Burning-glass

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A burning-glass is a large convex lens, which can focus the sun's rays on a small area and so ignite materials. Used in 18th century chemical studies for burning materials in closed glass vessels where the products of combustion could be trapped for analysis, the burning-glass was a useful contrivance in the days before electrical ignition was easily achieved. Burning mirrors achieve a similar effect using an array of reflecting surfaces to focus the light. Vases filled with water used to start fires were widely known in the ancient world, and metaphorical significance was drawn (by the early Church Fathers for instance) from the fact that the water remained cool even though the light passing through it would set materials alight.

The technology of the burning glass has been known since antiquity. Burning lenses were used to cauterise wounds and to light sacred fires in temples. Plutarch describes a burning mirror made of joined triangular metal mirrors installed at the temple of the Vestal Virgins.Aristophanes mentions the burning-lens in his play The Clouds (424 BC), and Archimedes, the renowned mathematician, was said to have used a burning glass (although it is more likely that a large number of angled hexagonal mirrored surfaces were used) in 212 BC when Syracuse was besieged by Marcus Claudius Marcellus. By means of it the Roman fleet was incinerated, though eventually when the city was taken, he was found among the slain.

Macrobius describes the defense of the Phoenician city of Gades against an attacking fleet in the 5th century BC by burning mirrors or lenses.

The legend of Archimedes gave rise to a considerable amount of research on burning glasses and lenses until the late 17th century. Successful recreations were been performed by Anthemius of Tralles (C6 A.D.), Proclus (C6 A.D.) (who by this means purportedly destroyed the fleet of Vitellus beseiging Constantinople), Roger Bacon (C13), Giambattista della Porta and his friends (C16), Athanasius Kircher and Gaspar Schott (C17), the Comte du Buffon in 1740 in Paris and Ioannis Sakas in the 1970s in Greece and others. Sakas was able to ignite a wooden boat at some distance in only seconds. Buffon using only 48 small mirrors was able to melt a six pound tin bottle, and ignite wood from a distance of 150 feet. These recreations show the plausiblity of Archimedes' achievement.

The pop science TV program Myth Busters attempted (in a rather slip-shod fashion) to model this event by using mirrors to ignite a small wooden boat covered with tar, with only partial success, as they found it too difficult to focus light from their hand-held mirrors onto a small enough point. However, their simulation was constrained by the budgets of the show and differed in many respects from accounts of Archimedes' feat. Apparently the producers of the show were unaware of the many previous successful attempts to construct burning mirrors.

Recent excavations at the Viking harbor town of Fröjel, Gotland in Sweden have revealed that this technology of fire-starting was known in the Viking Age as well. Rock crystal lenses produced at Fröjel in the 11th to 12th century via turning on pole-lathes have been found that have an imaging quality comparable to that of 1950s aspheric lenses. The Viking lenses quite effectively concentrate sunlight enough to ignite fires.

Similar technology was used in ancient Ireland (the Liath Meisicith is an ancient Irish burning lens) and quite possibly ancient Egypt.

Solar kilns are used in industry to produce extremely high temperatures without the need for fuel or large supplies of electricity. They employ a large parabolic array of mirrors (some facilities are several storeys high) to focus light to a high intensity.