This consists in the skillful bundling of the iron to be welded, in the use of an extraordinarily large forgehammer, in employing a bulling-furnace, instead of a hollowfire or chafery, and in passing the iron, reduced to a melting heat, through grooved mill rollers of different shapes and sizes, as required.
_Welding Steel, or Iron and Cast Steel._
Melt borax in an earthen vessel, and add 1-10th of pounded sal ammoniac. When well mixed, pour it out on an iron plate, and as soon as it is cold, pulverize and mix it with an equal quantity of unslaked lime. To proceed to the operation, the iron or steel must be first heated to a red heat, and the powder strewed over it; the pieces of metal thus prepared are to be again put in the fire, and raised to a heat considerably;ower than the usual welding one, when it is to be withdrawn and well beaten by a hammer till the surfaces are perfectly united.
_Welding by Pressure._
Soft metals can be welded cold by great pressure and recently hydraulic pressure has been applied by M. Duportail to the welding of heated masses of iron. The advantage of pressure over hammering, is that it reaches the centre of the bar and produces a homogeneous weld.
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