From my research about the history of mirror technology, I think a mirror made of brittle white metal would be a reasonable conjecture but would have been very expensive.
My main point of curiosity now: where I'm from (the US), there's the superstition that breaking a mirror is bad luck. I have no particular reason to think this would be the case in the setting, but mirrors are such a common point of superstition, I wanted to check up on it. In my own online searches, I found a few articles where Koreans replied that they also had the bad luck superstition, but it was piecemeal enough that I wasn't confident about going with it and thought maybe the superstition had been recently imported.
So, does anyone know if there are any longstanding superstitions in Korea against breaking mirrors or otherwise involving mirrors?
For the mirror itself, I'm looking mainly at the Wikipedia entries for "Mirror," "Bronze Mirror," and "Speculum Metal."
For the superstition, I Googled "korea mirror superstition."