With the help of relatives, Rothschild secured an apprenticeship under Jacob Wolf Oppenheimer, at the banking firm of Simon Wolf Oppenheimer in Hanover, in 1757. The grandson of Samuel Oppenheimer taught Rothschild useful knowledge in foreign trade and currency exchange, before he returned to his brothers' business in Frankfurt in 1763. He became a dealer in rare coins and won the patronage of Crown Prince Wilhelm of Hesse (who had also earlier patronised his father), gaining the title of "Court Factor" in 1769. Rothschild's coin business grew to include a number of princely patrons, and then expanded through the provision of banking services to Crown Prince Wilhelm, who became Wilhelm IX, Landgrave of Hesse-Kassel in 1785. Business expanded rapidly following the French Revolution when Rothschild handled payments from Britain for the hire of Hessian mercenaries.