Many of us know the legend of the burning mirror of Archimedes. When Marcellus [The Roman General] had placed the ships a bow shot off, the old man [Archimedes] constructed a sort of hexagonal mirror. He placed at proper distances from the mirror other smaller mirrors of the same kind, which were moved by means of their hinges and certain plates of metal. He placed it amid the rays of the sun at noon, both in summer and winter. The rays being reflected by this, a frightful fiery kindling was excited on the ships, and it reduced them to ashes, from the distance of a bow shot. Thus the old man baffled Marcellus, by means of his inventions.
Crafty old man, indeed, but did it really happen? The ability of concentrated mirrors to concentrate the sun and obtain high temperatures is no myth, as any kid who used a magnifying glass to burn scraps can attest. This year, Morocco opened the largest concentrated solar power (CSP) plant in the world which will generate enough electricity to power the homes of one million people. CSP plants typically use 12m high parabolic mirrors that reflect sunlight onto pipework that contains a heat transfer fluid (HTF), typically thermal oil. This increases the temperature of the fluid to almost 400°C. The HTF is then used to heat steam in a standard turbine generator. Some CSPs heat the target tower to temperatures in excess of 1,000 degrees Fahrenheit (537 degrees Celsius), so it’s easy to imagine how Archimedes might have pulled something similar to burn enemy ships.
The real question isn’t whether it’s possible per se, but whether Archimedes actually made a burning mirror system using the tools and resources at his disposal two thousand years ago.
Apparently in 1973 a Greek scientist, Dr. Ioannis Sakkas, became curious about whether Archimedes could really have used a “burning glass” to destroy the Roman fleet and set up an experiment involving 60 Greek sailors each using an oblong 3′ by 5′ flat mirror to focus light on a wooden rowboat 160 feet away. According to sources, he had no problem getting the wood to catch fire very quickly.
In 1973, a Greek scientist, Dr. Ioannis Sakkas, made an experiment in which he directed 70 mirrors against a Roman ship mock-up. Each 3″ by 5″ flat mirror was held by a Greek sailor who had to focus the beam on the wooden boat 160 feet away. The boat was set on fire fairly quickly, though it’s mentioning the boat was coated in tar paint which is highly flammable. Tar paint was used frequently to coat ships back in the day.
More recently, however, when the Mythbusters made their own re-enactment things didn’t go quite as smoothly. In 2010, 500 flat mirrors controlled by 500 volunteer middle and high school students were focused on the sail of a ship, which should have combusted at 500 degrees F. After an hour, no more than 230 degrees F could be reached, so the team classified this as ‘inconclusive’. Jamie Hyneman, who was stationed on the mockboat for the duration of the experiment, did say that he could barely see, though. He suggests that Archimedes’ burning mirrors might have been real, but used more for dazzling enemies than burning boats.