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PLASTER WASTE MOULDING
As already mentioned, the delivery of a well prepared master pattern is an increasingly rare event in the modern art foundry. This is in so small part due to a decline in the practice of waste moulding of CLAY models. For a variety of reasons, such as time constraints, or simply the lack of a taught skill – it is far more common for artists to submit soft clay models rather than hard copy plasters for casting.
The waste mould is a relatively simple and straightforwards method of creating a hard plaster copy from a soft clay model. The plaster copy is easily transported, can be reproduction moulded with little fear of sustaining serious damage. The resulting plaster cast can also be sharpened and refined by the artist in a to a degree not always possible in clay. The sculptor can even tint the plaster cast with a pigment to help determine the most appropriate patina colour for the finished work.
Some of the finest examples of waste moulding can be seen in the plaster works of the French sculptor August Rodin (1840-1917). Rodin’s plaster sculptures were specifically worked either for dispatch to a foundry as a master pattern, or for sale as part of an edition of plaster copies. In either case, the plasters produced are virtuoso examples of studio craftsmanship. What makes Rodin’s work particularly interesting is his common habit of leaving shim division lines across the surface of the cast plaster sculpture as an intrinsic feature. From these seam lines, the viewer can deduce the decision making process used to during the construction of the waste mould. Many plasters copies of Rodin’s sculptures are still in existence, unfortunately not all of these are believed to be ‘authentic’ (which is to say, copies produced to the sculptor’s direction and studio supervision).The best places to view examples of Rodin’s plaster work include the Museé Rodin, and nearby Museé D’Orsay in Paris. Many large collections in the US and Europe also include examples of Rodin bronze casts, the cast metal versions of these sculptures will also retain any evidence of waste moulding in the master pattern.
THE BASIC WASTE MOULDING TECHNIQUE
The first stage in forming a waste mould is to divide the surface of the clay model into convenient sections using a thin brass strip or ‘shim’ (fig 1). Once the clay model has been divided and registration marks inserted, a fluid plaster mix is prepared (see DIRECT PLASTERWORK). Colour, usually from a powder pigmentation, is added to the plaster during mixing. This initial application of plaster is known as the ‘warning layer’, and is flicked with the fingers over the clay model’s surface, carefully avoiding the entrapment of air bubbles between the clay and plaster surfaces (fig 2).
Once the warning layer has hardened slightly, a second (un-coloured) plaster layer is added. This second layer is built up to form the full thickness of the mould case – usually to between 1 and 2 in (25-50 mm) in depth, according to the scale of the model (fig 3). This process is repeated for each divided section or ‘cap’ on the clay model. Unlike other plaster constructions, no jute scrim (a coarse cloth) reinforcement is set into a waste mould case. Once all the plaster sections are set hard, each is cap removed in turn from around the clay.
The mould parts undergo a thorough cleansing process to remove any clay stuck to the inside of the mould (fig 4). This process should result in a clean, negative impression of the clay design on the inside of the case.
Once clean, the interior of the plaster case is treated with a release agent; either clay slip, vegetable oil or petroleum jelly will suffice. After treatment, a facing layer of plaster or other filler (for instance a gel coat for RESIN copies), is prepared and carefully applied to the interior of the mould. This first layer will determine the quality of surface reproduction, so every effort is made to fill mould detail, exclude air bubbles, and minimise the potential for defects in the finish.
The piece mould sections are then reassembled in sequence using case registration marks to ensure their correct alignment. The reunited mould may then be secured using clamps, and/or a temporary plaster and scrim bandage. Once secured, a small mould can be filled with plaster and left to harden. Larger moulds are part filled and ‘slushed’ with fluid plasterto create a lightweight hollow copy. If a base mount/fixing or internal reinforcement is desirable, these items are set into the mould before the plaster hardens (fig 5).
The now redundant plaster case is removed from the underlying hard copy by carefully chipping it away with the aid of a wood chisel. A gentle ‘V’ pattern of cutting into the plaster’s surface ensures a controlled and regular removal of waste material. As the surface of the underlying plaster pattern is approached the coloured warning layer alerts the sculptor to the proximity of the cast. This prevents damage to the reproduction through over cutting into the plaster cast’s surface (fig 6).
Eventually a positive plaster cast of the clay model is revealed. This in turn can be further worked into, refined and retouched to a sharp finish that is difficult to achieve and preserve in plastic clay. Sealing the finished plaster with a shellac or paint coating helps prevent any later applied moulding compound from adhering to the plaster’s naturally porous surface, the plaster can also be coloured to imitate a patina, this can help determine the ideal finish for a cast version.
As we can see, waste moulding involves a little effort, time, some skill and a little experience helps, however waste moulding is a technique the serious clay modeller and foundry user is well advised to acquire and perfect.
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