Because copper itself has a certain role in resolving evil spirits and town houses. Combined with a mirror, the effect is better. In particular, the current polishing technology is higher, which can polish the mirror surface of bronze mirror more smoothly and clearly, and the effect is better than that of ancient times.
Although the mercury mirror also has the function of reflection, it is not as effective as the bronze mirror because of its weak role in exorcising evil spirits.
However, if only from the perspective of reflecting brake gas, both can be used, and the effect is not much different. It's just that bronze mirrors have more additional effects, so I say bronze mirrors have better effects. For reference.