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Reflections on Mirror Coating Materials

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Reflections on Mirror Coating Materials HISTORICAL NOTE mirrors made of solid metal, a highly re­ flective alloy of tin and copper called "spec­ Reflections on Mirror ulum," which became the standard for as­ tronomical mirrors for the next two centu­ ries. In the mid-19th century, telescope maker Coating Materials Sir Howard Grubb wrote: "The composi­ tion of metallic mirrors of the present day differs very little from that used by Sir Isaac The Historical Note in the October 1988 mirror making eventually slipped out of Newton (1643-1727). Many and different MRS BULLETIN described the background the tightly controlled Venetian guilds, and alloys have been suggested, some includ­ and problems involved in casting some of London and Paris also became important ing silver or nickel or arsenic, but there is the giant glass mirror blanks used in the mirror manufacturing centers. But mirrors little doubt that the best alloy [speculum] is world's largest reflecting telescopes. This themselves remained extremely expen­ made with four atoms of copper, and one Historical Note takes an about-face to look sive, especially large ones. of tin." at the other side of a mirror—the reflective In the 16th century, Venetian mirror coating that makes it what it is. Reflective makers introduced a major new technique coating materials for mirrors have changed when they began to use an amalgam of tin through the ages, in material and process, and mercury as a reflective backing. The art of mirror making going from polished metallic surfaces to Guildsmen would lay out a sheet of tin foil thin metal films applied by chemical and on a horizontal blanket over a flat surface, seemed at a standstill physical methods. and then spread clean mercury on the foil. until 1835, when German People have been using various reflect­ A covering of paper kept the mercury ing tools to see themselves since the first clean. Workers then cleaned a sheet of chemist Baron Justus von human looked at his image in a still pool of glass and lowered it onto the paper—but Liebig invented water. The Egyptians used polished they drew away the paper just as the glass bronze, gold, and silver mirrors as early as was about to touch, which allowed the "silvering." 2500 B.C. The ancient Greeks added han­ clean glass to contact the clean mercury. dles to their mirrors, while the Etruscans Heavy weights placed on the glass sheet used round and oval mirrors (6th Century squeezed out the excess mercury, and the Speculum metal disks were difficult to B. C.). The Romans copied both styles. In amalgam adhered to the glass. Along with configure, though. The alloy was brittle the first century A.D. the Roman naturalist improved methods of pouring plate glass and difficult to cast, and still reflected only Pliny the Elder described how the inhabit­ sheets, this technique created highly re­ about 60% of the incident light even when ants of the Phoenician city of Sidon had flective mirrors of considerable size and freshly polished. Efforts to improve the learned how to coat glass with silver or tin, quality. casting and figuring of speculum mirrors but this type of mirror seems to have been While perfect for most popular uses, continued long after Newton's death in little used, and the art died out in the Dark glass mirrors backed with reflective coat­ 1727. British astronomer William Herschel Ages. ings were useless for precise scientific pur­ made his own speculum mirrors because poses. For careful measurements the they were too expensive for him to buy, reflective coating must be on the front of a and in so doing he advanced the art of In the late 12th and early mirror, not the back, to eliminate light working with the alloy. The largest spec­ losses and distortions, as well as spurious ulum mirror ever made was 72 inches in 13th centuries, craftsmen reflections and aberrations, caused by the diameter, constructed by Irish amateur as­ learned how to apply thickness of the glass. Using the tin and tronomer Lord Rosse in 1845. mercury amalgam method to coat the front The major problem with speculum was metallic backings to flat surfaces of scientific mirrors yielded results that the copper caused the polished sur­ glass, using steel and that were dull and irregular. face to tarnish rapidly, necessitating fre­ In 1663, Isaac Newton began experi­ quent repolishings. These repolishings silver almost exclusively. menting with optics and reflections. He destroyed the careful configuring of the operated under the assumption that all curved mirror surfaces—every time the substances possessed the same "disper­ mirror was polished it had to be remade, in In the late 12th and early 13th centuries, sive power," which (erroneously) led him effect. In addition to the cost involved, this craftsmen learned how to apply metallic to conclude that it was impossible to elimi­ removed an astronomer's telescope from backings to flat glass, using steel and silver nate or suppress chromatic aberration in service for weeks, causing disastrous gaps almost exclusively. The city of Nuremberg any instrument consisting of a system of in long-term observations. formed its own guild of mirror makers in lenses. He turned his attention toward us­ The art of mirror making seemed at a 1373 and, by the time of the Renaissance, ing magnifying mirrors as the main optical standstill until 1835, when the highly suc­ Nuremberg, along with Venice, had component of his telescope. cessful German chemist Baron Justus von earned a reputation as a major center of Magnifying mirrors formed on curved Liebig invented "silvering," a chemical mirror production. The mirrors produced pieces of glass had initially appeared in the process of coating a glass surface with me­ in Venice were famed for their ornate 13th century. Newton produced his first re­ tallic silver. During his career, Baron von frames and beveled edges, and Venice be­ flecting telescope in 1668, but it was very Leibig made so many important contribu­ came Europe's major supplier of mirrors small—6.25 inches long, with a mirror only tions to organic chemistry, biochemistry, for several centuries. The techniques of 1.25 inches in diameter! He worked with agricultural chemistry, and chemistry edu- 68 MRS BULLETIN/APRIL 1989 Downloaded from https://www.cambridge.org/core. IP address: 170.106.40.40, on 25 Sep 2021 at 12:31:28, subject to the Cambridge Core terms of use, available at https://www.cambridge.org/core/terms. https://doi.org/10.1557/S0883769400055147 HISTORICAL NOTE cation that his invention of the silvering In some optical and physical applica­ to optimize reflection in one specific wave­ process receives only incidental mention in tions, mirrors must be only partly silvered length range at the expense of others. his biographies. to reflect and transmit specific amounts of Reknowned French optician Colonel However, for mirror makers the silvering the incident light. This must be done Charles Deve wrote in 1942: "The silvering process revolutionized the industry. In sil­ slowly enough for the manufacturer to of mirrors has become such a normal prac­ vering a mirror, the glass is coated with an control the precise amount of coating de­ tice that it might be believed to be finally ammonium hydroxide-silver nitrate solu­ posited. "Sputtering," or high-voltage dis­ settled. Nevertheless other, very recent, tion that is reduced to metallic silver. The charge between electrodes in a vacuum, is methods of metallisation seem likely to reducing agent can be invert sugar, Ro- also used to deposit metallic coatings on modify the technique of mirror making chelle salt, or formaldehyde, which then mirrors. profoundly." Indeed, the chemical silver­ precipitates a layer of metallic silver onto Many other types of coatings are used in ing process is still used today, although it the glass. The process was originally quite mirrors and scientific optical systems, in­ has been supplanted with other more pre­ hazardous because the reaction created cluding antireflection coatings, protective cise techniques. highly unstable ammonium nitrate. coatings for the aluminized surfaces of mir­ Copper and gold can be deposited on rors, or reflective coatings of other metals KEVIN J. ANDERSON mirrors by similar chemical reduction tech­ niques. The thickness of gold films can be manipulated by controlling the heating of an organic gold compound. A film of lead sulfide deposited on glass will yield a dark- colored mirror. The silvering process could be used to Anatech's R.F. Planar Magnetron and D.C. Sputtering System provide a uniform coating over any shape of glass surface. In 1856 the German as­ tronomer Karl August von Steinheil devel­ THIN FILM DEPOSITION SYSTEM oped a method to use silvering to coat glass blanks for telescope mirrors. The major ad­ vantage of using silvered glass blanks in­ stead of solid speculum mirrors, von Steinheil found, was that when the silver film became tarnished, it could be quickly removed with acid and a bright new coat­ ing applied—all without affecting the care­ fully configured surface of the glass blank. Fresh silver coatings have a reflectivity of about 96% throughout the visible spec­ trum, the highest of any metal, but begin to drop off at a wavelength of about 4,500 A and become useless in the ultraviolet re­ gions. In 1935 this, along with the speed that silver tarnishes, prompted John Strong in California to develop a process to deposit aluminum films on mirrors. Aluminum can be deposited on a mirror surface by electrically heating an alumi­ num filament in a vacuum and allowing the aluminum to evaporate onto the glass. Anatech LTD offers the Research XII RFPM research sputtering system Although aluminum has a slightly lower for this film deposition.
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