The first man-made mirrors were polished metal. They did not reflect incredibly well (compared with more modern counterparts), but could be functional. They were expensive, so not many people owned one.
Later, people began coating one side of a pane of glass with substances. Metals became the norm for this, including lead in cheap mirrors and silver in expensive mirrors. I heard of murcury used in some cases. Today, a different backing is used (more of a metallic paint than any precious metal).
But those metals would corrose over time. The flaws within the incredibly thin surface may not be noticeable in bright light, or when other strong reflections are the focus of the person looking at the mirror. However, low light conditions, focusing at the mirror's surface, long-exposure near-dark photography, and the like, could make these flaws more apparent.