Anonymous alchemical poetry, which flourished during the fourteenth and fifteenth centuries in England, consisted of laboratory texts mainly concerning recipes and practical notes about the production of the philosophers’ stone or elixir. These texts, which are not part of the authoritative writings of the famous alchemists of the time, re-purposed the Opus in a prosaic and pragmatic way. They made use of the same mysterious lexicon of the well-known alchemical treatises, but they often revisited and modified it according to their own needs. Very little attention has been given to this body of writing or to the processes of adaptation of the original Latin figurative alchemical vocabulary during vernacular translation. This paper focuses on a particular lexical item related to a transmutative procedure identifiable in a collection of fifteenth-century English recipes for making the elixir. It highlights the strategy an anonymous writer employed to coin a metaphorical formula to describe cryptically the mysterious aqua mercurialis. This formula was handed down in a corpus of manuscripts without being fully understood by copyists and users. In fact, later translators, in rendering it from English into Latin, turned it into an unsuitable expression, which led to the loss of its semantic implications.