This article suggests that institutional workshops of assay were significant experimental sites in early modern London. Master assayers at Goldsmiths’ Hall on Foster Lane, in the heart of the city, and at the Royal Mint, in the Tower, made trials to determine the precious-metal content of bullion, plate and coinage. The results of their metallurgical experiments directly impacted upon the reputations and livelihoods of London's goldsmiths and merchants, and the fineness of coin and bullion. Engaged in the separation and transformation of matter, assayers and the affairs of their workshops were also a curiosity for those interested in the secrets of nature. Making use of a wide-ranging body of sources, including institutional court minutes, artisanal petitions, mercantile guidebooks, recipe books and natural-philosophical treatises, this article uncovers a complex culture of metropolitan expertise. We first examine the workshop spaces in which assayers undertook their professional activities, and their secretive corporate cultures. We turn next to the manuscript culture through which assayers codified and communicated knowledge, ‘secrets’ and techniques to broader urban audiences. Finally, we assess exchanges and tensions between assayers and the wider community of Londoners engaged in scientific knowledge production and dissemination.