This section is from the book "Spons' Mechanics' Own Book: A Manual For Handicraftsmen And Amateurs", by Edward Spon. Also available from Amazon: Spons' Mechanics' Own Book.
In soft soldering, the 6oldering-iron is only used for thin sheet metals, because, in order to unite 2 metals by soldering, their temperature must be raised to the melting-point of the solder, and a heavy body of metal cannot be sufficiently heated with the soldering-iron without making the latter too hot, which is apt to burn off the coating of tin, or to cause it to be absorbed by the copper, as in superficial alloying, and the solder will not adhere to the tool, and cannot be spread along the joint by it. In soft soldering heavy work, the work is first filed or scraped perfectly clean at the points to be soldered, and is dipped into a bath of liquid solder, which is covered with a little melted sal-ammoniac to prevent oxidation, and also to act as a flux for uniting the metals. In dipping the work into the bath, it first comes into contact with the flux, and is coated by it before it is subjected to the heat; when dipped into the solder, the tin readily adheres to it; and after heavy pieces of metal have been tinned in this way, or by the process of dry-tinning with mercury, they may be soldered with the soldering-iron. When tinning thin pieces of brass or copper alloys for soldering, it is usually done by rubbing a few drops of solder over the part to be tinned with the soldering-iron; and if tinned by dipping into a bath, it must bo quickly dipped, or there is a risk of the thin (direst being melted by the solder.
When tinning iron or steel, the work must bo allowed to remain in the bath, for some time, so as to be thoroughly heated by the bath, or the tin will not be completely united to the iron or steel, and may peel off when coll. Large pieces of iron or steel that are inconvenient to dip into a bath are tinned by heating in an open fire, and rubbing the solder on with the soldering-iron, using either sal-ammoniac. or rosin as a flux. When tinning in this way, the lowest heat that will fuse the solder should be used.
Hard solder differs from soft solder in that the "hard" is an alloy of silver and brass, while the "soft" is of bismuth, lead, etc.; the mode of working differs also. With hard solder, an intense and glowing heat is absolutely necessary to cause fusion of the metals, but with soft solder a comparatively low heat will suffice. It must be evident that by the former mode, where fusion takes place, there is a more complete union made than by the latter, where there is little more than cohesion. The latter mode of repairing has, however, these advantages, that as many articles are built up, so to speak, of pieces, and in such ways that only experienced workmen can handle them satisfactorily, the amateur may attempt repairing them with greater confidence and assurance of success, and he has no need to provide himself with a variety of chemicals, for the purpose of restoring the colour to the article that has been rendered unsightly by the heat. Apart from these advantages there are others, as soft soldering may be accomplished by the blowpipe, the soldering " bit," or actual contact with the flame. Preference is given to one method by one worker, to another by another; no absolute rule can be laid down; all three modes can be used as the necessities of the work in hand may require.
Rosin, sal-ammoniac, solution of hydrochloric acid and zinc, and in some cases fats, are used as a flux. Generally speaking, hydrochloric acid (spirits of salts) killed by zinc will answer all purposes : to make the solution, procure a pennyworth of spirits of salts, and place it in an open glass or glazed earthenware vessel; and having a number of small pieces of zinc, throw in a few. As they become consumed, throw in more until all chemical action has ceased. So soon as the zinc is put in, a violent action commences, and it is well to set the vessel down, as it becomes intensely hot, and emits a pungent vapour which it is wise not to inhale. When all turbulence has ceased, strain off the clear liquid and add twice its quantity of clear water, decanting all into a stoppered or well-corked bottle. A piece or two of zinc may be dropped in to kill any remaining salts. A soldering bit may be made by taking a piece of stout brass wire, say, rather thinner than a common wood penholder, and about 6 in. long, and hammering one end into the form of an abrupt spear-point; inserting the other into a wooden handle. Solder of a pure and easy-flowing kind should be procured; preference being given to that sold by dealers in jewellers' requisites.
A pair of tweezers or long slender pliers should also be got. Armed with these, no fear of burnt fingers need be entertained.
As an example to illustrate the operation, we may take the movable top of a silver-plated candlestick. It often happens that a too-low burning candle melts the solder away from the connections. To repair this, carefully remove all dirt and grease from the parts in contact, and scrape them bright with a knife or other tool. Then take the "bit" and file the end clean; dip it in the zinc solution, and, holding the afterpart in the gas flame, run a little solder all over the tip to "tin" it. Next, run a bead of solder on the end; then, taking either part of the broken top in the tweezers, apply, by means of a peg or piece of brass wire, a little of the solution to the part where the solder is required. Proceed to warm the metal top in the edge of the flame, at the same time holding the "bit" obliquely in the gas and in contact with the top. The solder will quickly melt, and attach to it, and whilst in a molten state must be thinly distributed all round on that part only which has to be connected with the socket. This has to be "tinned" in the same way. This done, lay aside the "bit' and take the blowpipe.
Holding the top inverted, place the socket in its position, and after putting a little more solution to the parts, direct a small flame all round the joint, turning the article about to do this. If the top has an ornamental filled edge to it, keep the heat as much as possible away from that part, or the filling, which is only lead or solder, will run out. A sufficient heat having been got, the solder, at the points of contact, will melt and run together. When it has run all round, press the socket gently down, and hold until the solder is seen to "set," and the union is then completed. Cool, and swill in water. If there is an excess of solder, and it has run out into a bead, a sharp knife-edge will detach it, and an oiled leather buff will remove the stain. A little cleaning with rouge will finish the work. Experience only in these matters teaches one how much or how little solder is required: use too little rather than too much at first. Do not let the solution spatter upon, or come in contact with, or vaporize near to steel tools, or they will soon have a coating of rust upon them.