Burning glass

Burning glass technology has been known since antiquity, as described by Greek and Roman writers who recorded the use of lenses to start fires for various purposes.

[1] Pliny the Elder noted the use of glass vases filled with water to concentrate sunlight heat intensely enough to ignite clothing, as well as convex lenses that were used to cauterize wounds.

Have you ever seen a beautiful, transparent stone at the druggists', with which you may kindle fire?The Hellenistic Greek mathematician Archimedes was said to have used a burning glass as a weapon in 212 BC, when Syracuse was besieged by Marcus Claudius Marcellus of the Roman Republic.

[4] The legend of Archimedes gave rise to a considerable amount of research on burning glasses and lenses until the late 17th century.

[9] Burning lenses were used in the 18th century by both Joseph Priestley and Antoine Lavoisier in their experiments to obtain oxides contained in closed vessels under high temperatures.

When Syracuse was besieged by Marcus Claudius Marcellus, the Roman fleet was supposedly incinerated by the use of not glass per se, but a concave mirror made of brass focusing sunlight.

Thus, Saint Boniface explained to Pope Zachary that he produced the new fire of Holy Saturday by means of a crystal lens concentrating the rays of the sun.

[19] There have been several real-world tests to evaluate the validity of the legend of Archimedes described above (see § War: since the legend of Archimedes) over the centuries, including a test by Comte de Buffon (circa 1747), documented in the paper titled "Invention De Miroirs Ardens, Pour Brusler a Une Grande Distance", and an experiment by John Scott, which was documented in an 1867 paper.

It concluded that although the theory was sound for stationary objects, the mirrors would not likely have been able to concentrate sufficient solar energy to set a ship on fire under battle conditions.