Rust (βρῶσις, ἰός) occurs as the translation of two different Greek words in Mt 6:19-20 and Jas 5:3. In the former passage the word βρῶσις, which is joined with σής, "moth," has by some been understood to denote the larva of some moth injurious to corn, as the Tinea granella (see Stainton, Insecta Britan. 3, 30). The Hebrew עָשׁ (Isa 1:9) is rendered βρῶσις by Aquila (comp. also Epist. Jerem. 5:12, ἀπὸ ἰου καὶ βρωμάτων, "from rust and moths;" A.V. Bar. 6, 12). Scultetus (Exerc. Evang. 2, 35; Crit. Sac. vol. 6) believes that the words σὴς καὶ βρῶσις are a hendiadys for σὴς βρώσκων. The word can scarcely be taken to signify "rust," for which there is another term, ἰός, which is used by James to express rather the "tarnish" which overspreads silver than "rust," by which name we now understand "oxide of iron." βρῶσις is no doubt intended to have reference, in a general sense, to any corrupting and destroying substance that may attack treasures of any kind which have long been suffered to remain undisturbed. The allusion of James is to the corroding nature of ἰός on metals. Scultetus correctly observes, "Erugine deformantur quidem, sed non corrumpuntur nummi;" but though this is strictly speaking, true, the ancients, just as ourselves in common parlance, spoke of the corroding nature of "rust" (comp. Hammond, Annotat. in Mt 6:19). — Smith. Moreover, various writers agree that the gold and silver coins of antiquity were much more liable to corrosion than those of the present, being much more extensively adulterated with alloys.
The word translated "scum" (חֶלאָה, chelah) in Eze 24:6,11-12 means the rust or corrosion of the pot of brass (or rather copper) which typified Jerusalem Copper is more liable to corrosion than the other metals, each of which has its own dissolvent; but copper is acted upon by all those dissolvents, and the corrosion of the copper pot symbolizes the aptitude of Jerusalem to corruptions, which, being shown by Ezekiel to be removed only by the agency of fire, was a type of the awful punishments and fiery purgation awaiting Jerusalem.