What were the "Mar'os ha'Tzov'os"?
Rashi and Ramban (citing Targum Onkelos) Seforno and Targum Yonasan: They were the copper mirrors that the women, who in a display of disdain for worldly desires, donated, which were now accepted and used to manufacture the copper basin and its stand. 1
Da'as Zekenim, Moshav Zekenim, Hadar Zekenim and Rosh: The Pasuk is not discussing what was used to make the Kiyor but its location - between the Mizbe'ach and the Ohel Mo'ed, drawn slightly towards the north 2 , in order that women who go to the Ohel Mo'ed will see it, and take to heart that a Sotah drinks from its water. (Da'as Zekenim - they will see Sotos drink, and be chastised.)
Which explains why later, in Pesukim 39-40, the Torah does not include the basin and the stand among the items that were manufactured from the donations of copper (Rashi and Seforno).
But not actually in the north of the Azarah, since in the Gemara in Zevachim 59a, all opinions agree that the north must be devoid of Keilim, and the Tana'im argue only about whether or not the Mizbe'ach may be in the north? (PF).
What is the word "ha'Tzov'os" referring to?
Rashi: It is referring to the episode in Egypt when the women encouraged their husbands, who were worn out from the back-breaking work, to be intimate with them, by bringing them food and drink and enticing them via the mirrors. 1 And they subsequently gave birth to babies in the fields - who ultimately formed 'the hosts of Yisrael'. 2
Ramban #2 (citing Targum Onkelos), Seforno and Targum Yonasan: It refers to the women who now congregated at the entrance of the Azarah. 3
As the Pasuk hints when it writes in Shir ha'Shirim 5:5 "Beneath the apple-tree I aroused you" (Rashi).
See also Ba'al ha'Turim.
Why did the women congregate at the entrance of the Ohel Mo'ed?
Rashi, Ramban #1 and Seforno: In order to donate their mirrors (To demonstrate that they no longer needed them).
Ramban #2 (citing Targum Onkelos): In order to Daven.
Seforno: In order to hear the word of Hashem. 1
Targum Yonasan: They were the modest women who had given birth and who were standing by their Korban Yoledes to Daven and to praise and thank Hashem. 2
Why did Moshe initially refuse to accept the mirrors?
Rashi and Ramban: Because he considered mirrors to be tools of the Yeitzer-ha'Ra, and therefore disgusting.
Moshav Zekenim #1: Because the entrance of the Heichal was open 1 , and Yisre'elim in the Azarah would be able to see [via the mirrors] the Avodah of the Kohen Gadol in the Heichal - and the Torah wrote "ve'Chol Adam Lo Yih'yeh be'Ohel Mo'ed ... ."
Moshav Zekenim #2: Because when the Kohanim would subsequently bend down to wash from the Kiyor, it would look as if they were bowing down to their own reflections.
Based on the Magen Avraham: Perhaps it is because he thought that it is forbidden to designate a K'li Chulin for Kodesh even if its shape has been changed. 2
This did not apply in the Mishkan, since there was a screen at the opening of the Ohel Mo'ed! Perhaps we must say that Moshe objected having in mind the Beis ha'Mikdash. Also there, why is seeing a reflection worse than Kohanim standing in front of the Heichal and looking inside? (PF).
The Magen Avraham, in Sh. A., O.Ch, 147:5 rules that, although one may not make a mantle for a Seifer-Torah from cloths that were used for Tzorech Hedyot, one may if one alters its shape, it is permitted - as we see from the Mar'os ha'Tzov'os. See Torah Temimah, note 1.
How will we reconcile Moshe's reaction here, with his his reaction in 35:22, when he accepted the women's donations of Kumaz, which was even more disgusting than the mirrors?
Ramban: Because there he accepted the Kumaz together with many other ornaments, which were all melted down and used for whatever it was needed. But to make an entire K'li out of such ornaments was something that he was not initially willing to do.
Moshav Zekeinim: Because, as opposed to the Kumaz ornaments, the mirrors were not melted down but joined together intact.
If Moshe considered the mirror, tools of the Yeitzer ha'Ra and disgusting, why did Hashem tell him to accept them?
Rashi: They were precious in the eyes of Hashem, because the righteous women used them to raise the hosts of Yisrael. 1
Ramban (citing Targum Onkelos), Seforno and Targum Yonasan: Because giving away their mirrors was an act of righteousness, in that they overcame the Yeitzer ha'Ra of desire.
Moshav Zekenim: Because, although a Kohen must offer a woman's Korban Lishemah (in her name), 2 he may not look at her face. Therefore, the Kiyor needed to be made of mirrors, so that he could see her reflection. 3
Which is crucial by a Chatas.
Why must he see her at all? We offer Korbanos sent through Sheluchim, or bought with money placed in boxes, without seeing the owner! And what is the Isur to see a woman's face? According to Kabalah, the Kohen must see the owner's face, in order to know the severity of his sin. And due to the Kedushah of the Kohen, seeing her face could cause damage. (PF, heard from R. Meir Eliyahu).
