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 a thick and comparatively massive shape is required, it is often easier to cast it at once. For thinner and lighter forms it is usually more convenient to treat the ingot or crude product of the furnace as mere raw material for a long series of workings under the hammer, or its patent mechanical equivalents, the rolling and pressing mills of modern mechanics. The choice is further influenced by the toughness generally characteristic of wrought metal, whereas the alloys which yield the cleanest castings are by no means universally the best in other respects. Iron is the extreme instance of this: ordinary cast iron being an impure form of the metal, which is too brittle to be worked under the hammer, but is readily cast into moulds, being fluid at a temperature which, though high, is easily obtained in a blast furnace. Wrought iron, however, which is usually 72