Carbon arc is an old welding method which uses relatively low voltage, high amperage electricity to heat metal when an arc is established between a carbon electrode and the work (single-carbon arc) or between two such electrodes (twin-carbon arc). The single-carbon and twin-carbon processes have gone out of fashion, at least in the United States, other processes having replaced them. Few welders today have experience with carbon arc's use in welding, brazing, or heating, although the process retains its utility for those who acquire the particular skills involved.

Carbon arc welding is often confused with the widely-used arc-air gouging process, which is used for metal removal rather than metal joining. Arc-air employs a single carbon in a special handpiece that is supplied with electrical power and compressed air, and is used for removing flawed welds, gouging out cracks for welding, beveling the edges of plates, and other spot-removal problems. An arc is struck between the carbon and the metal to be removed; when the metal reaches a liquid state, the operator triggers a jet of compressed air which blows out the liquid metal. It should be noted that this action differs in principle from gouging, beveling, and cutting of steel as done by oxygen/acetylene and oxygen/propane torches. In that process, the steel is heated with a flame, but not to the liquid state; instead an oxygen jet is triggered which makes an exothermic reaction with free ferrite in the steel, liquifying it and blowing it out of the base metal.

Single-carbon arc welding, brazing, and heating utilizes a direct-current (DC) power source, usually connected with the electrode negative ("straight polarity"). The process was thought to have particular merit in welding galvanized sheetmetal, having a relatively concentrated heat which minimized distortion of the metal. The single-carbon handpiece was designed to withstand considerably higher electrode temperatures than do common arc welding handpieces designed for coated "stick" welding electrodes, and some industrial versions were water-cooled. A significant problem with single-carbon welding was the likelihood of the weld puddle being contaminated by bits of carbon from the electrode. Carbon contamination is much less likely with twin-carbon arc.

Twin-carbon arc handpieces are unlike any other welding handpieces (often loosely referred to as "torches"). These have two arms which affix the twin electrodes, or carbons, at a range of angles to each other, and which are manipulated by the operator to vary the distance between the tips of the electrodes, or "gap," this usually being done while the work is in progress. Welding electricity usually and preferably is from an alternating-current (AC) power source. Holding the handpiece, the operator manipulates the moving arms to touch the electrode tips together momentarily to initiate current flow, then moves them apart to achieve the desired gap. The resultant electric arc across the gap serves as a heat source to bring the workpiece to a temperature at which welding or brazing filler rod can be added.

The electrodes for either carbon arc process were made of baked carbon or pure graphite in a tubular copper jacket. The electrodes, often called "carbons," are not intended to be consumed as a part of the process, although they gradually erode and must be replaced. Welding carbons were commonly 6" long and ranged in diameter from 3/16" to 1/2". The arc temperature was in the area of 5000C/9000F degrees, emitting an extremely bright light which in earlier times found use in street and stage lighting, in searchlights and lighthouses, and a little later in photo-engraving and movie theater projection equipment. The brightness of this arc calls for the protection of a welding helmet with a minimum darkness rating of #12, with #14 recommended, and protective clothing is essential to protect the skin from the intense ultra-violet radiation of the arc. Carbon electrodes for arc-air gouging are readily available, and those for old theater projectors are still manufactured, but the current availability of new welding carbons is unclear.

Both the single and twin-carbon arc processes were first patented in Europe and the United States at the end of the 19th century, and achieved widespread use in the first three decades of the 20th century. Advances in the shielded metal arc welding process (abbreviated SMAW and often referred to as "stick" welding) during the second world war, particularly to the coating materials on the consumable electrodes, made it a superior process for nearly every application. SMAW quickly became popularly synonymous with arc welding and remained so for many years, and completely replaced carbon arc. Today, in the United States, the UK, And Australia, carbon arc joining as an industrial process exists only in a very few highly specialized and automated applications.

The special handpieces for carbon arc welding appear to have gone out of production, at least in the United States. The few current users of such equipment tend to employ the carbon arc most often for heating metal for bending and forming, sometimes for brazing, and often for heating purposes such as thawing frozen water pipes. It could also be used for melting lead and babbit, and even the hard-surfacing of digging and earthmoving equipment, using a special hardfacing paste. The twin-carbon arc torch in particular was often offered as an accessory with small AC transformer welders sold to amateur welders and farmers, serving some of the same purposes as an oxy-acetylene gas welding outfit which would have cost these occasional users hundreds of dollars more.

--Philip H. Smith, Jr.

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