Among professional model makers the English term Clay refers to a special modelling material that will not dry out in the air and which can be machined at room temperature (15 °C to 25 °C). Clay (sometimes called Design Clay), then, is primarily used in the automobile industry for designing auto bodies. In order to keep costs and the weight of the model low, the rough shape made with the clay is transferred onto a base body made from RIGID FOAM (e.g. polystyrene foam) or wood. In the process, the clay, which is relatively hard at 20 °C, is pre-warmed in an oven heated to, depending on the clay, 50 °C to 65 °C so that it will become more pliable and easier to apply on the model.
After the applied clay has cooled off and thereby become hard again, it can, unlike normal plastiline, be machined. The desired shape is then slowly arrived at by means of grating or scratching. In industry, this work is done by a computer driven milling machine whereby the surface of the model is subsequently scanned again and retraced according to the CAD data. If too much material has been removed, warmed clay can be added to the surface after it too has been warmed by means of a hot-air gun.
In the automobile industry, finished model studies are usually overlain with very special films or paints so as to give the look of painted coachwork. If necessary, these special coatings can be removed in order to make any later changes in the shape. Modulor does not yet have these special materials in its product range - clay surfaces, however, can be also successfully treated with TONING PIGMENT or RESIN-DISPERSION INTERIOR PAINT.