An incandescent gas mantle, gas mantle, or Welsbach mantle is a device for generating bright white light when heated by a flame. The name refers to its original heat source, existing gas lights, which filled the streets of Europe and North America in the late 19th century, mantle referring to the way it was hung above the flame. Today they are still used for portable camping lanterns and pressure lanterns.
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The mantle is a roughly pear-shaped ramie-based artificial silk or rayon fabric bag made from silk or fabric impregnated with rare-earth metallic salts that will convert to solid oxides after being burned (heated). A mantle will glow brightly in the visible spectrum while emitting little infrared radiation. The rare earth oxides (cerium) and actinide (thorium) in the mantle have a low emissivity in the infrared (in comparison with an ideal black body), but have high emissivity in the visible spectrum. This is because of candoluminescence. Hence, when heated by a kerosene or liquified petroleum gas flame, the mantle emits radiation that is weighted less heavily in the infrared and more heavily in the visible spectrum, leading to an enhanced output of useful light.
The mantle shrinks after all the fabric material has burnt away and becomes very fragile after this first use.
The mantle also aids the combustion process, keeping the flame small and contained inside the mantle at higher fuel flow rates than in a simple lamp. This concentration of combustion inside the mantle, in turn, improves the transfer of heat from the flame to the mantle.
For centuries, artificial light had been generated using open flames. Limelight had been invented in the 1820s, but the temperature required was too high to be practical for small lights. In the late 19th century several inventors tried to develop an effective alternative based on heating a material to a lower temperature but using spectral lines to simulate white light.
Many early attempts used platinum-iridium gauze soaked in metal nitrates, but were not successful because of high cost materials and poor reliability.
The first effective mantle was the Clamond basket in 1881, named after its inventor. It was exhibited in the Crystal Palace exhibition of 1883. This device was made from a mixture of magnesium hydrate, magnesium acetate and water which was squeezed through holes in a plate to form threads, which were then moulded into a basket shape and ignited. The acetate burnt, the combustion products forming a matrix to support the magnesium oxide formed as the hydrate decomposed. The fragile structure was supported by a platinum wire cage and heated by a coal gas flame.
The modern gas mantle was one of the many inventions of Carl Auer von Welsbach, a chemist who studied rare earth elements in the 1880s and who had been Robert Bunsen's student. Ignaz Kreidl worked with him on his early experiments to create the Welsbach mantle. His first process used a mixture of 60% magnesium oxide, 20% lanthanum oxide and 20% yttrium oxide, which he called Actinophor, and patented in 1885.
The original mantles gave off a green-tinted light and were not very successful, and his first company, which established a factory in Atzgersdorf in 1887, failed in 1889. In 1890 he discovered that thorium was superior to magnesium, and in 1891 perfected a new mixture of 99% thorium dioxide and 1% cerium dioxide that gave off a much whiter light and produced a stronger mantle. After introducing it commercially in 1892 it quickly spread throughout Europe. The gas mantle remained an important part of street lighting until the widespread introduction of electric lighting in the early 1900s.
To produce a mantle, cotton is woven into a net bag and impregnated with the soluble nitrates of these metals and then heated; the cotton burns away and the nitrates are converted to nitrites, which fuse together to form the solid mesh. As the heating continues, the nitrites decompose into the final solid, (but fragile) very high melting point oxides.
Early mantles were sold in the unheated cotton mesh condition, since the oxide structure was too fragile to transport easily, and the purchaser carried out the conversion when it was first used. The cotton quickly rotted because of the corrosive nature of the acidic metal nitrates (although was later reduced by soaking the mantle in ammonia solution to neutralise the excess acid).
Later mantles were made from guncotton (nitrocellulose) or collodion rather than ordinary cotton, since extremely fine threads of it could be produced; it was converted back to cellulose before heating (since these materials are highly flammable or explosive) by dipping in ammonium sulfide.
It was discovered that the finished mantle could be strengthened sufficiently by dipping in a solution of collodion, which would coat it with a thin layer of the material to be burnt off when the mantle was first used, although modern mantles are now usually sold in their original fabric condition.
Early mantles often had a binding thread of asbestos for tying onto the lamp fitting, but because of its carcinogenic properties it has been replaced with wire or ceramic fiber thread in modern mantles.
Since thorium is radioactive and produces a radioactive gas, radon-220, as one of its decay products, there are concerns about the safety of thorium mantles. Some nuclear safety agencies make recommendations about their use [1]. A study in 1981 estimated that the dose from using a thorium mantle every weekend for a year would be 0.3-0.6 millirems, tiny in comparison to the normal annual dose of a few hundred millirems, although a person ingesting an entire mantle would receive a comparable dose of 200 mrem (2 mSv; [2], [3]). However, the radioactivity is a major concern for those people involved with the manufacture of mantles and with contamination of soil around some former factory sites [4]. All of these issues have meant that alternatives, usually yttrium or sometimes zirconium, are used in some countries although they are either more expensive or less efficient.
One potential cause for concern is that particles from thorium gas mantles "fall out" over time and get into the air, where they may be ingested in food or drink. These particles can also be inhaled and remain in the lungs or liver. Also of concern is the release of thorium-bearing dust if the mantle shatters due to mechanical impact.
Secondary decay products of thorium include radium, actinium, and radon gas.