To be suitable for mirrors usually the best quality of glass produced was used. To become a mirror it needs to have a reflective backing. The 16th century glass makers on the island of Murano are generally credited with the process of slivering the back of mirrors with an amalgam of tin and mercury. The majority of mirrors were produced in this manner until the mid 1800s. In this process, paper thin sheets of tin were placed on large flat tables and then covered with mercury. The glass was placed on top, the tin and mercury reacted together and bonded to the glass. The glass was turned over, the surface smoothed and the excess mercury was drained off. This produced a near perfect reflection with any limitations being in the glass itself. This was a hazardous production and today one must use caution and protection when handling unframed pieces of mercury mirror.
Improvements in the European glass making process continued and in the mid 1800s a new process of silvering allowed a thin reflective film using an aqueous solution of silver to be deposited on the glass. This did not completely replace the mercury silvered mirror process until the early 20th century. In America most mirror glass was imported from Europe until the mid 1800s.
Often until fairly recently, antique mirror glass was not valued for it’s aesthetic qualities. The silvering often deteriorates becoming darkened or cloudy over time through oxidation. This tarnishing can be over the whole surface or localized as blotches or streaks. Sometimes abrasions or scratches on the the unprotected back of the mirror will lead to further degeneration. All too often these antique mirrors were replaced with new mirrors. This is particularly unfortunate when the new mirror is set into an antique frame.
Unless the mirror silvering is completely destroyed, a mirror even with substantial discoloration can be seen as having desirable qualities and appeal. The character of an old mirror harmonizes with the aged surface of an antique frame while a new mirror can seem too bright and gaudy which can detract from the charm of the frame. If possible, the best recourse is to find an antique mirror glass to fit into an antique frame but these are often very difficult to find. Another option is to find reproduction antique mirror which is readily available. The quality of these mirrors can vary considerably so it is helpful to study the appearance of authentic antique mirrors to select the most appropriate. This is not to suggest that a new mirror is entirely wrong for an antique frame but to offer some considerations and options.
To help identify an antique mirrored glass one can look at the glass itself to see if it has any imperfections such as waviness or small bubbles but this is not always a sure sign as occasionally one will encounter modern blown glass that has been silvered. Oxidation of the silvering is not necessarily an indication of age due to the reproduction mirrors on the market but the true antique piece will often be tarnished more unevenly with a more random and natural looking variation. The oxidation can be more pronounced on the bottom due to the years of cleaning with too much moisture collecting at the bottom of the mirror being kept damp through proximity with the frame. Back scratches or surface scratches even if they are very fine from cleaning can indicate an older mirror. Mercury silvered mirrors age with a distinctive sparkling effect that almost looks like crystals. This can be seen overall or in localized areas, again likely at the bottom of the mirror. Once the appearance of this condition is understood it is relatively easy to identify and will be a positive identification of an antique mirror glass but not of it’s exact vintage. The degree of the darkening or oxidation of an antique mirror may give some indication of age since the process is cumulative and the darker the image the older the mirror may be. Studying true examples of antique mirror glass will lead to a better understanding of the various ways they age and lead to greater appreciation.