Throughout history mirrors have been prized and valuable objects. Framed antique mirrors may still retain their original mirror glass which is highly desirable even if the silvering has deteriorated over time. A basic understanding of historic mirrors and how they were made will kelp identify and further the appreciation of them.
Since the earliest times mirrored surfaces have been made from various materials. The history of mirrors and their production is a fascinating subject full of rivalries, intrigue and the entanglements of governments and royalty.
The best undistorted images require highly reflective, flat smooth surfaces. The earliest mirrors were polished rocks. By 3000 BC polished metals such as silver, tin and bronze became more common. The Roman author Pliny the elder in 77 AD refers to mirrors being made with blown pieces of glass with a layer of a metal like zinc or silver on one side which would be the similar to the fabrication of mirrors of today. Until about 1500 the majority of European mirrors were still made of polished metal.
In the 16th century improvements in the glass making process made possible increasingly larger and better quality clear flat glass which led to the improved reflectivity of mirrors. There was great competition between the glassmakers to produce the largest and clearest plates.
There were two basic ways of making a flat piece of glass at that time. One process used glass blowing techniques to blow an elongated molten glass cylinder. The ends were cut off the cylinder while still hot and then the cylinder was cut along it’s length, flattened and annealed, which is a slow cooling process that relieves the internal stresses in the formed glass. Then it could be further rolled out or smoothed. The disadvantage of this process is the relatively small size of glass produced.
The second process invented around 1687 used a casting process which involved pouring the molten glass onto a flat metal surface with low sides the thickness of the plate. To be suitable for mirrors the glass when hardened was laboriously ground with coarse sandstone and other abrasive materials through successively finer grits and finally polished with Tripoli powder.
For more information on the second process, see Antique Mirrored Glass Part II.