Stone became friends with Joseph Priestley as a member of Richard Price's church in Hackney; these associations also radicalized him. He was a member of the London Revolution Society and in February 1792 he offered to help Talleyrand secure British neutrality in the European wars involving France. He and his wife moved to Paris in April to open a sal ammoniac factory and established himself as part of the British expatriate community, including Helen Maria Williams with whom he had a subsequent long love affair. Stone published his opinions on the 1792 military campaigns as part of Williams' "Letters Written in France" in 1793. Stone, while saddened by the September Massacres, believed they were necessary for the revolutionary to succeed and he and other expatriates celebrated French military victories. In addition to his factory, Stone established a printing house, Imprimerie Anglaise, which printed Joel Barlow's epic, "The Vision of Columbus".