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The requirement on glass is rabbinic, but it still warrants a bracha. (We say brachas on rabbinic mitzvas all the time.) The only time we do no-bracha is if there's a doubt whether it's required, e.g. a difference of opinion among poskim. – ShalomMay 7 '10 at 14:16
I believe the aharonim say it is still better to have something metal that you are toveling to remove you from safek bracha because there are those who hold glass does not require tevilah. Correct me with sources if I remember wrong. – YahuMay 7 '10 at 18:02
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Yahu: Shach (120:5) says this about lead-plated dishes, because Rema (ibid. :1) says that these should be toveled without a berachah (and as Taz ibid. :3 explains, the reasoning is that the lead plating is only decorative). But I'm not aware of anyone who says this about glass dishes, since it's stated explicitly in the Gemara (Avodah Zarah 75b, Rav Ashi) that they do require immersion. – AlexMay 7 '10 at 20:23