Midrasch zu Jeschijahu 48:4

If you should be of the opinion that the brass was not affected because it was thick, R. Nehemiah declared that the overlay was but the thickness of a dinar. R. Phinehas the son of Hama said: If you should believe that the fire descended from on high only for a short time, and that is why the altar was not damaged, the fact is that this altar was so important that the fire was not removed from it either by day or by night, as it is said: The fire shall be kept burning on the altar continually. Why was the overlay made of brass? In order to atone for the brazen brow (i.e., Israel’s stubbornness),¹⁹See Kiddushin 70b. as it is said: And thy neck is an iron sinew, and thy brow brass (Isa. 48:4).