§ 94 Preparing a Soldering Bit.
The wedge-shaped edge of one of the forms of bit shown in the sketch is filed to shape and the bit heated in a fire or on a gas heater. A bit of rough sandstone, or even a clean soft brick, or a bit of tin plate having some sand sprinkled over it, is placed in a convenient position and sprinkled with resin.
As soon as the bit is hot enough to melt solder it is withdrawn and a few drops of solder melted on to the brick or its equivalent. The iron or bit is then rubbed to and fro over the solder and resin till the former adheres to and tins the copper head. It will be found advisable to tin every side of the point of the bit and to carry the tinning back at least half an inch from the edge.
If the solder obstinately refuses to adhere, the cause is to be sought in the oxidation of the copper, or of the solder, or both — in either case the result of too high a temperature or too prolonged heating. The simple remedy is to get the iron hot, and then to dress it with an old file, so as to expose a bright surface, which is instantly passed over the resin as a means of preserving it from oxidation. If the process above described be now carried out, it will be found that the difficulty disappears.
Before using the iron, wipe off any soot or coke or burned resin by means of an old rag. An iron tinned in this way is much to be preferred to one tinned by means of chloride of zinc.
A shorter and more usual method is carried out as follows: The solution of chloride of zinc is prepared by adding bits of zinc to some commercial hydrochloric acid diluted with a little (say 25 per cent) of water. The acid may conveniently be placed in a small glazed white jar (a jam pot does excellently), and this should only be filled to about one-quarter of its capacity. An excess of zinc may be added.
It may be fancy, but I prefer a soldering solution made in this way to a solution of chloride of zinc bought as a chemical product. The jar is generally mounted on a heavy leaden base, so as to avoid any danger of its getting knocked over, for nothing is so nasty or bad for tools as a bench on which this noxious liquid has been upset (Fig. 78).
To tin a soldering bit, a little of the fluid is dipped out of the jar on to a bit of tin plate bent up at the edges—a few drops is sufficient — and the iron is heated and rubbed about in the liquid with a drop of solder. If the iron is anything like clean it will tin at once and exhibit a very bright surface, but quite dirty copper may be tinned by dipping it for a moment in the liquid in the pot and then working it about over the solder. An iron so tinned remains covered with chloride of zinc, and this must be carefully wiped off if it is intended to use the iron with a resin or tallow flux in lead soldering.
One disadvantage of this process is that the copper bit soon gets eaten into holes and requires to be dressed up afresh. On the other hand, an iron so tinned always presents a nice clean solder surface until the next time it is heated, when it generally becomes very dirty and requires to be carefully wiped before using.
In my experience also an iron so tinned is more easily spoiled as to the state of its surface, “detinned,” in fact, by overheating than when the tinning is carried out by resin and friction. When this happens, the shortest way out of the difficulty is the application of the old file so as to obtain a perfectly fresh surface. No one who knows his business ever uses an iron that is not perfectly clean and well tinned.
The iron may be cleaned from time to time by heating it red hot and quenching it in water to get rid of the oxide, which scales off in the process.