Patternwork

A necessary part of the process above is the creation of the pattern. It must be tapered so you can extract the pattern from the mould. If all the curves go one way (as in the bucket from which you get sand castles) then the pattern can be a simple one-piece affair. These can be small – like the xyz here, or large like the patterns for the portlights in Liverpool Tate. The common factor is that all the tapers run one way.

Life gets more fun – or more complex – when you need to have truly parallel surfaces, for example where an aperture needs to be moulded in, or where some of the shape runs in a different direction. In these cases “cores” are used. These are shapes made from sand which are placed into the mould after the main pattern is extracted. This then modifies the shape which the metal assumes when poured in, allowing undercuts or parallel shapes to be formed. An example is the square shaft shown here.
For more complex shapes or one-off pieces like sculpture, making a mould would be nearly impossible, so another technique is used: lost wax or investment casting, or fabrication.