The Gemara teaches that the prohibition of immersing Keilim on Yomtov only applies to vessels that have Biblical Tumah. A vessel that became impure with a Rabbinic Tumah may be immersed on Yomtov. Rashi explains that whereas purifying from Biblical Tumah resembles “fixing”, purifying from Rabbinic Tumah does not, since Biblically the vessel was considered pure.
The Maggid Mishna uses this to explain why the Rambam permits Tevilas Keilim on Shabbos. Since the Rambam rules that Tevilas Keilim is only Rabbinic, it is not considered a Tikkun. In contrast, the Rashba who is stringent, holds that immersing Keilim is a Biblical obligation. This is the opinion of the majority of Rishonim, including Rashi and Tosfos.
Based on this, even according to the stringent opinion, the prohibition of immersing Keilim on Shabbos should only apply to metal Keilim whose immersion is Biblically required. Since immersing glass Keilim is only a Rabbinic requirement, it should be permitted on Shabbos. The Rishonim do not mention this distinction.
Even though this distinction is not brought in the Rishonim, many Acharonim write that one may immerse glass Keilim on Shabbos. These include the Shaagas Aryeh, Pri Megadim and Panim Meiros. Other Poskim rule that Tevilas Keilim for glass is also prohibited on Shabbos. Their view is based on the Maggid Mishna.
The Maggid Mishna writes that the distinction in the Gemara between Tumah Deoraisa and Tumah Derabanan only applies for immersion for purification. This is because Keilim that are Tameh may technically be used for non-Terumah food. Therefore, we can be lenient on Shabbos to immerse Keilim that have only Rabbinic Tumah. This would not apply to Tevilas Keilim, even for glass, since we may not use vessels acquired from a non-Jew that have not been Toiveled.
The Alter Rebbe references this Maggid Mishna and writes that even according to those who hold that Tevilas Keilim is only Rabbinic, it could still be considered a Tikkun and thus forbidden on Shabbos. As such, it should apply to glass as well. The Mishna Berura is also stringent with glass Keilim.
The Alter Rebbe rules that since the Ikar is like the view that Tevilas Keilim is Mideoraisa, it may not be performed on Shabbos.
If one did immerse on Shabbos, the Alter Rebbe rules that the Keilim may still be used. In this case we rely on the lenient opinion which permits Tevilas Keilim on Shabbos. This ruling is based on the Magen Avraham, however the Magen Avraham does not write it with certainty, but rather “perhaps”.
In contrast to the Alter Rebbe, the Tzemach Tzedek writes that on Shabbos one may only be lenient if the vessels were immersed unknowingly. If it was done intentionally on Shabbos, one may not use them.
Where there is a doubt whether Tevilas Keilim is required at all, the Mishna Berura writes that one may rely on the lenient opinion to immerse them on Shabbos. However, the Ketzos Hashulchan argues with this leniency.
