The correspondence relates to seven 'Sun Pictures' now in the Science Museum's Pictorial Collection (inv. no. 1884-16) five of which have been identified as 'mechanical paintings', executed between 1778 and 1781 by an unpatented process of James Watt's partner, the entrepreneur Matthew Boulton, and his craftsman at the Birmingham Soho Works, Francis Eginton. The process is significant for its use of an albumen surface and may be regarded as aquatint at an intermediate stage ready for transfer printing processes akin to those used in ceramics.