[Letter] 1777 Feb 10, No. 45 great Titchfield Street, Oxford Road, London [to] Mattw. Boulton Esqr. at Soho near Birmingham, Warwickshire / P-J de Loutherbourgh. [2p. on 1 leaf. Seeking help in locating a particular mineral to enable him to prepare 'one of the finest colours I use in my Pictures']