Because he was a mineralogist, Jeff was soon pressed into service by the Daleks to help them find a solution to the rust problem they were having on that moist world. They quickly discovered that a new lead-enriched paint would protect their outer casings. Stone worked out that if he combined an amount of lead salt with sodium azide — both of which happened to be around the lead mine where he was working for the Daleks — he would get lead azide.