Silvering

Silvering is the chemical process of coating glass with a reflective substance. When glass mirrors first gained widespread usage in Europe during the 16th century, most were made of an amalgam of tin and mercury, but by the 19th century mirrors were commonly made through a process by which silver was coated onto a glass surface.^[1] Today, sputtering powdered aluminium or other compounds are more often used for this purpose, although the process maintains the name "silvering".

History

The earliest mirrors were made from polished obsidian during the Stone Age.^[2] But by the Bronze Age most cultures had adopted mirrors made from polished discs of bronze, copper or other metals.^[2] Such metal mirrors remained the norm through to Greco-Roman Antiquity and throughout the Middle Ages in Europe.^[3] It was not until the 1st century CE that glass mirrors began to show up, now believed to have originated in Sidon, in what is modern-day Lebanon.^[4] In Ptolemaic Egypt small glass mirrors were made backed by lead, tin, or antimony. In the early 10th century, the Iranian scientist al-Razi described ways of silvering and gilding in a book on alchemy, but this was not done for the purpose of making mirrors.^[2]

In 1835 German chemist Justus von Liebig developed a process for depositing silvering on the rear surface of a piece of glass that gained wide acceptance after improvement by Liebig in 1856.^[5][6] The process was further refined and made easier by French chemist Petitjean (1857). This reaction is a variation of the Tollens' reagent for aldehydes. A diamminesilver(I) solution is mixed with a sugar and sprayed onto the glass surface. The sugar is oxidized by silver(I), which is itself reduced to silver(0), i.e. elemental silver, and deposited onto the glass.

In 1856-57 Karl August von Steinheil and Léon Foucault introduced the process of depositing an ultra thin layer of silver on front surface of a piece of glass, making the first optical quality first surface glass mirrors, replacing the use of speculum metal mirrors in reflecting telescopes.^[7] These techniques soon became standard for technical equipment.

Modern silvering process

In modern aluminum silvering, a sheet of glass is placed in a vacuum chamber with electrically heated nichrome coils that can sublime aluminum. In a vacuum, the hot aluminum atoms travel in straight lines. When they hit the surface of the mirror, they cool and stick. Some mirror makers evaporate a layer of quartz on the mirror; others expose it to pure oxygen or air in an oven so that it will form a tough, clear layer of aluminum oxide.

Mirrors made by this method are classified as either back-silvered, with the silvered layer viewed through the glass; or front-silvered, (called a first surface mirror) with the reflective layer on the surface towards the incoming light or image. Most common mirrors are back-silvered, since this protects the fragile reflective layer from corrosion, scratches, and other damage. However, precision optical surfaces normally need the reflective material on the front surface of the glass to avoid introducing optical aberrations. First surface mirror use the substrate to keep form. There are optical mirrors such as mangin mirrors that are back-silvered (reflective coating on the rear surface) as part of their optical design.

See also

• Dielectric mirror
• List of telescope parts and construction
• Mercury glass
• Mercury silvering
• Metallizing
• Optical coating

References

1. ^ "mirrors". designboom. http://www.designboom.com/history/mirror.html. Retrieved 2009-08-14. 
2. ^ ^{a b c} Fioratti, Helen. "The Origins of Mirrors and their uses in the Ancient World". L'Antiquaire & the Connoisseur. http://www.lantiquaire.us/origins-of-mirrors.html. Retrieved 2009-08-14. 
3. ^ "A Brief History of Mirrors". Encyclopedia Britanica. http://www.mirrorresilvering.com/a_brief_history_of_mirrors.htm. Retrieved 2009-08-14. 
4. ^ Holland, Patricia. "Mirrors". Isnare Free Articles. http://www.isnare.com/?aid=387476&ca=Advice. Retrieved 2009-08-14. 
5. ^ Justus Liebig (1835). "Ueber die Producte der Oxydation des Alkohols". Annalen der Chemie 14 (2): 133. doi:10.1002/jlac.18350140202. 
6. ^ Justus Liebig (1856). "Ueber Versilberung und Vergoldung von Glas". Annalen der Chemie und Pharmacie 98 (1): 132–139. doi:10.1002/jlac.18560980112. 
7. ^ mazing-space.stsci.edu - Era of Huge Reflectors

External links

• diy mirror / mirroring / silvering