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.
(I) The soldering of 2 metallic surfaces together implies something more than mere mechanical union, and probably depends in some measure upon the formation of an alloy between the solder and the metals joined by it: hence the necessity for clean contact, and therefore perfectly bright inoxidized surfaces. To ensure this condition, various solutions are used just at the moment of soldering. The most common is hydrochloric acid "killed" with zinc (i. e. in which zinc is dissolved until the acid takes up no more), forming zinc chloride, which runs over the surface exposed to it, removing any existing oxide, and preventing its further formation by the action of the air. Sal-ammoniac (ammonium chloride) sometimes replaces the zinc chloride, or is used in conjunction with it. Powdered rosin applied to the heated metallic surface forms a protective varnish which excludes the air and prevents oxidation. With the same object, borax (sodium biborate) is mingled with granulated hard solder just before use, either by crushing the borax and mixing dry, or by dissolving the borax in water and making a paste of the solution and the powdered solder.
(2) "Hard" or "strong" solder is commonly known as "spelter," a term properly applied to commercial zinc ingots. For some kinds of work, commercial spelter is not so well suited as other brasses; for ordinarily it consists of equal weights of zinc and copper, and in certain cases it is advisable to use a harder solder than is obtained by these proportions. The admixture of copper and zinc produces a series of alloys differing considerably in their qualities, and when tin is introduced, the increase or decrease of the zinc and tin produces a compound metal, the properties of which are widely different according to the relative quantities of the ingredients used in its pro-duction. Spelter when home-made is best prepared by melting the copper and zinc in separate crucibles, the copper being in a crucible large enough to hold the zinc as well. When both metals are thoroughly melted, the zinc is poured into the copper crucible, the two being stirred well, so as to ensure thorough admixture, when the alloy is poured out on to a bundle of birch twigs or pieces of coarse basket-work, supported over a tub of water, the object being to obtain the solder in the form of fine grains with an irregular crystallization.
If, when taken from the water, the spelter is not sufficiently uniform in size of grain, it is passed through a sieve, and the large particles are crushed in a cast-iron mortar or any suitable appliance, and again passed through the sieve, for fineness and uniformity of size are essential to the accomplishment of some examples of brazing in a thoroughly satisfactory manner. Manufacturers of hard solder, however, usually cast it into ingots, delaying the cooling in order to develop as much as possible the crystallization, which is found to facilitate the subsequent crushing and sifting of the spelter. The term "brazing" is often applied to the operation of "hard soldering," from the fact that the solder used is really a brass.
(3) The solder found in commerce generally is known as "coarse," " common," and "fine"; and the respective proportions of the metals are supposed to be - for coarse, 2 parts lead to 1 of tin; for common, equal parts; and, for fine, 2 parts tin to 1 of lead. These proportions can generally be detected in the manufactured article, for coarse solder exhibits on its surface small circular spots, caused by a partial separation of the metals on cooling; but these are wanting when the tin exceeds the lead, as in fine solder. In the ordinary solder of commerce, it is very rare that the tin exceeds the lead, and No. 1, or hard solder, of the shops, will, as a rule, be found to vary between 11/2 and 2 of lead, to 1 of tin. The commoner stuff - that which plumbers use for making wiped joints in leaden pipes - contains 2 1/2 to 3 parts lead and 1 of tin.
(4) Solder will sometimes get contaminated with zinc, burnt tin, lead, iron, etc, which causes it to "work short," "set," or crystallize, contrary to the general rule. This is known by the solder quickly curdling or setting and working rough, with the tin separating, and looking like so much sawdust, except in colour, which, if disturbed when cooling, is a kind of grey-black. This is often caused by overheating the metal, viz. by making it red hot or by dipping brasswork into the pot for tinning, and also when soldering brasswork to lead, when, if brasswork be dipped into the pot too hot, the zinc leaves the copper and the tin takes it up, because tin and zinc readily mix. A small portion of zinc will also cause the lead and tin to crystallize or separate. If you have any idea that there is zinc in your solder (the least trace is quite sufficient), heat it to about 800° F. (427° C), or nearly red hot, only just visible in the dark (if visible, or red hot, in the day time, it will be at least 1100° F.: red-hot irons do not improve solder). Throw in a lump of brimstone (sulphur), which melts at 226° F. (108° C), but at a greater heat, between this and 430° F. (221° C.) - just below the melting-point of plumbers' solder, it thickens, and from 480° to G00° F. (249° to 315° C.) remelts, and again becomes thinner.
At 773° F. (412° C.) the zinc melts, and being lighter than lead or tin, has a chance to float, especially with the aid of sulphur. The sp. gr. of lead is 11.45; tin, 7.3; zinc, 6.8 to 7 (just enough to rise); and sulphur, 1 • 98. The last named readily mixes with the zinc, etc, and carries the lot of foreign matter to the surface. It also brings up all the oxidized lead and tin in the form of a whitish powder called " putty powder," which may be in the pot, or makes it fly to the iron. Skim the solder well, and after the heat is brought down to about 400° F (204° C), or just below working-point, stir the lot well up in plenty of tallow, which will free the sulphur, and your solder will be clean. A good lump of rosin will improve it; and add a little tin. If you have very much zinc present, the best way will be to granulate the solder as follows: - Just at setting point, turn it out of the pot and break it up with the dresser, like so much mould or sand. Put it into an earthernware basin or jar, or back into the pot, and cover it with hydrochloric acid; let it soak for a day or so, then well wash the lot, and serve it as above. This will effectually take the zinc out.
Afterwards add a little more tin to compensate for that destroyed by the excessive heat and the acid. A little arsenic very readily carries zinc through the solder.
Overheating solder renders it "burnt," i.e. much of the various metals present is oxidized, producing a cloggy dull mass; this is remedied by the process just described which eliminates the injurious oxides. When there is only a small quantity of bad solder, it is best to make it up into fine solder, or use it for repairing zinc roofs. Do not put bought fine solder into plumbers' solder, as it may contain all sorts of metal. (P. J. Davies.)