Classifications of Clay Types
This section is an introduction to the various families of clays that available to the artist, modeler, sculptor and ceramist and its applications.
Oil-based clays are made from various combinations of oils, waxes and clay minerals. Because the oils do not evaporate as water would. Oil-based clays remain malleable even when left for long periods in dry environments. Articles made from oil-based clays cannot be fired, and therefore are not ceramics. Because the viscosity of oils decreases as temperature rises, the malleability is influenced by heating or cooling the clay. Oil-based clays can be heated, melted and then poured. Oil-based clay is not soluble in water. It can be re-used and so is a popular material for animation artists who need to bend and move their models. It is available in a multitude of colors and is non-toxic. However, some oil-based clay contains sulfur which prevents certain silicone mold rubbers from curing. In which case, non-sulfur oil-based clay should be substituted. Popular brands of oil-based clays include, DeMilano, Chavant and Roma
Water-based clays- Originally developed for use in sculpting Disney's animatronics models, WED (Walter E. Disney) clay is an extremely versatile water-based clay with properties similar to many of today's most popular oil based clays. It's smooth, slow drying, extremely pliable, and works great for sculptors seeking a "fast sculpt" medium. WED clay is still commonly used by professionals in the entertainment industry for masks, modeling, mockup, and large scale sculptures. Like any water based clay, WED clay needs to be kept moist, preferably with a spray bottle or a damp towel. Saran wrap is also commonly used. It is not formulated to be fired.
Polymer clays –This is a type of modeling clay based on the polymer polyvinyl chloride (PVC) which can be hardened. Polymer clay is generally used for making arts and craft items, and is also used in commercial applications to make decorative parts. Art made from polymer clay can now be found in major museums. Polymer clay remains workable until cured generally from between 265 F (129 C) to 275 F (135 C) for 15-minutes per 1⁄4-inch (6.4 mm) of thickness. This temperature is significantly less than for mineral clays and can be achieved using a home oven. The clay does not shrink when cured. Brands of polymer clay include Fimo, Sculpey, Premo, Cernit, Formello, Modello,Du-Kit and Kato Polyclay. Polymer clay safety is the subject of concern specifically the long-term effects of exposure to certain phthalate plasticizers that have been classified a endocrine disruptors.
Dough clay-is a modeling clay, which may be edible or inedible whichresembles the product PlayDoh®, and is often, in fact, called playdough. Playdoughs are easily made at home in both cooked and uncooked versions, and are less expensive than some of the other types of clay. They are made of such ingredients as flour, cornstarch, cream of tartar, oil, and water. They can be colored when made, for example, with food coloring, or have color added after.
One of the useful features of dough modeling clay is that it reusable, though, for example, in the case of a gingerbread house, baking is used to set and preserve the form. Flour-based products — including PlayDoh®, which clearly states that it is meant to be used and reused rather than employed to make lasting items — have a tendency to crack as they dry.
Ceramic clays are classified into five classes; earthenware clays, stoneware clays, ball clays, fire clays and porcelain clays. The three most commonly used ceramic clays are earthenware clay bodies, mid-fire stoneware clay bodies, and high-fire stoneware clay bodies. All three are available commercially in moist, ready-to-use form. Clay bodies can also be produced by mixing dry clays and additives with water to create your own desired clay body.
- Eartheware clays- were some of the earliest clays used by potters, and it is the most common type of clay. The clays are easily worked and can be sticky. Earthenware clays contain iron and other mineral impurities which cause the clay to reach optimum hardness at between 1745°F and 2012°F (950°C and 1100°C).
- Stoneware clays - are plastic and are often grey when moist. Their fired colors range through light grey and buff, to medium grey and brown. Fired colors are greatly affected by the type of firing.
- Ball clays - cannot be used by themselves due to their excessive shrinkage during drying and firing. They are extremely useful, however, when added to other clays to increase workability and plasticity.
- Fire clays - vary widely in their characteristics. The hallmark is their high firing range. They mature at about 2696°F (1500°C). Although relatively free from mineral impurities, they tend to have spots of iron which lend a speckled appearance once fired.
- Kaolin clays - due to their mineral purity, kaolin clays are used for porcelain. Although kaolin clays do have some range in color, they are all very light in color. While moist, they will be light grey and will fire in the range between a very light grey or buff, to near-white and white.
Paper clays sometimes referred to as fiberclay is any clay body to which processed cellulose fiber (paper being the most common) has been added. Earthenware, terra cotta, stoneware, porcelain and bone china clay bodies can be made into paper clay. The fiber increases the tensile strength of the dry clay and enables dry-to-dry and wet-to-dry joins. Commercial paper clays air-dry to a firm, lightweight sculpture, with minimal shrinking during the drying process. Paper clay can be used as an unfired body in craft and doll making. It is used in ceramic art studios as sculptural and functional studio pottery. Commercially paper clay is used to retard warping in clay, reduce weight of clay objects, and to lessen the cost of producing clay objects by replacing part of the clay with paper fibers which costs less. This is a common practice in the brick making industry.
Plasticine or Plastillina Clay