The difficulty in getting solder to adhere to aluminium is caused by a metallic film (probably an oxide) which forms on the surface of the metal while heated, and which prevents ordinary soft solders alloying to form a strong joint. A flux might he used to render the surface of the aluminium pure during the soldering operation, or the film might be removed by mechanical means, or a solder devised that would dissolve, or combine with the film on the surface of the metal while both solder and aluminium were heated. The composition of a really reliable flux for soft soldering has not been made public, consequently either of the two latter methods must be adopted. For working with a tinned copper bit the solder should melt at a moderate temperature, and should contain only small proportions of brittle metals, solders containing much brittle metal usually showing decreased malleability and ductility. Alloys of an easily fusible nature are recommended for soldering aluminium by the following authorities. Frishmuth, of Philadelphia, says: tin '.5 parts, and bismuth 5 parts; or tin 97 parts, and bismuth 3 parts •, while J. Richards recommends aluminium 2'5 parts, zinc 25.25 parts, phosphorus '25 parts, and tin 72 parts.

Other alloys for this purpose are aluminium 1 part, tin 9 parts; or cadmium 5 parts, zinc 2 parts, and tin 3 parts. Also phosphor tin (in variable proportion); or tin 20 parts, and zinc 1 part; or tin 99 parts, and copper 1 part; or tin 90 parts, copper 9 parts, and bismuth 1 part. Any of these can readily be fused with a copper bit, which, to ensure success, should be of a wedge shape bent round to, roughly, a quarter circle. Its edge is then at right angles to the aluminium, and, by lightly moving the bit backwards and forwards while in contact with the aluminium and flowing solder, the impure film is removed. The coated surface can then be soldered with an ordinary shaped copper bit. Phosphor tin probably owes its adhesiveness to the affinity of phosphorus for oxygen, a molten alloy containing phosphorus placed on aluminium tending to absorb oxygen from the impure film as well as from the surrounding air. When soldering, everything should be perfectly clean, the soldering being performed quickly, as if the surface is not coated at the first attempt the aluminium surface is injuriously affected, and good soldering becomes almost impossible unless the affected surface is removed by scraping or filing.