The composition and properties of solders are a matter of considerable interest to all metal workers, but the subject is of especial importance to plumbers, because on the quality and purity of solder depend in a large measure the reliability and good appearance of their work. Nothing is more annoying, nor is there anything so productive of bad work, waste of time, and consequent irritability and bad temper, as the trying to do good work with bad material, particularly if that material is wiping or plumbers solder. Until recent years it was invariably the practice for plumbers to make their own solders, either from the pure lead and tin, or, old joints and solders were melted down, and tin added in proportion. Of late years it is becoming quite unusual for plumbers to know anything about solder-making. Plumbers consider it more economical to buy it, already made, from firms who make solder-making a branch of their manufacturing trade. Another advantage is, that if supplied by a firm of good standing it can generally be depended upon for purity and uniform ouality.
Good plumbers' solder should consist of two parts of lead to one of tin, but the proportions, of course, vary according to the quality of the constituent parts. Tin, for instance, varies very much in quality, and no fluxing or a superabundance of the tin will make good solder if this metal is of an inferior kind. It is, therefore, far the most economical in the long run to use tin of the very best quality.
As the exact proportions, as they are generally given, depend to a very great extent upon the condition of the two metals, it follows that the mere mixing of certain quantities of tin and lead does not necessarily make a composition that will serve the purpose that it is intended for, but a plumber with an experienced eye can detect at a glance the inferiority and usefulness of such solders when required for the execution of good work.
Although it is not absolutely necessary that a good solder-maker should be a plumber, it is important that he should have a considerable knowledge of the appearance of solder in proper condition. In the absence of a practical test, there are certain indications by which the solder may be judged, whether it is good or bad. The most common practice is to run out a strip of solder on a smooth level stone. As soon as the strip is nearly cold, the quality of the solder or the proper proportion of tin and lead can be determined by the appearance of both surfaces. It is important, before running the solder out on the stone, that it should be at such a heat as to allow the solder to run freely. A temperature just below red heat is the most suitable for this purpose, if the solder is not hot enough, it will have a dull white look, whether it is good or bad.
If it is in good condition, it should have a clean, silvery appearance, bright spots should also form on the surface from an eighth to a quarter of an inch in diameter. As a rule, the larger the spots the finer is the solder, although some kinds of tin will not show large spots, however much is used. In such cases they should appear more numerous.
If the strip has a dull, dirty appearance and a mottled surface, it is evident the solder is not as pure as it should be. It probably contains some mineral impurities, which can generally be removed by well heating the solder in the pot, and stirring into it a quantity of resin and tallow. These substances have but very little, if any, chemical effects, either upon the solder or the foreign matters it may contain, but the action that seems to take place is that they combine with the lighter mineral matters by what may be called adhesive attraction, and cause them to rise to the surface, where they can be skimmed off. There are some earthy impurities that get into the solder, the specific gravities of which are probably much lighter than the solder itself, but which will not rise to the surface until assisted by means of fluxes. It must be remembered that although tin has a specific gravity of 7.3 and lead 11.445, it is therefore, necessary to well stir the solder while it is being poured into the moulds, as the tin will continually rise to the top, yet if it were not stirred at all after it was once mixed, the lower portion would not be wholly deprived of tin, showing that the greater specific gravity of the one does not wholly displace the other. The same is true of certain impurities, which are not removed until they are washed out, as it were, by means of fluxes such as resin and tallow.
The greatest enemy to plumbers' solder is zinc. If the slightest trace of this metal gets into a pot of solder, it is almost a matter of impossibility to wipe joints with it, especially underhand joints.
When zinc is present, the strip of solder has a dull, crystallized appearance on the surface. The tin spots are also very dull and rough, and not at all bright and clean. When solder of this kind is being used for wiping, the first thing noticed is that a thick, dirty dross forms on the surface directly after it is skimmed. It is im-possible to keep the surface clean for even a second. When it is poured on a joint, it sets almost instantly, and it matters not at what heat it is used. As soon as one attempts to move it with the cloth, it breaks to pieces, and falls off the joint.
In the case of branch joints when an iron is used, the solder cools in hard lumps, and breaks away like portions of wet sand. There are two or three ways of extracting zinc from solder, one is to partly fuse it, and when it is nearly set to pulverize it until the particles are separated as much as possible. The whole is then placed in a pot or earthenware vessel and saturated with hydrochloric acid, commonly called muriatic acid. The acid dissolves the zinc and produces chloride of zinc; the latter can be washed out with clean water and the solder returned to the pot in a comparatively pure state. This method cannot be recommended as a certain cure, because of the difficulty there exists in dividing the particles to such an extent as to expose the whole of the zinc that may be contained in it, and considering the small amount of zinc that is sufficient to poison a pot of solder it is doubtful if the acid process is radical enough in its action to thoroughly eradicate the zinc without repeated applications.