כֻּלָּם֙ סָרֵ֣י סֽוֹרְרִ֔ים הֹלְכֵ֥י רָכִ֖יל נְחֹ֣שֶׁת וּבַרְזֶ֑ל כֻּלָּ֥ם מַשְׁחִיתִ֖ים הֵֽמָּה׃
Tous sont d’incorrigibles rebelles, propagateurs de calomnies, [durs comme] l’airain et le fer; tous sont malfaisants.
Rashi
Will iron break iron from the north I say that iron that comes from the north is harder than other iron, and this is its interpretation: They were compared to iron (above 6:28), “Going tale bearing like copper and iron.” And concerning Jeremiah it is stated (above 1:18): “I have made you... into an iron pillar and into copper walls,” and his is stronger than theirs, for Nebuchadnezzar will come upon them according to his words.
Rashi
לא תלך רכיל THOU SHALT NOT GO ABOUT AS A TALE BEARER — I say that because all those who sow discord between people and all who speak slander go into their friends' houses in order to spy out what evil they can see there, or what evil they can hear there so that they may tell it in the streets —they are called הולכי רכיל which it the same as הולכי רגילה, "people who go about spying"; espiement in O. F. A proof of my statement is the fact that we do not find anywhere the term רגיל used in Scripture except in connection with the expression הלך "to go". Examples are: the phrase here, לא תלך רכיל; (Jeremiah 6:28) "[They are all] walking as spies: they are brass and iron". But as for any other expressions for “slander”, the verb הלך is not used with them. Examples are (Psalms 101:5) "whoso privily slandereth his neighbour”; (Psalms 120:2) "false tongue"; (Psalms 12:4) "the tongue that speaketh proud things (slander)". For this reason I say that this expression (הולך רכיל) means "going about ומרגל, and spying out” (רגל = רכל), because the כ may interchange with ,ג since all letters the pronounciation of which are of the same place in the organs of speech may interchange with each other e. g., בי"ת with גימ"ל ,פ"א with כ"ף or with נו"ן ;קו"ף with זי"ן ;למ"ד with צד"י. And in a similar sense we have, (II Samuel 19:28) "He spied against thy servant [to my lord]” which implies, "he spied me out with subtly in order to speak evil about me to my lord״ (and thus וירגל comes to mean "to slander”). Similar is (Psalms 15:8): לא רגל על לשונו which means, "he has not spied out in order to have evil on his tongue”. Similarly the רוכל, the trader, is one who goes round and searches for (spies out) all kinds of merchandise, and so also the seller of perfumes which women use to make themselves nice, because he constantly goes about in the villages, he is called רוכל, which has the same meaning as רוגל. And its translation in the Targum לא תיכול קורצין, has the same meaning as (Daniel 3:8) "and they slandered (אכלו קורציהון) the Jews”; and as (Berakhot 58a) "he slandered him (אכל ביה קורצא בי מלכא) to the king”. It seems to me that people had the custom to eat a little snack in the house of him who listened to their slanderous words, and this served as the final confirmation that his (the slanderer's) statements were well founded and that he would maintain the truth of them. This "snack” was called אכילת קורצין, the word קורצא being connected in meaning with the root קרץ in (Proverbs 6:13) "He winketh (קורץ) with his eyes”, for it is the manner of all who go about slandering to wink with their eyes and to suggest their slanderous statements by innuendos in order that others who happen to hear them should not understand them.
Rabbeinu Bahya
לוטש כל חורש נחשת וברזל "who knew how to sharpen metal instruments made of copper or iron." He was a true craftsman. At this point in time the word נחושת means steel (acero in Spanish) which combines very well with iron. Proper copper is called alambre in Spanish and does not make a good alloy when combined with iron. In Jeremiah 6,28 the prophet speaks about the incompatibility of iron and copper when he writes כלם סרי סוררים הולכי רכיל נחשת וברזל כלם משחיתים המה “They are copper and iron; they are all stubbornly defiant, they deal basely all of them act corruptly.” The prophet was chastising the people for committing slander and character-assassination. He compared their conduct to the relationship of copper to iron. Even when you combine them such a combination will not stand the test of time. The end of the verse in Jeremiah emphasizes that true cooperation between people who practice slander is no more possible than alloying copper and iron. In the end, both metals will become useless. Another anomaly that you will find after you have combined iron and copper is that you can never separate these two metals again. This is the reverse of what we find with alloys containing silver and gold, for instance. The verse in Jeremiah therefore uses the example of these two metals to demonstrate the totally negative fallout of anything involving slanderous statements. The very name of the word נחשת which is derived from נחש, a word suggesting experimentation, putting something to a test, is already an allusion to its meaning. Lavan told Yaakov in Genesis 30,27 נחשתי ויברכני ה' בגללך, “I have learned by means of experimentation [in an idolatrous fashion. Ed.] that G’d has blessed me on your account.” No craftsman in the world can tell if a certain alloy contains steel or iron unless he has made an attempt to separate the component parts of the alloy. If the metal breaks, he knows it must contain steel. If it hardens and does not break this is proof that it is iron. [I have translated the text using a Spanish dictionary to describe the word אציי'ר found in the text, though it seems obvious to me that the “steel” the author speaks of is not the steel we know today. Ed.]