There is no solder which operates with aluminium in the same way that ordinary solders operate with copper, tin, etc. There are two reasons for this.

First. Aluminium does not alloy readily with solders at temperatures as low as the other metals require, and it is consequently necessary, in soldering aluminium, to use a much higher temperature. Furthermore, aluminium alloys with lead only with great difficulty and with but a small proportion of lead at that; consequently lead solders are useless with aluminium.

Second. The surface of all aluminium is covered with a thin invisible coating of aluminium oxide. This coating forms instantly on the surface of aluminium and is very refractory, and its presence is responsible for the high resistance of aluminium to corroding' agents, since, although aluminium itself is soluble in a great many chemical compounds, this protective coating of oxide is insoluble in almost everything excepting hydrofluoric acid. While in general this coating of oxide is beneficial, in that it forms a perfect protection to the aluminium underneath, it is, by reason of its efficiency in this particular, responsible for the principal portion of the difficulty which occurs in soldering aluminium, as naturally no solder will alloy with aluminium oxide.

In soldering aluminium, therefore, it is necessary that this oxide be removed before the soldering can take place; and as it forms again instantly after removal, it is necessary that the removal of the oxide and the covering with solder shall be simultaneous. In soldering other metals, the oxide can be removed chemically. With aluminium this is not possible, and it must be removed mechanically by abrasion.

Bearing these facts in mind, it will be readily understood how aluminium soldering must be done. All the surface to which it is intended that the solder shall adhere must first be tinned. This is accomplished by heating the metal to a temperature above the fusion point of the solder used, and then rubbing the surface with a stick of the solder, thus rubbing the oxide off the surface with the solder itself, and covering the exposed points with melted solder, all in the same motion. In order to make sure that the tinning is thorough, it-is better to rub the surface with a steel or brass scratch brush while the solder on this surface is still molten. This insures a thorough job of tinning. After the edges to be united are thus tinned, they may be sweated together with pure block tin. with the aid either of a soldering iron or blast lamp.

With regard to the composition of aluminium solders, zinc appears to alloy with aluminium more readily than any other metal available for the constituent part of the solder; consequently all solders which will readily tin aluminium contain zinc in varying proportions. The solders which we have found to be most satisfactory are composed usually of tin, zinc, and a very small proportion of aluminium. These solders do not run very freely nor fuse as readily as ordinary solders, and it is necessary, a- stated above, to use a higher temperature - so high in fact that extreme dfhculty is found in using these solders with a soldering iron, and it is generally necessary to use a blast lamp.

Another thing which must be borne in mind is that solder will not flow into an aluminium joint, even when tinned, by capillary action as it does into copper or tin joints, and it is therefore necessary to place on the surfaces to be united all of the material necessary to sweat them together before the edges are brought into contact. In soldering aluminium joints it is necessary that both the tinning and sweating shall be most thoroughly done; otherwise the joint will not be durable.

On account of the presence of zinc in the tinning solder, the solder is decomposed by moisture, and unless the work is so well done that the joint is absolutely waterproof, it will not be durable. The quality of the workmanship has more influence than anything else on the permanence of the work.