In the 1870s, microscopy societies began to proliferate in the United States. Most of these societies attracted microscopists from surrounding cities, but the American Postal Microscopical Club, modelled on the British Postal Microscopical Society, used the postal system to connect microscopists scattered across the country. Club members exchanged microscope slides and notes following a chain-letter system. The main objective of the club was to teach its members how to make permanent slides. Preparation and mounting methods required technical skill, which was, as even club members had to admit, difficult to learn without personal instruction. Yet members developed ways to share craft knowledge through the post. Drawing on the private notes of a member and published reports on the slides circulated, this paper challenges the widespread assumption that the generation of craft knowledge depended on the co-location of artisans. It argues that microscopists' knowledge of preparation methods was intertwined with their skill in building and navigating information infrastructures, and that by tracing these infrastructures we gain a better understanding of how craft knowledge travelled in the late nineteenth century.