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In the National Museum of China we find "mirrors" from various Chinese dynasties:

A photo I took of "mirrors" at the National Museum of China in Beijing
A photo I took of "mirrors" at the National Museum of China in Beijing

Notably, these mirrors are not reflective, the identifying property of mirror. I was thinking maybe they could be laid on the ground and have a reflective liquid added. Or perhaps on the reverse side there was a mirror, but it was not preserved over time.

Question: How do these "mirrors" in the National Museum of China function?

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They’re displaying the back of the mirrors. Here are a couple of images I found via Google that shows both sides:

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Chinese Museums typically exhibit the back of these mirrors, because doing so showcases the craft and design of the objects more.

The reflective side, aside from not being particularly interesting, are generally not actually reflective anymore due to corrosion and wear.

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    Just to add that the reflective side would have been highly polished when in use, which may not be obvious in a Han dynasty bronze mirror that hasn't been polished for 2000 years.
    – Adam Burke
    Dec 31, 2021 at 3:01
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The black color indicates the mirrors are silver. The silver will blacken with sulfide. If coal was being used for heating as common in modern times, the black silver sulfide (from sulfur in coal) would have formed relatively quickly ( less than a year). Periodic polishing would have been needed for use. Bronze or brass would develop greenish corrosion deposits (patina) that are chemically more complex.

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    It would appear that these mirrors are bronze...Mirrors of Eternity: A Cultural Exhibition of Ancient Chinese Bronze Mirrors
    – justCal
    Dec 28, 2021 at 18:36
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    Bronze has a blue/green corrosion product in air ; I se no blue \/green in photo
    – blacksmith37
    Dec 28, 2021 at 19:14
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    I'm just going by the description the museum posts for the display. Perhaps the backs have been treated somehow...they are the decorative side after all. Some of them obviously look painted or enameled.
    – justCal
    Dec 28, 2021 at 19:16
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    @JustCal is correct, the vast majority of Chinese mirrors were indeed bronze. Bronze often but does not necessarily oxidize blue or green platina. For such mirrors, black platina is particularly treasured and generally thought to be due to high tin content in the bronze alloy used.
    – Semaphore ♦
    Dec 29, 2021 at 9:41
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    @blacksmith37 I've seen several bronze cannons that are black with green around edges and recesses but you have to look closely to see the green (your face literally inches from the surface). From any normal distance all the bronze cannons I've seen are black. Maybe if bronze is old enough the patina turns black? I don't know, I'm not a chemist. All I know is all the bronze cannons I've in real life seen are black.
    – slebetman
    Dec 30, 2021 at 14:07

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