I have this question on Rashi every year in Ki Savo. Deuteronomy 28:23 says: "And the heavens above your head will be like copper and the earth that is beneath you, iron." Rashi says that the heavens will be “mazi’in” as copper is “mazi’a” and as a result there will not be destruction (avadon) through dryness (chorev) in the world (my translation). What does Rashi mean? How is copper “mazi’a” in a way that iron is not?
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This site argues, somewhat convincingly (based on Rashi and Maharsha), that the reference is to the patina (oxides) that emerges on the surface of copper. Iron in those days always had a layer of rust, and therefore was not considered to "sweat" any kind of substance.