When electrochemically machined, mild steel and cast iron have shown contrasting characteristics. Whilst the former has a high current efficiency (105 to 96° for current densities ranging from 75 to 1200 A/in.3), and a good surface finish ( e.g. 30 micro-in. at 300 A/in.2), the latter has a poor efficiency ( e.g. 3.5° at 150 A/in.2) and a poor finish (invariably greater than 200 micro-in. ). This contrast has been attributed mainly to the differences in their carbon contents. Further tests on stools with different carbon contents have shown that an increase in carbon content does reduce the current efficiency.