The first mirrors used by people were most likely pools of dark, still water, or water collected in a primitive vessel of some sort. The requirements for making a good mirror were a surface with a very high degree of flatness (preferably but not necessarily with high reflectivity), and a surface roughness smaller than the wavelength of the light. The earliest manufactured mirrors were pieces of polished stone such as obsidian, a naturally occurring volcanic glass. Examples of obsidian mirrors found in Anatolia (modern-day Turkey) have been dated to around 6000 BC.[1] Mirrors of polished copper were crafted in Mesopotamia from 4000 BC,[1] and in ancient Egypt from around 3000 BC.[2] Polished stone mirrors from Central and South America date from around 2000 BC onwards.[1] In China, bronze mirrors were manufactured from around 2000 BC,[3] some of the earliest bronze and copper examples being produced by the Qijia culture. Mirrors made of other metal mixtures (alloys) such as copper and tin speculum metal may have also been produced in China and India.[4] Mirrors of speculum metal or any precious metal were hard to produce and were only owned by the wealthy.[5] These stone and metal mirrors could be made in very large sizes, but were difficult to polish and get perfectly flat; a process that became more difficult with increased size; so they often produced warped or blurred images. Stone mirrors often had poor reflectivity compared to metals, yet metals scratch or tarnish easily, so they frequently needed polishing. Depending upon the color, both often yielded reflections with poor color rendering.[6]
Glass was a desirable material for mirrors. Because the surface of glass is naturally smooth, it produces reflections with very little blur. In addition, glass is very hard and scratch resistant. However, glass by itself has little reflectivity, so people began coating it with metals to increase the reflectivity. Metal-coated glass mirrors are said by the Roman scholar Pliny the Elder to have been invented in Sidon (modern-day Lebanon) in the first century AD, although no archeological evidence of them date from before the third century.[7] According to Pliny, the people of Sidon developed a technique for creating crude mirrors by coating blown glass with molten lead.[8][9] Glass mirrors backed with gold leaf are mentioned by Pliny in his Natural History, written in about 77 AD.[10] Because there were few ways to make a smooth piece of glass with a uniform thickness, these ancient glass-mirrors were made by blowing a glass bubble, and then cutting off a small, circular section, producing mirrors that were either concave or convex. These circular mirrors were typically small, from only a fraction of an inch to as much as eight inches in diameter.[11] These small mirrors produced distorted images, yet were prized objects of high value. These ancient glass mirrors were very thin, thus very fragile, because the glass needed to be extremely thin to prevent cracking when coated with a hot, molten metal. Due to the poor quality, high cost, and small size of these ancient glass mirrors, solid metal-mirrors primarily of steel were usually preferred until the late nineteenth century.[12]
Parabolic mirrors were described and studied in classical antiquity by the mathematician Diocles in his work On Burning Mirrors.[13] Ptolemy conducted a number of experiments with curved polished iron mirrors,[14] and discussed plane, convex spherical, and concave spherical mirrors in his Optics.[15] Parabolic mirrors were also described by the physicist Ibn Sahl in the 10th century,[16] and Ibn al-Haytham discussed concave and convex mirrors in both cylindrical and spherical geometries,[17] carried out a number of experiments with mirrors, and solved the problem of finding the point on a convex mirror at which a ray coming from one point is reflected to another point.[18] By the 11th century, clear glass mirrors were being produced in Moorish Spain.[19][verification needed]
In China, people began making mirrors by coating metallic objects with silver-mercury amalgams as early as 500 AD. This was accomplished by coating the mirror with the amalgam, and then heating it until the mercury boiled away, leaving only the silver behind.[20]
The problems of making metal-coated, glass mirrors was due to the difficulties in making glass that was very clear, as most ancient glass was tinted green with iron. This was overcome when people began mixing soda, limestone, potash, manganese, and fern ashes with the glass. There was also no way for the ancients to make flat panes of glass with uniform thicknesses. The earliest methods for producing glass panes began in France, when people began blowing glass bubbles, and then spinning them rapidly to flatten them out into plates from which pieces could be cut. However, these pieces were still not uniform in thickness, so produced distorted images as well. A better method was to blow a cylinder of glass, cut off the ends, slice it down the center, and unroll it onto a flat hearth. This method produced the first mirror-quality glass panes, but it was very difficult and resulted in a lot of breakage. Even windows were primarily made of oiled paper or stained glass, until the mid-nineteenth century, due to the high cost of making clear, flat panes of glass.[21]
The method of making flat panes of clear glass from blown cylinders began in Germany and evolved through the Middle Ages, until being perfected by the Venetians in the sixteenth century. The Venetians began using lead glass for its crystal-clarity and its easier workability. Some time during the early Renaissance, European manufacturers perfected a superior method of coating glass with a tin-mercury amalgam, producing an amorphous coating with better reflectivity than crystalline metals and causing little thermal shock to the glass.[22] The exact date and location of the discovery is unknown, but in the sixteenth century, Venice, a city famed for its glass-making expertise, became a center of mirror production using this new technique. Glass mirrors from this period were extremely expensive luxuries.[23] For example, in the late seventeenth century, the Countess de Fiesque was reported to have traded an entire wheat farm for a mirror, considering it a bargain. These Venetian mirrors were limited in size to a maximum area of around 40 inches (100 cm) square, until modern glass panes began to be produced during the Industrial Revolution.[24] The Saint-Gobain factory, founded by royal initiative in France, was an important manufacturer, and Bohemian and German glass, often rather cheaper, was also important.
The invention of the silvered-glass mirror is credited to German chemist Justus von Liebig in 1835.[25] His process involved the deposition of a thin layer of metallic silver onto glass through the chemical reduction of silver nitrate. This silvering process was adapted for mass manufacturing and led to the greater availability of affordable mirrors. In the modern age, mirrors are often produced by the wet deposition of silver (or sometimes aluminum via vacuum deposition)[26] directly onto the glass substrate.
