- You may be looking for the short story of the same name.
When the Sheriff of Nottingham had captured Ian Chesterton and was going to execute him, the First Doctor offered to show him the secrets of alchemy in exchange for Ian's release. (PROSE: The Thief of Sherwood)
Abel Gantz was an alchemist, who could change his flesh to any element or shape. He acquired this ability after a laboratory accident inadvertently caused when the TARDIS escaped from the Funhouse (COMIC: Funhouse)