“old smith’s water”
Yes it's a technical term. Aqua fabrorum crops up here and there as an ingredient for surgical remedies, rendered as "smiths' water" in contemporary translations. See Juhani Norri, Dictionary of Medical Vocabulary in English, 1375–1550..
Decision: since the recipe is kept in Latin in the TL, with an editorial note providing the translation, I have translated "old smiths' water", with an editorial note on the Latin term "Aqua fabrorum antiquae" explaining that aqua fabrorum is "smiths' water" (see my comment above). This should disambiguate that this is old water used by smiths, not water used by old smiths.