What is the black staining on my wheelhead when I'm throwing?

Most wheelheads on the market today are made from an aluminium bearing alloy, because aluminium generates a protective oxide barrier when it comes into contact with oxygen. This barrier helps to protect the aluminium from pitting/corrosion. 

When you are throwing, the slightly abrasive nature of the clay in a slip form and the motion of your hands wears away the aluminium along with the oxide coating and because the particles are so fine they don’t reflect light very well and therefore appear black.

This oxide layer reforms quickly once the wheelhead is cleaned and dried, protecting the metal underneath.