Jamieson, Fausset and Brown on Jeremiah 6:27

27. tower ... fortress--(Jr 1:18), rather, "an assayer (and) explorer." By a metaphor from metallurgy in Jr 6:27-30, Jehovah, in conclusion, confirms the prophet in his office, and the latter sums up the description of the reprobate people on whom he had to work. The Hebrew for "assayer" (English Version, "tower") is from a root "to try" metals. "Explorer" (English Version, "fortress") is from an Arabic root, "keen-sighted"; or a Hebrew root, "cutting," that is, separating the metal from the dross [Ewald]. Gesenius translates as English Version, "fortress," which does not accord with the previous "assayer."