A quick test to identify types of ferrous metal. A sample of the metal is pressed against a grinding wheel, and the pattern of sparks thrown off indicates of the type of metal. When wrought iron which contains no carbon is held against the wheel, the sparks stream out in a straight line. The lines of sparks grow wider and brighter as they stream out behind the wheel.

Mild steel, with its low carbon content, has a spark pattern similar to that of wrought iron, except some of the sparks have tiny forks on them. The spark pattern for tool steel with its high carbon content is similar to that for mild steel, except there are more forks in the stream, and the forked sparks form close to the wheel. Cast iron produces a large number of sparks close to the wheel, but they die out quickly. The spark pattern for cast iron is very short.