A Short History of the Mirror: From Polished Metal to Front-Surface

3 days ago

7 min read

By Rocco (Roi) Ramon - licensed architect, founder of Studio Yabaye

The history of mirrors turns on one question: where does the reflection happen? For several thousand years it happened on the exposed face of a polished stone or metal plate. For about five centuries after that it happened behind a sheet of glass, on a metal layer protected by the glass itself. In optical instruments it has moved back to the front.

This article follows that sequence in five steps, from polished stone to the front-surface mirror used in kaleidoscopes. The main sources are Rossing and Chiaverina, "Light Science" (Springer, 2019), chapter 3, and David Brewster, "A Treatise on the Kaleidoscope" (Edinburgh, 1819).

Why metal reflects and glass mostly does not

When light reaches the boundary between two materials, part is reflected and part is transmitted. Rossing and Chiaverina explain that the split depends on the electrical properties of the materials. A good electrical conductor such as silver or aluminium reflects nearly all the light. An insulator such as glass transmits a substantial share of it.

Citing the experiments of Bouguer, Brewster writes in Chapter III of the Treatise that when light is reflected perpendicularly from good plate glass, "only 25 rays are reflected out of 1000". At an angle of incidence of 60 degrees the figure rises to 112. Almost every mirror in this history therefore depends on a metal surface.

Polished stone and metal: the mirrors of the ancient world

The oldest manufactured mirrors usually cited are polished obsidian, a volcanic glass, found at Catalhoyuk in Anatolia and dated to around 6000 B.C. Polished copper mirrors are reported from Mesopotamia from about 4000 B.C. and from Egypt from about 3000 B.C.

Rossing and Chiaverina date the first Egyptian mirrors earlier, to wet slate used around 4500 B.C. The dates differ between sources.

A polished metal plate is a front-surface mirror in the plain sense of the term: the light meets the reflecting metal directly, with nothing in front of it. Its limits are material ones: copper and bronze reflect less visible light than silver or aluminium, copper reflects red light more strongly than blue, and the exposed surface tarnishes in air and has to be polished again at intervals.

Venetian glass and the tin-mercury amalgam

Rossing and Chiaverina place the next step in the Renaissance. Venetian glassmakers learned to produce clear, flat, colourless glass and to apply a reflective backing to it. The backing was a mixture of tin and mercury, applied in a process the authors call "foiling" the glass.

Here the reflecting layer moved to the back. The glass plate provides a flat, smooth surface for the metal layer to copy, and it seals that layer from air and handling.

By the 16th century Venice dominated this production, and the tin-mercury amalgam remained the usual mirror backing in Europe until chemical silvering replaced it in the 19th century.

This was still the ordinary mirror in 1819. In Chapter VI Brewster writes that the plate glass used for kaleidoscope reflectors "may be either quick-silvered or not". Quicksilver is the old name for mercury, so a quick-silvered plate is a tin-mercury mirror.

19th-century chemical silvering

Rossing and Chiaverina place silver coatings on glass in the mid-nineteenth century. The process is usually credited to the German chemist Justus von Liebig. The date given is 1835, and the technique gained wide acceptance after he improved it in 1856. A silver compound in solution is chemically reduced, and metallic silver deposits as a thin film directly on the glass.

The silver film replaced the tin-mercury amalgam, but the geometry stayed the same. The silver is still on the rear face of the glass, usually protected by a layer of paint behind it.

Vacuum-deposited aluminium

The next method does not use a chemical bath. Aluminium is evaporated in a vacuum chamber and condenses on the glass as a thin film. The physicist John Strong at Caltech used this technique to make the first aluminium-coated telescope mirrors in the 1930s, and reflecting telescopes then moved from silver to aluminium.

Aluminium keeps its reflectance in air better than silver, which tarnishes. That makes it practical to put the metal on the front of the glass and leave it there.

What a front-surface mirror is

In a rear-surface mirror, light passes through the glass, reflects from the metal at the back, and passes through the glass again. In a front-surface mirror, also called a first-surface mirror, the metal film is on the face that the light meets first. The glass is only a flat support.

A rear-surface mirror has two reflecting surfaces: the glass face, which reflects a small fraction of the light, and the metal behind it, which reflects most of it. The two images are separated by the thickness of the glass.

Brewster describes the problem in Chapter III of the Treatise. An object that touches a common looking-glass does not touch its own image, which is separated from it "by a space equal to the thickness of the glass", because the reflection takes place at the rear surface. In Chapter II he states the requirement directly: the image "must be formed by reflection from the first surface of the mirror", so that the direct and the reflected image can join.

In the same chapter he notes that at small angles of incidence the images from the glass face are faint, and at very oblique angles they become the bright ones.

Why kaleidoscopes use front-surface mirrors

A kaleidoscope is a demanding case for a rear-surface mirror, for two reasons.

Oblique viewing. The eye looks along the length of the mirrors, so the light meets them at a large angle of incidence. This is the range in which, by Brewster's account, the reflection from the glass face competes with the reflection from the metal.

Repeated reflection. The pattern is built from reflections of reflections. In my reading of Brewster, every reflection from a rear-surface mirror carries the same offset, and the offsets appear at the seams where sectors should meet.

Brewster had no aluminised front-surface glass. His options in 1819 were these:

Polished metal. In Chapter V he names "finely polished steel" and "the best speculum metal" as mirror materials. Speculum metal is a copper-tin alloy that was also used for telescope mirrors. The polyangular kaleidoscope made by Mr Bate, described in Chapter IX, was made with metallic reflectors.
Glass with the rear face disabled. In Chapter VI he describes plate glass whose rear surface is ground, or covered with black wax or varnish, so that only the front face of the glass reflects. This works because bare glass reflects a much larger share of the light at oblique angles than at perpendicular incidence.

What to check: a checklist

These points can be read in the field of a kaleidoscope.

The reflecting layer is on the front face of the glass, not behind it.
Lines in the pattern cross the sector seams without a step.
Bright edges show no displaced second copy, which would indicate reflection from two surfaces.
Sectors seen after several reflections remain sharp. Each reflection loses some light, so they show the mirror quality first.
The instrument is viewed in direct, preferably strong light when judging these points.

FAQ

Who invented the mirror?

No single inventor is recorded. Polished obsidian mirrors from Catalhoyuk in Anatolia are dated to around 6000 B.C., and polished copper mirrors are reported from Mesopotamia and Egypt between about 4000 and 3000 B.C. The clear glass mirror with a metal backing is credited to Venetian glassmakers of the Renaissance.

How were Venetian mirrors made?

Venetian glassmakers of the Renaissance produced clear, flat, colourless glass and applied a reflective backing made from a mixture of tin and mercury, a process described in "Light Science" as foiling the glass. The metal layer sat behind the glass, which protected it and gave it a flat surface to follow.

When did silvered glass mirrors appear?

Chemical silvering of glass is credited to the German chemist Justus von Liebig, with the date usually given as 1835, and it came into wide use after he improved the process in 1856. A thin film of metallic silver on the rear face of the glass replaced the earlier tin-mercury backing.

What is a front-surface mirror?

A front-surface mirror, also called a first-surface mirror, carries its reflecting metal film on the face that light reaches first. Light does not pass through the glass at all. A household mirror is the opposite arrangement: the metal is on the rear face, so light crosses the thickness of the glass twice.

Why do kaleidoscopes use front-surface mirrors?

A rear-surface mirror reflects from two planes, the glass face and the metal behind it. Brewster wrote in 1819 that the image is then separated from the object "by a space equal to the thickness of the glass". In a kaleidoscope the pattern is built from repeated reflections, so that offset shows at the sector seams.

What mirrors did Brewster use in his kaleidoscopes?

Brewster's 1819 Treatise names several options: finely polished steel, speculum metal, and plate glass. For glass he describes grinding the rear surface or covering it with black wax or varnish to remove its reflective power, so that only the first surface of the glass forms the image at oblique angles.

About Studio Yabaye

At Studio Yabaye I build kaleidoscopes and teleidoscopes with front-surface mirror systems in several configurations, each in a brass tube, cut, fitted and hand-finished in the studio. Each object wheel is poured and composed by hand, so no two pieces are alike. The studio is based in Israel and ships worldwide. The current range is at all products.

References

T. D. Rossing and C. J. Chiaverina, "Light Science", Springer, 2019, sections 3.3 and 3.9.
David Brewster, "A Treatise on the Kaleidoscope", Edinburgh, 1819, Chapters II, III, V, VI and IX.