An influential strand of English alchemy was the pursuit of the "vegetable stone," a medicinal elixir popularized by George Ripley (d. ca. 1490), made from a metallic substance, "sericon." Yet the identity of sericon was not fixed, undergoing radical reinterpretation between the fifteenth and seventeenth centuries as Ripley's lead-based practice was eclipsed by new methods, notably the antimonial approach of George Starkey (1628-65). Tracing "sericonian" alchemy over 250 years, I show how alchemists fed their practical findings back into textual accounts, creating a "feedback loop" in which the authority of past adepts was maintained by exegetical manipulations--a process that I term "practical exegesis."