"persistent boiling pitch" is a mistranslation of "à poix bouillante tenacé”, based on the erroneous gloss “to a pitch of tenacious boiling”. On this one, Lawton seems closer to the target, both with respect to the meaning of"tenacé" and to the object it qualifies: Lawton's "tenacious" at least can mean (indeed, like "tenacé") something along the lines of "clinging" or "viscous", which as it happens are pretty decent adjectives for "pitch" (rather than "boiling") if you think about it.