Rashi on Exodus 27:2/רש"י על שמות כ״ז:ב׳

1ממנו תהיין קרנתיו ITS HORNS SHALL BE OF THE SAME — i. e. he shall not make them separately and afterwards attach them to it (to the altar).

2וצפית אתו נחשת AND THOU SHALT OVERLAY IT WITH COPPER — to atone for sins committed with effrontery (more lit., with impudence of forehead, the Hebrew expression for effrontery being, “having a copper forehead,”) as it is said, (Isaiah 48:4) “[Because I know that thou art obstinate] and that thy forehead is copper” (cf. our English expression “brazen-faced”) (Midrash Tanchuma, Terumah 11).