Mirrors have been around for millenia, but until the 1st century C.E. they were made out of highly-polished metal or volcanic glass. These mirrors produced a poor reflection and were prone to tarnishing over time. The upkeep required to maintain a metallic mirror's quality was extensive, and so this style of mirror is almost unheard-of these days.
Experiments with glass-face mirrors were made, but after the fall of the Roman Empire much of the knowledge was lost and wasn't rediscovered until the 11th century in the Moorish Empire. A thousand-years of mirror development was lost to the world!
To make a perfect glass mirror, there were two major technological hurdles to overcome. First, a perfectly clear sheet of glass was necessary to protect the mirror surface while still allowing light to pass through cleanly. Second, a suitable reflective surface needed to be developed that would cling to the glass and the back of the mirror.
The second problem was solved with the discovery of a tin-mercury amalgam that hardened into a reflective surface. No one knows where this technique was first developed, but it soon became widespread. All glass mirrors made prior to the 19th century were made using mercury and tin. Later, much safer techniques using silver and aluminum replaced the mercury.
The problem of producing clear plate glass is where Venice comes in. Venice, and especially the nearby island of Murano, were the foremost centers of glass production for centuries. It was Venetian artists who perfected making clear plate glass. They also made modifications to the tin-mercury amalgam, adding gold and bronze in special proportions to make the images reflected even more beautiful. Early Venetian mirrors that were made using all these techniques cost about as much as a ship, and were highly sought after by royalty.
Once the technical problems of mirror production were conquered, it was time for the artisians to work their magic to make Venetian mirrors the highpoint of luxury. Early mirror frames used carved materials such as wood or ivory. Artists got to be so talented at this that sumptuary laws were passed forbidding certain carving styles from being possessed by lower-class people.
Murano Island was the focal point for Venice's glass production. By the 16th century, nearly half of the island's 7,000 inhabitants were involved in the glass-making industry, and it is still a glass-making center today. Murano glass began making its way into the frames of their high-quality mirrors, using a variety of lampworking techniques such as smalto and millefiore.
Murano's techniques were so prized that master glass artisans were forbidden to leave by law, and it was only with great bribes that a few were convinced to travel to other countries and share their secrets. Today, Murano glass has its own trademark of origin, certifying that the glass came from this historical location.