As we have already stated in the article on Iron, welding is in reality a species of autogenous soldering. And, as in soldering or brazing, it is necessary to keep the surfaces that are to be united, free from dirt and oxide, so in welding, the surfaces must be perfectly clean or the joint will be imperfect. In welding common iron, sand is the flux generally used. When it is required to weld steel to iron, the steel must be heated to a less degree than the iron, as it is the most fusible. The surfaces should be thoroughly cleaned before they are brought together. Sal ammoniac cleans the dirt from the steel, and borax causes the oxide to fuse before it attains that heat which will burn the steel; consequently, a mixture of these two substances forms one of the best materials for welding.
The best mode of preparing this mixture is as follows: Take ten parts of borax and one part of sal ammoniac and grind them together. Then melt them together, and when cold reduce the mixture to tine powder, and preserve in a well-stopped jar or bottle.