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.
Soft soldering is the art of soldering or uniting 2 of the fusible metals or 2 pieces of the same metal. The solder used is a more soft and fusible alloy than the metals united, and the mode of applying the heat is consequently different from that employed in hard soldering. The soft solders are prepared in different forms to suit the different 'classes of work for which they are intended. Thus for tin soldering, the solder is cast into bars of 10 or 12 in. long by 1 in. wide, and by some it is cast into cakes 10 or 12 in. long by 3 or i in. wide. Plumbers' solder is generally cast into small ingots or cakes, 2 in. square or more, according to the work for which they are intended, and size of pot they are to be melted in. Some of the very fusible solders that are destined for very light work are trailed from the ladle upon an iron plate, so as to draw the solder into thin or large bars, so that the size of the solder may always suit the work that it is used upon.
In soft soldering, it is very essential that the parts to be united should be perfectly clean and free from metallic oxides, and for this reason they are generally wet with a little zinc chloride before applying the solder; and when the metal is old or very dirty, it must be scraped on the edges intended to be united before applying the solder. When soldering leaden pipe, sheet lead, etc, the plumber first smears a mixture of size and lampblack around the intended joint to prevent the melted solder adhering to the metal at the point where it is not wanted. The parts to be united are then scraped quite clean with the shave-hook, and the clean metal is rubbed over with tallow. The wiped joints are usually made without using the soldering-iron. The solder is heated in the plumbers' pot rather beyond its melting-point, and poured plentifully upon the joint to heat it. The solder is then moulded into the proper shape, and smoothed with cloth or several folds of thick bed-ticking, which is well greased to prevent burning, and the surplus solder is removed by the cloth. In forming the striped joint, the soldering-iron and cloth are both used at the commencement in moulding the solder and heating the joint.
Less solder is poured on when forming this joint than when forming the wiped joint, and a smaller quantity remains upon the work. Striped joints are not so neat in appearance as wiped joints, but they are often claimed to be sounder, from the solder having been left undisturbed when in the act of cooling; but in wiped joints, the body of solder is heavier, and the shrinkage of it around the pipe is sufficient to unite with the pipe. In forming joints in leaden pipe, the cloth is always used to support the fluid solder when poured on the pipe.
Light leadwork that requires more neatness than the ordinary plumbing is usually soldered with the common tinners' soldering-iron. This is made of a square piece of copper weighing 3 or 4 oz. to 3 or 4 lb., according to the size of the work it is intended for. This piece of copper is drawn down to a long square point, or to a flat wedge, and is riveted into an iron shank fitted to a wooden handle. The copper bit or soldering-iron is then heated in the tinners' firepot with charcoal to dull redness, and is then screwed in the vice and hastily filed to a clean metallic surface. It is next rubbed with a piece of sal-ammoniac, or on some powdered rosin, and then upon a few drops of solder in the bottom of the soldering-pan. In this way the soldering-iron is thoroughly coated with tin, and is then ready for use. In soldering tin-plate work, the edges are slightly lapped over each other, and the joint or seam is strewed with powdered rosin, which is usually contained in a small box set in the soldering-pan. The soldering-iron, which has been heated in the firepot, is then drawn over the cake of solder; a few drops are melted and adhere to the soldering-iron, and are distributed by it along the joint or seam.
In large work, the seams are first tacked together, or united by drops of solder so as to hold the seams in proper position while being soldered; but this is seldom done in small work, which can be easily held together with the hands. Two soldering tools are generally employed, so that while one is being used for soldering, the other is being reheated in the firepot, thus avoiding the delay of waiting for the tool to heat. The temperature of the tool is very important: if it is not hot enough to melt the solder, it must be returned to the fire; and if it gets too hot, the tinning will be burnt oft", the solder will not hang to it, and the tool must be retinned before it can be used. In soldering tinware, the tool is usually passed only once over the work, being guided by the contact with the fold or ledge of the seam; but when the operator is not an expert, he usually runs the tool backward and forward over the work 2 or 3 times. This makes slow work.
Sheet copper, in common work, is soldered with the soldering-iron in the same manner as sheet tin; but the finer or more important work is brazed or hard soldered. In soft soldering copper, as well as sheet iron, the flux generally used is powdered sal-ammoniac, or a solution of sal-ammoniac and water. A piece of cane, the end of which is split into filaments to make a stubby brush, is used for laying the solution oa the work, and powdered rosin is subsequently applied. Some workmen mix the powdered sal-ammoniac and rosin together before applying it to the work. This they claim is better than putting them on separately; but so long as the metals are well defended from oxidation, either of the modes is equally good, for the general principle is the same in both. Zinc is the most difficult metal to solder, and the joints or seams are seldom so neatly formed as in tin or copper. Zinc will remove the coating of tin from the soldering tool in a very short time. This arises from the superior affinity of copper for zinc than for tin, and the surface of the tool is freed from tin, and is coated with zinc.
Sal-ammoniac is sometimes used for a flux in soldering zinc, but the most common flux employed for zinc is zinc chloride, which is made by dissolving fragments of zinc in hydrochloric acid diluted with about an equal amount of water. This solution is put in a wide-mouthed bottle, and small strips of zinc are dropped into it until they cease to be dissolved. The solution is then ready for use; it is likewise resorted to for almost all the other metals, as it can be employed without such strict necessity for clean surfaces as when some of the other fluxes are availed of.