubber as the working end; these can be useful. I’ve also found that a small piece of stiff leather makes a versatile tool which can be shaped and sanded to the contour required. A soft “chamois” leather can also be handy for smoothing, while some recommend nylon stockings, “pantyhose”, or pieces of muslin sheet. These all are best used with some kind of lubrication, to avoid smearing the surface. I’ve used liquid dishwashing soap, rubbing alcohol, and even spit for this. Some people recommend stronger solvents, such as turpentine, naptha, and lighter fluid, but I’ve found they tend to leave the surface sticky, (although it will evaporate after a while) and they are best avoided for health reasons as well.
Although oilclay can be purchased in many different grades, it is also possible to make ones own. A classic recipe is as follows:
- 20 lbs. microcrystalline wax
- 1¼ gal. #10 weight oil
- 7 lbs. plain automotive grease
- 50 lbs. gritless dry clay powder (Such as Kentucky ball clay, or Gordon clay)
Melt wax, oil, and grease together in an electric frying kettle; stir clay in slowly once melted. Pour into shallow microwave-safe plastic containers, or into a wet plaster mold.
Adapted from “Sculpture Casting,” by Kowal and Meilach, © 1972 Crown Publishers, NY ISBN 0-517-500590.
Although this will work, and is inexpensive, I prefer a variation I worked out which uses beeswax for half of the wax component, substitutes petroleum jelly for the grease, and purified mineral oil for the 10 wt. motor oil. (You might also want to reduce the quantity proportionally to fit your electric frying kettle). This variation smells better, and doesn’t have a problem with the rubber mold compounds I use, like some proprietary oil-clays do. Modifying the proportions of the constituents slightly will yield harder or softer clays. More wax and clay makes it harder, which will be an advantage if highly detailed forms are being modeled in a warm place; more oil and grease makes it softer, which can be helpful if one works in an unheated space in the winter. A correspondent in Brazil sent in a similar recipe:
- 480 gms. bee wax
- 200 ml. purified mineral oil
- 192 gms. petroleum jelly
- 1200 gms. talc (white)
In an electric pan (60° C) mix beeswax + mineral oil + petroleum jelly. Before they become entirely liquid, mix in the talc.
When mixing in dry ingredients such as clay powder and talc, make sure to wear a dust mask, and make sure you have adequate ventilation, preferably an exhaust hood over the melting pot. The clay contains silica, which can cause a lung disease called silicosis; and talc has been implicated in scarring of the lungs. Make sure to use an electric pot with a low-range temperature control, so the wax, oil and grease won’t overheat and cause a fire. Use loose-fitting rubber gloves to avoid burning your hands.
I find that if one is working intensively with a material, having ones hands covered in it and ones face in close proximity all day, little things like a better smell can make a big difference. By the way, to get it off my hands, I use vegetable oil on paper towels before washing up with soap and warm water; the vegetable oil removes most of the clay; and the soap then takes off the remaining oil.