Modelling clay

Clay has been used for modelling from the beginning of civilisation. The original earthen clay used by modellers and seemingly forgotten by those who use more recently developed products is dug from the ground. Colours vary from bright red, yellow, terracotta and cream to black. Classically, terracotta was the preferred material; it is still very popular. Modellers also prefer grey which dries light grey and then can readily be painted. Earthen clays can be fired but are often air-dried. In layman’s terms it is made up of very finely ground hydrous silicate particles which is technically described in Wikipedia Clay.

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A relatively new product variation of earthen clay is clay reinforced with filaments of Nylon. It is used by modellers who do not intend to fire their works. This is designed to bond the clay more tightly and therefore reduce the incidence of cracking. Modelling large models with earthen clays is made difficult because clay shrinks unevenly when it dries. It is therefore essential that the model is dried very slowly otherwise fatal cracking occurs. Cracking can also be minimised by using an internal supporting frame (armature) of wood, wire mesh, polystyrene or similar materials made to the rough shape of the intended object and then finishing the model with a layer of clay no more than 15mm thick.

Modelling clays both natural and Nylon reinforced can be fired at a kiln temperature in the range of 1000°C to 1250°C. When firing a model, modellers should take care to ensure that their armature (frame) can withstand these temperatures without breaking down or releasing explosive or noxious fumes. Models can then be glazed in the same way as pottery clay. The Nylon does not affect the glazing process.

Notable amongst these is Plasticine which was invented by art teacher William Harbutt of Bathampton, near Bath, England in 1897. He wanted a non-drying clay for use by his sculpture students. Plasticine has since become a generic term in Commonwealth English. The great advantage to artists of Plasticine is that it is very slow drying, it is not soluble in water as it is oil based. When warmed to between 30 and 40 deg C (body temperature), it is easily workable. It is an ideal material for animation artists who need to rework their models. It can be reused. It is available in a multitude of colours. When cooled to room temperature of say 18 deg C. It stiffens and is very stable. It is non-toxic and is therefore suitable for use by children. There is now a variety of similar products. Notable amongst these in the U.K. are Newplast and Colour Clay.

Generic Plasticine being readily workable in fine detail is also suitable for creation of an original work from which a mould (molding)can be made. Castings and reproductions in a much more durable material can then be produced.

Some modellers and craftsmen want a modelling material which hardens permanently at low temperature without significant distortion. Polymer clay is the solution. Polymer clay hardens by curing at temperatures created in a typical home oven generally at 265 to 275 °F (129 to 135 °C) for 15 minutes per 1/4" (6 mm) of thickness and does not shrink or change texture during the process. When properly cured, most items won't break if dropped or normally stressed. Polymer clay also comes in liquid form and in permanently flexible solid form. Polymer clay is sold in craft, hobby, and art stores, and is used by artists, hobbyists and children. Leading brands of polymer clay include Premo, FimoClassic and FimoSoft, Kato Polyclay, Sculpey Cernit, Formello and Modello and Das.

Under the classification of Clay, it is necessary to include another air drying clay which does not shrink noticeably when drying. This is paper clay. Production of Paper Clay for sculptures and fine art is, however, an art in itself. An example of how not to make paper clay is at http://paperclay.blogspot.com. Paper clay can be painted, varnished, drilled, sawn, cut and glued. This makes it a great fun product for children and an extremely versatile and challenging product for fine artists. There is brand of paper clay called Fun Fair Paper Clay which is aimed at children. Model Magic by Crayola is a similar product.