(Leviticus 6:6) "A perpetual fire" — perpetual, even on the Sabbath; perpetual, even in a state of ritual impurity; "it shall not be put out" — even during the journeys. What did they do for it? They turned a copper vessel over upon it; these are the words of Rabbi Yehudah. Rabbi Shimon says: even at the time of the journeys they would clear its ashes, as it is said (Numbers 4:13) "and they shall clear the ashes from the altar and spread a purple cloth over it."
A Perpetual Fire That Burns on Sabbath and on the March
Yalkut Shimoni on Torah 484:4
(ויקרא ו ו) אֵשׁ תָּמִיד, תָּמִיד, אַף בְּשַׁבָּת. תָּמִיד, אַף בְּטֻמְאָה. לֹא תִכְבֶּה, אַף בַּמַּסָּעוֹת. מָה עוֹשִׂין לָהּ, כּוֹפִין עָלֶיהָ פְּסַכְתֵּר דִּבְרֵי רַבִּי יְהוּדָה, רַבִּי שִׁמְעוֹן אוֹמֵר אַף בִּשְׁעַת מַסָּעוֹת מְדַשְּׁנִין אוֹתָהּ, שֶׁנֶּאֱמַר (במדבר ד, יג) "וְדִשְּׁנוּ אֶת הַמִּזְבֵּחַ וּפָרְשׂוּ עָלָיו בֶּגֶד אַרְגָּמָן".