For centuries, the only method man had for metallurgically joining metals was forge welding, a crude and cumbersome blacksmith-type operation in which heated metals were pounded or rammed together until they bonded. No one knows when man first learned to use forge welding, but by the time of the Renaissance, craftsmen were highly skilled in forge welding. Parts go be joined were shaped and then heated in a forge or furnace before being hammered, rolled or pressed together. For a few centuries thereafter, ordinary fire remained the principle source of heat for welding. The traveling tinker carried with him a small charcoal furnace for heating his irons. During this era, tinsmiths and other workers in metal often used the heat of burning gases to braze and solder. Within the span of a few years prior to 1900, three new processes came into existence, arc welding, resistance welding and oxy-acetylene welding. Arc Welding is done when two high-voltage electric terminals are brought near each other causing an arc. This arc, which casts a bright light, and gives off considerable heat, can be struck and maintained at will. Its length and intensity can be varied within limits determined by the circuit voltage and by the type of terminals used. Resistance Welding refers to a group of welding processes such as spot and seam welding that produces coalescence of faying surfaces where heat to form the weld is generated by the resistance of the welding current through the work pieces. Small pools of molten metal are formed at the point of most electrical resistance (the connecting surfaces) as a high current is passed through the metal. In general, resistance-welding methods are efficient and cause little pollution, but their applications are limited to relatively thin material. The Oxy-acetylene Welding process is built upon two fundamental principles: first, that acetylene burned with oxygen produces a flame so intensely hot that it can be used to melt and fuse even the most refractory metals; second, that a stream of oxygen directed against a pieced of iron or steel that has been heated to its kindling temperature causes the metal to burn away and so can be used to cu or shape it as desired. In principle, oxy-acetylene welding is fairly simple. Two pieces of metal are brought together and the edges in contact are melted by the oxy-acetylene flame, with or without the addition of molten metal from a welding rod. The molten edges will flow together until each is completely fused with the other. After the metal has cooled, there is a single continuous piece with no seam. Of the three processes developed just prior to the Twentieth Century, arc welding has emerged as the most widely used and commercially important method.

References: The James F. Lincoln Arc Welding Foundation, The Procedure Handbook of Arc Welding - Fourteenth edition (2000) Linde Air Products Company, The Oxy-Acetylene Handbook (1951) Weman, Klas (2003) Welding Processes Handbook O'Brien, R.L. (Ed.) (1991) Welding Handbook Vol 2