John Gill Commentary Deuteronomy 28:23

"And thy heaven that is over thy head shall be brass, and the earth that is under thee shall be iron." — Deuteronomy 28:23 (ASV)

And the heaven that [is] over your head shall be brass
Or like brass, not for its clearness, brightness, and splendour, or for its being spread out like a molten looking glass which was of brass, (Job 37:18) ; but for its dryness and hardness, no moisture being in it, or passing through it; no showers of rain nor dew being let down from it:

and the earth that is under you [shall be] iron ;
or like iron, hard and impenetrable, into which the plough and spade will not enter; nor anything spring out of it, for want of rain and dew to moisten and soften it. The same is said in (Leviticus 26:19) ; only there is an inversion of the figures; there the heaven is said to be as iron, and the earth like brass, but signify the same thing.