Passage: Zollverein Coal Mine was founded by Duisburg-born industrialist Franz Haniel (1779–1868), who needed coke for steel production. Test drilling in the Katernberg region (nowadays a suburb of Essen) had revealed a very rich seam of coal, which was then named after the German Customs Union (Zollverein), established in 1834. In 1847, Haniel founded the bergrechtliche Gewerkschaft Zollverein (a special kind of Prussian corporation for the exploitation of natural resources) and distributed the shares of the new company amongst the members of his family and the owner of the land on which the future mine would be constructed.

Question: When was the rest drilling established?
1834