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WOOD PATTERNS
Wood is used extensively as a patternmaking material in commercial foundries, but is less common in the art foundry. This is partly because wood lends itself well to the needs of the commercial sand founder, who must be supplied with strong and accurate patterns that can withstand repeated heavy moulding operations. In contrast, a great deal of wood sculpture is carved, and intended for exhibition as finished work in it’s own right, not as a step towards making a metal cast.
Wood is sometimes used in sculpture patternmaking to construct support frameworks for plaster (see DIRECT PLASTER), resin, or laminated plywood finishes. These types of constructed wood pattern are well suited to the creation of large scale designs which have smooth regular facets, also support features such as plinths and bases.
There are of course many different wood products available, and the choice of which one to use depends upon many different factors, not least of which are the sculptor’s preferred construction methods.
In commercial patternmaking, the HEARTWOOD or centre portion of a cut timber is preferred because of it’s strength and durability, though in practice most sections of timber can be used. One of the problems encountered in using natural woods for patternmaking is the tendency for moisture loss through evaporation to shrink and warp cut timber. The moisture content of a wood product depends upon factors such as species and storage conditions. In most cases it is necessary to treat the timber and reduce it’s moisture content if a wood pattern is going to be of real use in the foundry.
The treatment of a wood to minimise movement and distortion is called SEASONING. Seasoning removes a large percentage of a wood’s moisture content through drying. The drying process can be done naturally (air drying), or artificially (kiln drying). The time taken and moisture content loss through air drying can vary greatly according to the wood species and storage environment. Cut lengths are stacked in such a way as to allow air to circulate through the stack, drying the wood until it reaches an appropriate level which can be as little as 10% of it’s original moisture content. Kiln drying has the advantage of accelerating the seasoning process, removing more moisture than possible by air drying alone. Kiln drying also reduces the chance of secondary damage to the wood by insect attack. Kiln seasoned wood is also generally better for use if the pattern is likely to be painted or glued in any way [ref].
PLYWOOD & MDF
Selected types of seasoned wood species are suitable for carving and other methods of sculpting, but if an artist wants to construct a regular form, it may be easier to use a prepared sheet such as plywood or MEDIUM DENSITY FIBERBOARD (MDF).
Plywood is a CROSSBANDED material, constructed by gluing alternate layers of thin wood veneer together, with the grain of each layer usually set at a right angle to the preceding layer (recalling the laminate method described for scrim and plaster constructions). Crossbanding not only produces a very strong material, but also one that can be steam worked over frames, and manipulated into curves, tubes, and other gentle breaks of form. Plywoods can be fixed by pinning, screws and gluing as well as by traditional carpentry joints. Well constructed and carefully stored plywood patterns should remain usable for many years.
Medium density fibreboard is a compressed wood dust consolidated in resin. Unlike natural wood products, MDF has no grain and requires no seasoning. The material is available in sheets with a near laminate thicknesses, through to heavy duty board. Because it has no grain, MDF is easily carved, sawn, machined and otherwise worked in any direction. Thin sheets can be gently bent to create curved surfaces. MDF constructions can be pinned, screwed and glued like plywoods, and the surface is easily primed and painted if needs be. Remember to countersink and fill any screws or pins used, unless you want these details to be seen on the casting itself.
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