Master pattern is the name given to a pattern having a double contraction or shrinkage allowance. The castings made from the master pattern are used by the patternmaker in making duplicate patterns that will have only a single contraction or shrinkage allowance for the particular metal required of the final casting. An allowance is made not only for the shrinkage of the metal pattern, but also, a second allowance is added for the shrinkage of the casting produced from the metal pattern. For example, a master pattern constructed for an aluminum pattern to be used in producing bronze castings has the shrinkage allowance for aluminum (5/32" per foot) plus the shrinkage allowance for bronze (3/16" per foot) or a total shrinkage allowance of 11/32" per foot. If the final casting is to be machine finished, further allowances must be made for finishing the metal pattern and final casting. The actual metal pattern is made in the foundry, the same as any other casting of similar complexity would be made.
The different types of material that can be used in the construction of master patterns are numerous. These types include wood, plaster, aluminum, brass, bronze, plexiglass, ren-shape, and more. As comparatively few castings are made from a master pattern, the cost of the materials is held to a minimum so that the pattern can be made as economically as possible. Therefore, for the general purposes of lower cost and of speed in the manufacture of a master pattern, poplar is the most logical material to use.