Exodus 27:2

ממנו 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). וצפית 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).

וְעָשִׂ֣יתָ קַרְנֹתָ֗יו עַ֚ל אַרְבַּ֣ע פִּנֹּתָ֔יו מִמֶּ֖נּוּ תִּהְיֶ֣יןָ קַרְנֹתָ֑יו וְצִפִּיתָ֥ אֹת֖וֹ נְחֹֽשֶׁת׃

And thou shalt make the horns of it upon its four corners: its horns shall be of the same: and thou shalt overlay it with brass.