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"The surface of parchment, also called sulfurized paper,"
From a cooking definitions page.
I must admit I was baffled by why anyone would want any kind of sulphur in close proximity to mirrored surfaces, or any metals, since sulfur tarnishes silver immediately and badly.
In this day and age "genuine" parchment, which is properly a skin not a paper, is expensive and hard to find. "Parchment paper" which is a paper grade that is similar to skin, is what you'll get from most paper merchants. I think I'd rather take a sheet of Tyvek, obtained free of charge from a CD sleeve, Fedex or USPS mailing envelope, or other source. Tyvek is a spun-bonded plastic (polyolefin?), soft, clean, impermeable, cheap, with nothing to emit. It should protect glass very nicely.
Some of the cooking parchments are also waxed or otherwise impregnated, no need to risk whatever that might be on your mirror.
