< WASTE MOULDING
DIRECT PLASTER WORK
Plaster of Paris is a highly versatile sculpting material that is well suited to use in the studio and foundry alike. When correctly prepared and applied, plaster is capable of forming strong and lightweight patterns with a potentially lengthy storage life. The raw materials are inexpensive and require little in the way of equipment for use. In sculpture making, plaster of Paris is used either as a filler to create waste moulded plaster casts, or else as a construction material in it’s own right for creating directly modelled plaster designs.
WHICH TYPE OF PLASTER?
There are numerous versions of the basic plaster product available, most of which are formulated for the construction industry. An artist might have some special reason for using a building product like a wall finishing plaster, but for normal studio and foundry purposes wall and ceiling plasters are generally inappropriate.
The group of plaster products that are of most use to the sculptor centre around the fine casting and dental casting types. The exact product name may vary according to manufacturer, also the formulation may vary somewhat, but a choice of one from this group of plasters should be adequate for nearly all your studio plastering needs. You may be tempted to experiment with special plasters, the high strength versions with trade names like Cystacal™ or Herculte™ for example, but for most sculptural uses, a correctly mixed dental type casting plaster will be more than adequate for most jobs.
PLASTER MIXING & APPLICATION.
Fill a container of water no more than 30% full of cold (not hot or warm), water. Open the plaster bag, and using your hand or a small container remove a handful or so of plaster. Sprinkle this evenly over the water in the container without disturbing the surface with your hand, allow the plaster to wet and then settle. Repeat and continue this process, allowing the plaster to settle until the level of deposited plaster becomes visible just below the water’s surface. When you are ready to use the plaster, place your hand in the container and gently stir the contents, making sure you scrape the container’s base and sides to ensure the plaster is fully distributed and mixed. The plaster should have a few minutes of working life until it begins to set hard.
It is good working practice when modeling to mix relatively small quantities of plaster in succession, rather that try and finish a bigger job with one large quantity. Provided you do not mix up the plaster by hand, a container of settled plaster will keep for some time, if you need large quantities in short order you should be able to prepare two or three containers in advance, then initiate the setting process by stirring the contents as and when needed. Warm water will lead to rapid setting times, and the addition of manufacturer’s proprietory agent will retard setting. Remember that mixing plaster with either warm water or additives (including colourings), can weaken the strength of a set plaster.
Freshly mixed plaster can be poured as a fluid material into impressed clay to make panel reliefs and other simple shapes; likewise waste and rubber moulds can be slushed (running wet plaster around a mould cavity) to create more complex hollow forms. As plaster begins to set, it reaches a ‘buttery’ consistency, in this state the material can easily be directly handled and modelled over a prepared framework or other form.
Frameworks and armatures supports for direct plaster working can be constructed in any number of ways. Mesh and ‘chicken wire’ can be manipulated into volumes, wood and steel can be used to create strong, rigid frameworks, and polystyrene foam blocks can be cut and carved into forms, then later skimmed with a plaster finish layer.
To make the best use of plaster’s light dry weight and strength it is best to construct reinforced hollow forms for all but the smallest of sculptures. The artist does not usually work straight onto a prepared framework with handfuls of mixed plaster. Instead, it is better to cut lengths of a coarsely woven fabric known as jute scrim, (or just ‘scrim’). The cut lengths are dipped into a container of wetted plaster, then passed lightly through the fingers to remove excess plaster before being applied to the framework. This builds up a thin but strong layer of plaster over the framework. The scrim/plaster process will often be repeated a number of times, with the artist ideally working the first layer of scrim in one direction (ie horizontally across the framework), the next layer at right angles (ie vertically up and down the framework), and so on. This technique produces a laminate construction with an excellent weight to strength ratio. It is usual not to apply scrim to the final outer layer(s) of plaster, this allows the sculptor to freely work back into the pattern’s surface later.
One of the most effective methods of creating a large scale foundry pattern is by working scrimmed plaster over an expanded polystyrene cut foam core. This combination can produce an easy to handle master pattern, which is usually strong enough for either rubber moulding or sand moulding (see SAND CASTING). A foam core can be roughed out into a volumetric form by hot wire cutting, then fine tuned with a cheese grater. The foam core is finished by skinning with scrim and plaster. If necessary, the core can be stiffened and reinforced by inserting steel bars into the polystyrene.
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