A welding torch can also be used to heat small areas such as rusted nuts and bolts. Other than the above, there are some additional processes of cutting: Plasma Cutting, Air Plasma cutting, Underwater cutting (Underwater welding and cutting has been already described in Part I). These are not used for cutting by hand since they need very accurate positioning above the work. This method works well for brazing, but higher-purity oxygen is necessary to produce a clean, slag-free kerf when cutting. The heat to keep the cut going—once it has started—is provided partly by the heating jet, and partly by the heat of the chemical action. Between the regulator and hose, and ideally between hose and torch on both oxygen and fuel lines, a flashback arrestor and/or non-return valve (check valve) should be installed to prevent flame or oxygen-fuel mixture being pushed back into either cylinder and damaging the equipment or causing a cylinder to explode. The cut should be wide enough so that the electrode can be used well down in it especially when the metal is thick. The melting point of the iron oxide is around half that of the metal being cut. The unburned carbon insulates the flame and drops the temperature to approximately 5,000 °F (2,800 °C). Rough cuts can be made quickly; smooth cuts require longer time. Fasten oxygen and acetylene cylinders in an upright position. Welding metal results when two pieces are heated to a temperature that produces a shared pool of molten metal. The oxygen and the hydrogen are led off the electrolysis cell separately and are fed into the two gas connections of an ordinary oxy-gas torch. Filler material selection depends upon the metals to be welded. The feather is adjusted and made ever smaller by adding increasing amounts of oxygen to the flame. This keeps the oxygen from reaching the clean metal and burning it. The surface of the metal to be cut should be free of grease, oil and rust and the heating flame held above the edge of the metal to be cut.  Or from a non-pressurised tank with the fuel being drawn into the torch by venturi action by the pressurised oxygen flow. In recent decades it has been superseded in almost all industrial uses by various arc welding methods offering greater speed and, in the case of gas tungsten arc welding, the capability of welding very reactive metals such as titanium. The reducing flame is typically used for hard facing operations or backhand pipe welding techniques. This causes the acetone inside the acetylene cylinder to come out of the cylinder and contaminate the hose and possibly the torch. An excess of acetylene creates a carbonizing flame. The heat is released because the molecules of the products of combustion have a lower energy state than the molecules of the fuel and oxygen. During the early 20th century, before the development and availability of coated arc welding electrodes in the late 1920s that were capable of making sound welds in steel, oxy-acetylene welding was the only process capable of making welds of exceptionally high quality in virtually all metals in commercial use at the time. The metal is cut entirely by exothermic chemical action. Types of this sort of torch: Methylacetylene-propadiene (MAPP) gas and LPG gas are similar fuels, because LPG gas is liquefied petroleum gas mixed with MPS. The carbons specially designed for the process are a mixture of carbon and graphite, covered overall with a thin sheath of copper. The amount of heat applied to the metal is a function of the welding tip size, the speed of travel, and the welding position. Hydrogen has a clean flame and is good for use on aluminium. The cutting torch only heats the metal to start the process; further heat is provided by the burning metal. The excess pressure of oxygen is then released and this area burnt out—this will produce a lag in the cut. This flame type is observed when welders add more oxygen to the neutral flame. In the 1940s cobalt melters’ glasses were borrowed from steel foundries and were still available until the 1980s. Oxygen lances are used in scrapping operations and cut sections thicker than 200 mm (8 inches).  Manufacturers have developed custom tips for Mapp, propane, and propylene gases to optimize the flames from these alternate fuel gases. The oxygen flow rate is critical; too little will make a slow ragged cut, while too much will waste oxygen and produce a wide concave cut. The Steel and Iron is immediately oxidised to magnetic oxide of iron (Fe3O4). A single-stage regulator will tend to allow a reduction in outlet pressure as the cylinder is emptied, requiring manual readjustment. Special safety eyewear must be used—both to protect the welder and to provide a clear view through the yellow-orange flare given off by the incandescing flux. The outer jets are for preheat flames of oxygen and acetylene. In oxy-fuel cutting, a torch is used to heat metal to its kindling temperature. It is approximately 6,000 °F (3,300 °C) and provides enough heat to easily melt steel. Acetylene is the primary fuel for oxy-fuel welding and is the fuel of choice for repair work and general cutting and welding. The welder will modify the speed of welding travel to maintain a uniform bead width. Oxy-propane torches are usually used for cutting up scrap to save money, as LPG is far cheaper joule for joule than acetylene, although propane does not produce acetylene's very neat cut profile. See oxyhydrogen. A double-hose or twinned design can be used, meaning that the oxygen and fuel hoses are joined together.