Gas welding is a technique that uses a high temperature flame produced by igniting a mixture of two gases, usually oxygen and acetylene. The main advantage of gas welding is the flexibility of the equipment and the small investment it involves. The disadvantage is it requires considerable more skill from the operator and operating costs are higher. Thermit welding is basically a casting process. The method employs the chemical reaction obtained by igniting a mixture of finely divided aluminum and iron oxide. During the reaction, the oxygen leaves the iron oxide to combine with the aluminum and the free iron is drawn off into a mold previously prepared around the parts to be welded. A big advantage in the method is its simplicity. There is no expensive machinery or equipment to buy. All you need is a pair of formed expendable shell molds, the correctly prepared thermit mixture, and a luting compound to seal the mold to the steel bar. Kits are available commercially for this use.