All molten metal shrinks when solidifying. The amount of shrinkage varies in different metals and also in different castings of the same metal. A cylinder, for example, will shrink more in length than in diameter, largely because of the resistance of the central core. Therefore, patterns must be made larger than the required castings. The usual allowances for different metals are as follows:

When the molten metal cools to the point at which it is about to become solid, it suddenly flashes, expands, and then immediately contracts. The flash is called a "higher heat." This sudden expansion makes it possible to get impressions of small, fine lines on the casting.