Silvering
Encyclopedia
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. Today, sputtering
powdered aluminium
or other compounds are more often used for this purpose, although the process may either maintain the name "silvering" or be referred to as aluminising.
during the Stone Age
, but by the Bronze Age
most cultures had adopted mirrors made from polished discs of bronze
, copper
or other metals. Such metal mirrors remained the norm through to Greco-Roman Antiquity and throughout the Middle Ages
in Europe
. 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
. In Ptolemaic Egypt
small glass mirrors were made backed by lead
, tin, or antimony
. In the early 10th century, the Iran
ian 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.
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. 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 telescope
s. These techniques soon became standard for technical equipment.
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 mirror
s that are back-silvered (reflective coating on the rear surface) as part of their optical design.
The silvering on a second surface mirror is often actual silver.
The "silvering" on precision optical instruments such as telescopes is usually aluminum, even though silver has the best reflectivity in the visible spectrum and oxidizes slower.
Such instruments are required to use front-surface mirror to avoid seeing a "ghost" star next to every bright star.
Silver alone is unsuitable for front surface mirrors because it quickly oxidizes to a low-reflectivity tarnish.
Although aluminum oxidizes even more quickly, the thin aluminum oxide (sapphire) layer is transparent, and so the high-reflectivity underlying aluminum stays visible.
The "silvering" on infrared instruments is usually gold. It has the best reflectivity in the infrared spectrum, and has high resistance to oxidation and corrosion.
Chemistry
Chemistry is the science of matter, especially its chemical reactions, but also its composition, structure and properties. Chemistry is concerned with atoms and their interactions with other atoms, and particularly with the properties of chemical bonds....
process of coating glass
Glass
Glass is an amorphous solid material. Glasses are typically brittle and optically transparent.The most familiar type of glass, used for centuries in windows and drinking vessels, is soda-lime glass, composed of about 75% silica plus Na2O, CaO, and several minor additives...
with a reflective substance. When glass mirrors first gained widespread usage in Europe during the 16th century, most were made of an amalgam
Amalgam (chemistry)
An amalgam is a substance formed by the reaction of mercury with another metal. Almost all metals can form amalgams with mercury, notable exceptions being iron and platinum. Silver-mercury amalgams are important in dentistry, and gold-mercury amalgam is used in the extraction of gold from ore.The...
of tin
Tin
Tin is a chemical element with the symbol Sn and atomic number 50. It is a main group metal in group 14 of the periodic table. Tin shows chemical similarity to both neighboring group 14 elements, germanium and lead and has two possible oxidation states, +2 and the slightly more stable +4...
and mercury
Mercury (element)
Mercury is a chemical element with the symbol Hg and atomic number 80. It is also known as quicksilver or hydrargyrum...
, but by the 19th century mirrors were commonly made through a process by which silver
Silver
Silver is a metallic chemical element with the chemical symbol Ag and atomic number 47. A soft, white, lustrous transition metal, it has the highest electrical conductivity of any element and the highest thermal conductivity of any metal...
was coated onto a glass surface. Today, sputtering
Sputtering
Sputtering is a process whereby atoms are ejected from a solid target material due to bombardment of the target by energetic particles. It is commonly used for thin-film deposition, etching and analytical techniques .-Physics of sputtering:...
powdered aluminium
Aluminium
Aluminium or aluminum is a silvery white member of the boron group of chemical elements. It has the symbol Al, and its atomic number is 13. It is not soluble in water under normal circumstances....
or other compounds are more often used for this purpose, although the process may either maintain the name "silvering" or be referred to as aluminising.
History
The earliest mirrors were made from polished obsidianObsidian
Obsidian is a naturally occurring volcanic glass formed as an extrusive igneous rock.It is produced when felsic lava extruded from a volcano cools rapidly with minimum crystal growth...
during the Stone Age
Stone Age
The Stone Age is a broad prehistoric period, lasting about 2.5 million years , during which humans and their predecessor species in the genus Homo, as well as the earlier partly contemporary genera Australopithecus and Paranthropus, widely used exclusively stone as their hard material in the...
, but by the Bronze Age
Bronze Age
The Bronze Age is a period characterized by the use of copper and its alloy bronze as the chief hard materials in the manufacture of some implements and weapons. Chronologically, it stands between the Stone Age and Iron Age...
most cultures had adopted mirrors made from polished discs of bronze
Bronze
Bronze is a metal alloy consisting primarily of copper, usually with tin as the main additive. It is hard and brittle, and it was particularly significant in antiquity, so much so that the Bronze Age was named after the metal...
, copper
Copper
Copper is a chemical element with the symbol Cu and atomic number 29. It is a ductile metal with very high thermal and electrical conductivity. Pure copper is soft and malleable; an exposed surface has a reddish-orange tarnish...
or other metals. Such metal mirrors remained the norm through to Greco-Roman Antiquity and throughout the Middle Ages
Middle Ages
The Middle Ages is a periodization of European history from the 5th century to the 15th century. The Middle Ages follows the fall of the Western Roman Empire in 476 and precedes the Early Modern Era. It is the middle period of a three-period division of Western history: Classic, Medieval and Modern...
in Europe
Europe
Europe is, by convention, one of the world's seven continents. Comprising the westernmost peninsula of Eurasia, Europe is generally 'divided' from Asia to its east by the watershed divides of the Ural and Caucasus Mountains, the Ural River, the Caspian and Black Seas, and the waterways connecting...
. It was not until the 1st century CE
Common Era
Common Era ,abbreviated as CE, is an alternative designation for the calendar era originally introduced by Dionysius Exiguus in the 6th century, traditionally identified with Anno Domini .Dates before the year 1 CE are indicated by the usage of BCE, short for Before the Common Era Common Era...
that glass mirrors began to show up, now believed to have originated in Sidon
Sidon
Sidon or Saïda is the third-largest city in Lebanon. It is located in the South Governorate of Lebanon, on the Mediterranean coast, about 40 km north of Tyre and 40 km south of the capital Beirut. In Genesis, Sidon is the son of Canaan the grandson of Noah...
, in what is modern-day Lebanon
Lebanon
Lebanon , officially the Republic of LebanonRepublic of Lebanon is the most common term used by Lebanese government agencies. The term Lebanese Republic, a literal translation of the official Arabic and French names that is not used in today's world. Arabic is the most common language spoken among...
. In Ptolemaic Egypt
Egypt
Egypt , officially the Arab Republic of Egypt, Arabic: , is a country mainly in North Africa, with the Sinai Peninsula forming a land bridge in Southwest Asia. Egypt is thus a transcontinental country, and a major power in Africa, the Mediterranean Basin, the Middle East and the Muslim world...
small glass mirrors were made backed by lead
Lead
Lead is a main-group element in the carbon group with the symbol Pb and atomic number 82. Lead is a soft, malleable poor metal. It is also counted as one of the heavy metals. Metallic lead has a bluish-white color after being freshly cut, but it soon tarnishes to a dull grayish color when exposed...
, tin, or antimony
Antimony
Antimony is a toxic chemical element with the symbol Sb and an atomic number of 51. A lustrous grey metalloid, it is found in nature mainly as the sulfide mineral stibnite...
. In the early 10th century, the Iran
Iran
Iran , officially the Islamic Republic of Iran , is a country in Southern and Western Asia. The name "Iran" has been in use natively since the Sassanian era and came into use internationally in 1935, before which the country was known to the Western world as Persia...
ian scientist al-Razi
Al-Razi
Muhammad ibn Zakariyā Rāzī , known as Rhazes or Rasis after medieval Latinists, was a Persian polymath,a prominent figure in Islamic Golden Age, physician, alchemist and chemist, philosopher, and scholar....
described ways of silvering and gilding
Gilding
The term gilding covers a number of decorative techniques for applying fine gold leaf or powder to solid surfaces such as wood, stone, or metal to give a thin coating of gold. A gilded object is described as "gilt"...
in a book on alchemy
Alchemy
Alchemy is an influential philosophical tradition whose early practitioners’ claims to profound powers were known from antiquity. The defining objectives of alchemy are varied; these include the creation of the fabled philosopher's stone possessing powers including the capability of turning base...
, but this was not done for the purpose of making mirrors.
In 1835 German
Germany
Germany , officially the Federal Republic of Germany , is a federal parliamentary republic in Europe. The country consists of 16 states while the capital and largest city is Berlin. Germany covers an area of 357,021 km2 and has a largely temperate seasonal climate...
chemist
Chemist
A chemist is a scientist trained in the study of chemistry. Chemists study the composition of matter and its properties such as density and acidity. Chemists carefully describe the properties they study in terms of quantities, with detail on the level of molecules and their component atoms...
Justus von Liebig
Justus von Liebig
Justus von Liebig was a German chemist who made major contributions to agricultural and biological chemistry, and worked on the organization of organic chemistry. As a professor, he devised the modern laboratory-oriented teaching method, and for such innovations, he is regarded as one of the...
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. The process was further refined and made easier by French chemist Petitjean (1857). This reaction is a variation of the Tollens' reagent
Tollens' reagent
Tollens' reagent is a chemical reagent most commonly used to determine whether a known carbonyl-containing compound is an aldehyde or a ketone. It is usually ammoniacal silver nitrate, but can also be other mixtures, as long as aqueous diamminesilver complex is present...
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
Silver
Silver is a metallic chemical element with the chemical symbol Ag and atomic number 47. A soft, white, lustrous transition metal, it has the highest electrical conductivity of any element and the highest thermal conductivity of any metal...
, and deposited onto the glass.
In 1856-57 Karl August von Steinheil and Léon Foucault
Léon Foucault
Jean Bernard Léon Foucault was a French physicist best known for the invention of the Foucault pendulum, a device demonstrating the effect of the Earth's rotation...
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
Speculum metal
Speculum metal is a mixture of around two-thirds copper and one-third tin making a white brittle alloy that can be polished to make a highly reflective surface. It is used primarily to make different kinds of mirrors including early reflecting telescope optical mirrors...
mirrors in reflecting telescope
Reflecting telescope
A reflecting telescope is an optical telescope which uses a single or combination of curved mirrors that reflect light and form an image. The reflecting telescope was invented in the 17th century as an alternative to the refracting telescope which, at that time, was a design that suffered from...
s. These techniques soon became standard for technical equipment.
Modern silvering process
In modern aluminum silvering, a sheet of glass is placed in a vacuumVacuum
In everyday usage, vacuum is a volume of space that is essentially empty of matter, such that its gaseous pressure is much less than atmospheric pressure. The word comes from the Latin term for "empty". A perfect vacuum would be one with no particles in it at all, which is impossible to achieve in...
chamber with electrically heated nichrome
Nichrome
Nichrome is a non-magnetic alloy of nickel, chromium, and often iron, usually used as a resistance wire. Patented in 1905, it is the oldest documented form of resistance heating alloy. A common alloy is 80% nickel and 20% chromium, by mass, but there are many others to accommodate various...
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
Quartz
Quartz is the second-most-abundant mineral in the Earth's continental crust, after feldspar. It is made up of a continuous framework of SiO4 silicon–oxygen tetrahedra, with each oxygen being shared between two tetrahedra, giving an overall formula SiO2. There are many different varieties of quartz,...
on the mirror; others expose it to pure oxygen
Oxygen
Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...
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
Optics
Optics is the branch of physics which involves the behavior and properties of light, including its interactions with matter and the construction of instruments that use or detect it. Optics usually describes the behavior of visible, ultraviolet, and infrared light...
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 mirror
Mangin mirror
In optics, a Mangin mirror is a negative meniscus lens with the reflective surface on the rear side of the glass forming a curved mirror that reflects light without spherical aberration...
s that are back-silvered (reflective coating on the rear surface) as part of their optical design.
The silvering on a second surface mirror is often actual silver.
The "silvering" on precision optical instruments such as telescopes is usually aluminum, even though silver has the best reflectivity in the visible spectrum and oxidizes slower.
Such instruments are required to use front-surface mirror to avoid seeing a "ghost" star next to every bright star.
Silver alone is unsuitable for front surface mirrors because it quickly oxidizes to a low-reflectivity tarnish.
Although aluminum oxidizes even more quickly, the thin aluminum oxide (sapphire) layer is transparent, and so the high-reflectivity underlying aluminum stays visible.
The "silvering" on infrared instruments is usually gold. It has the best reflectivity in the infrared spectrum, and has high resistance to oxidation and corrosion.
See also
- Dielectric mirrorDielectric mirrorA dielectric mirror is a type of a mirror composed of multiple thin layers of dielectric material, typically deposited on a substrate of glass or some other optical material. By careful choice of the type and thickness of the dielectric layers, one can design an optical coating with specified...
- List of telescope parts and construction
- Optical coatingOptical coatingAn optical coating is one or more thin layers of material deposited on an optical component such as a lens or mirror, which alters the way in which the optic reflects and transmits light. One type of optical coating is an antireflection coating, which reduces unwanted reflections from surfaces, and...
- Mercury glassMercury glassMercury glass is the common term for silvered glass, which describes glass that was blown double walled, then silvered between the layers with a liquid silvering solution, and sealed. Although mercury was originally used to provide the reflective coating for mirrors, elemental mercury was never...
- Mercury silveringMercury silveringMercury silvering or fire gilding is a silvering technique for applying a thin layer of precious metal such as silver or gold to a base metal object. The process was invented during the Middle Ages and is documented in Vannoccio Biringuccio's 1540 book De la pirotechnia...
- MetallizingMetallizingMetallizing is the general name for the technique of coating metal on the surface of non-metallic objects.Techniques for metallization started as early as mirror making. In 1835, Justus von Liebig discovered the process of coating a glass surface with metallic silver, making the glass mirror one of...
External links
- Tions.net, Diy mirror / mirroring / silvering