Magnesium Light

photography — The guide with carbide lamp and magnesium burning bright behind the ice.

Magnesium is a chemical element which has symbol Mg and atomic number 12, it belongs to the alkaline earth metals, group 2 of the periodic table. Magnesium is one of the ten most common elements in the earth’s crust. It is found in numerous minerals, in seawater and in the green leaves of plants. In its elementary form it does not occur in nature, which is the result of the fact it reacts readily with air and many other substances. But it is quite common in many rocks and ores. The fact that it reacts so easily is the reason why it is very useful for various purposes, including photography and cave lighting. A drawback however are the substances it produces.

Magnesium is flammable, burning at a temperature of approximately 3,100 °C, produces a brilliant white light that includes strong ultraviolet wavelengths. In photography powder is used to create a flash to illuminate the scene for a short time. In caves a ribbon is used, which is fixed at a certain place and then ignited. Depending on the length it takes several seconds to burn down and during this time it is extremely bright allowing to illuminate a huge chamber. It is usually placed so that visitors cannot see it directly, as it could dazzle them, and instead illuminates the cave. It is also placed behind transparent objects like cave formations or ice, which is then illuminated from behind and glows brightly showing the internal structure.

The drawback however is that the burning produces clouds of magnesium oxides and also a powder of magnesium oxide. Both pollutes the cave, although it is not poisonous or dangerous, but it might colour speleothems, and it may cloud view if there is no weathering in the cave. This is the reason why it is generally used only in rather huge caves where no other light is available, especially ice caves where the ice would destroy any other light system. While in rare occasions the guide may hold the ribbon with outstretched arm, in most cases there is a sort of hook and a bowl on the floor below to catch the residues.