Calx

noun

1. The substance which remains after a metal or mineral has been thoroughly burnt, once seen as being the essential substance left after the expulsion of phlogiston, but now recognised as being the metallic oxide (or, in some cases, the metal in a state of sublimation).
2. In the Eton College wall game, an area at the end of the field where a shy can be scored by lifting the ball against the wall with one's foot.