Since ancient times, mirrors have been produced, but it is most of the times small objects, made out of polished convex metal. During the 13th century, we saw the first mirrors to be covered with a led or silver layer. The technique of the mercury mirror was developped in Northern Europe during the 14th century, but it was the Venetians who, during the 15th century really developped the technique. It involved producing a flat and polished glass and covering it with a layer of mercury, that is to say a silver metal, that reflects. This silver covering was used on mirrors for 400 years.
Around 1672, France was able to produce high quality mirrors itself and thus stopped importing Venetian mirrors. One of the most famous examples in France is the hall of mirrors (in Versailles Palace) which has 357 mirrors made out of mercury, and of large dimensions, made by a manufacturer in Saint-Gobain around 1684. 80% are originals, we can tell this by the fact they they are not beveled.
However mercury is a chemical element that is very toxic and dangerous. The average life of a mirror manufacturer, subject to mercury vapors, was 10 years and fatal accidents were frequent. The technique was thus banned in 1850, coinciding with the discovery of another silver covering technique. Today, mirrors are covered with a layer of aluminium.