Oxyhydrogen is a mixture of hydrogen (H2) and oxygen (O2) gases. This gaseous mixture is used for torches for the processing of refractory materials and was the first gaseous mixture used for welding. Theoretically, a ratio of 2:1 hydrogen:oxygen is enough to achieve maximum efficiency; in practice a ratio 4:1 or 5:1 is required to avoid an oxidizing flame.
This mixture may sometimes be called by an old term knallgas (German; "bang-gas"), although some authors used to define knallgas to be a generic term for the mixture of fuel with precise amount of oxygen required for complete combustion, thus 2:1 oxyhydrogen would be called "hydrogen-knallgas".
Oxyhydrogen will combust when brought to its autoignition temperature. For a stoichiometric mixture at normal atmospheric pressure, autoignition occurs at about 570 °C (1065 °F). The minimum energy required to ignite such a mixture with a spark is about 20 microjoules. At standard temperature and pressure, oxyhydrogen can burn when it is between about 4% and 95% hydrogen by volume.
When ignited, the gas mixture releases energy and converts to water vapor, which sustains the reaction: 241.8 kJ of energy (LHV) for every mole of H2 burned. The amount of heat energy released is independent of the mode of combustion, but the temperature of the flame varies. The maximum temperature of about 2800 °C is achieved with a pure stoichiometric mixture, about 700 degrees hotter than a hydrogen flame in air. When either of the gases are mixed in excess of this ratio, or when mixed with an inert gas like nitrogen, the heat must spread throughout a greater quantity of matter and the temperature will be lower.
- electrolysis: 2 H2O → 2 H2 + O2
- combustion: 2 H2 + O2 → 2 H2O
William Nicholson was the first to decompose water in this manner in 1800. The energy required to generate the oxyhydrogen always exceeds the energy released by combusting it. (See Electrolysis of water#Efficiency).
Many forms of oxyhydrogen lamps have been described, such as the limelight, which used an oxyhydrogen flame to heat a piece of lime to white hot incandescence. Because of the explosiveness of the oxyhydrogen, limelights have been replaced by electric lighting.
Oxyhydrogen was once used in working platinum because at the time such a torch was the only device that could attain the temperature required to melt the metal (1768.3 °C). These techniques have been superseded by the electric arc furnace.
Oxyhydrogen blowpipe 
The oxy-hydrogen blowpipe was developed by English mineralogist Edward Daniel Clarke and American chemist Robert Hare in the early nineteenth century. It produced a flame hot enough to melt such refractory materials as platinum, porcelain, fire brick, and corundum, and was a valuable tool in several fields of science. It is used in the Verneuil process to produce synthetic corundum.
Oxyhydrogen torch 
Due to competition from the acetylene-fueled cutting torch and from arc welding, the oxyhydrogen torch is seldom used today, but it remains the preferred cutting tool in some niche applications—see oxy-fuel welding and cutting.
Water torch 
A "water torch" is a portable oxyhydrogen torch that combines a DC power supply and an electrolytic cell with a pressure gauge and flashback arrestor. Water is decomposed on-demand into oxyhydrogen, obviating the need for separate hydrogen and oxygen tanks. The original was designed in 1962 by William Rhodes and Raymond Henes of the Henes Manufacturing Co. (now Arizona Hydrogen Manufacturing, Inc.) and marketed under the trade mark "Water Welder". A hypodermic needle was originally used for the torch tip.
Fringe science and fraud 
"Brown's Gas" is simply oxyhydrogen with a 2:1 molar ratio of H2 and O2 gases, the same proportion as water. It's named after Yull Brown who claimed that it could be used as a fuel for the internal combustion engine. It's also called "HHO gas" after the claims of fringe physicist Ruggero Santilli, who claims that his HHO gas, produced by a special apparatus, is "a new form of water", with new properties, based on his (fringe) theory of "magnecules".
Oxyhydrogen is also often mentioned in conjunction with vehicles that claim to use water as a fuel. The most common and decisive counter-argument against producing this gas on board to use as a fuel or fuel additive is that the energy required to split water molecules exceeds the energy recouped by burning it. Additionally, the number of liters per minute of gas that can be produced for on-demand consumption through electrolysis is very small in comparison to the liters per minute consumed by an internal combustion engine.
An article in Popular Mechanics reports that Brown's gas can't even increase the miles per gallon (MPG) of your vehicle, and that the only real savings come from tampering with your engine, which may confuse the anti-smog controls.
- Howard Monroe Raymond (1916), "Oxy-Hydrogen Welding", Modern Shop Practice volume 1, American Technical Society
- Viall, Ethan (1921). Gas Torch and Thermite Welding. McGraw-Hill. p. 10.
- W. Dittmar, "Exercises in quantitative chemical analysis", 1887, p. 189
- Ball, Philip (September 14, 2007). "Burning water and other myths". Nature News. doi:10.1038/news070910-13. Retrieved 2008-12-08. "(...) Brown's gas — a modern chemical unicorn to rival phlogiston — in which hydrogen and oxygen are combined in a non-aqueous state called 'oxyhydrogen', in the same proportions in which they are found in water (2:1)."
- O'Connor, Ken. "Hydrogen". NASA Glenn Research Center Glenn Safety Manual.
- 1911 Encyclopedia. "Oxyhydrogen Flame." (Available here Accessed 2008-01-19.)
- Calvert, James B. (2008-04-21). "Hydrogen". University of Denver. Retrieved 2009-04-23. "An air-hydrogen torch flame reaches 2045 °C, while an oxyhydrogen flame reaches 2660 °C."
- "Adiabatic Flame Temperature". The Engineering Toolbox. Retrieved 2009-04-23. "Oxygen as Oxidizer: 3473 K, Air as Oxidizer: 2483 K"
- "Temperature of a Blue Flame". Retrieved 2008-04-05. "Hydrogen in air: 2,400 K, Hydrogen in Oxygen: 3,080 K"
- Tilden, William Augustus (1926). Chemical Discovery and Invention in the Twentieth Century. Adamant Media Corporation. p. 80. ISBN 0-543-91646-4.
- Griffin, John Joseph (1827). A Practical Treatise on the Use of the Blowpipe in Chemical and Mineral Analysis. Glasgow: R. Griffin & co.
- P. N. Rao (2001), "24.4 Oxyhydrogen welding", Manufacturing technology: foundry, forming and welding (2 ed.), Tata McGraw-Hill Education, pp. 373–374, ISBN 978-0-07-463180-5
- US 3262872 Apparatus for the electrolytic production of Hydrogen and Oxygen for the safe consumption thereof, William A. Rhodes, Raymond A. Henes, Filed March 23, 1962
- Ball, Philip (2006). "Nuclear waste gets star attention". news@nature. doi:10.1038/news060731-13. ISSN 1744-7933.
- Weimar, Carrie (May 7, 2007). "Snubbed By Mainstream, Scientist Sues". St. Petersburg Times. Retrieved 3 February 2011.
- Schadewald, Robert J. (2008). Worlds of Their Own - A Brief History of Misguided Ideas: Creationism, Flat-Earthism, Energy Scams, and the Velikovsky Affair. Xlibris. ISBN 978-1-4363-0435-1.
- Simpson, Bruce (May, 2008). "The proof that HHO is a scam". Aardvark Daily. Retrieved 12 Feb 2012.
- Water-Powered Cars: Hydrogen Electrolyzer Mod Can't Up MPGs, Mike Allen, August 7, 2008, Popularmechanics.com