Sulphur is the best thing to use for this purpose.
When a pot of solder has been found to be poisoned with zinc, it is heated to just below a red heat. Lump sulphur is broken up and granulated, it is then screwed up tight in three or four thicknesses of paper, and in this form is thrown into the pot and held below the solder with a ladle. As the paper burns the sulphur rises through the solder, combines with the zinc, and floats on the surface. The solder is well stirred so as to thoroughly mix the sulphur with the whole of the contents of the pot, the dross which is formed by this process is then skimmed off with a ladle and thrown away as useless.
In the case of the sulphur, although it is generally called a flux, the action that takes place is altogether different to that of resin and tallow. It may safely be inferred by reference to the results of chemical combinations that the zinc, having a great affinity for sulphur, as soon as it comes in contact, forms sulphide of zinc, this is really a substance similar to zinc blende, a common form of zinc ore. In this condition, the specific gravity being considerably reduced, it readily rises to the surface of the solder, where it can be skimmed off with a ladle.
The question naturally arises - why is it the sulphur does not combine with the lead to which it also has an affinity, and thus form sulphide of lead? If lead is heated only just above its melting point and then some sulphur is mixed with it, a substance would be formed similar to galena, or sulphide of lead. But if the temperature is raised several degrees higher the sulphide gives up the lead, and either floats to the top or passes off in the form of gaseous vapor, chemically termed sulphurous anhydride. Therefore, by heating the solder containing zinc to a temperature just below redness, it is hot enough to prevent the sulphur combining with the lead and tin, but not sufficiently heated to cause the sulphur to give up the zinc, which fuses at a temperature of 773 degrees Fahrenheit, whereas lead fuses at 612 degrees Fahrenheit, and in combination with tin as solder at 441 degrees Fahrenheit. The difference in the melting points is in all probability the principal cause of the sulphur attracting the zinc and leaving the lead and tin comparatively unaffected.
Another method of extracting the zinc from solder is to raise the temperature to a very bright red heat, if this is continued long enough the zinc vaporizes and passes off in a gaseous state.
The latter is a very wasteful process because it cannot be done without a large proportion of the tin becoming oxidized. The oxide gathers in the form of a powder on the surface, and is what is commonly known as putty powder. One of the most common means of spoiling solder is the last mentioned.
The flowing of solder, especially that used with the copper-bit, depends to a large extent upon the fluxes that are used for tinning purposes. For soldering lead only a very simple flux is necessary, namely, a little tallow and powdered resin. The same kind of flux is also very often used for tinning and soldering brass and copper, and there are many plumbers who use nothing else but a piece of common tallow candle, which seems to answer the purpose very well. For soldering iron, zinc, and tin goods, chloride of zinc, or what is commonly called killed spirit of salt, is generally used, although it is not necessary to kill the hydrochloric acid when zinc has to be soldered. Soldering fluids and preparations have been invented which have, to a very large extent, superseded the common fluxes. The disadvantage of spirit of salt is owing to the tendency it has to produce oxidation on iron, and chlorides on zinc, after the soldering is done.
It would be interesting to try and find out the reason why a combination of metals fuses at such a low temperature when compared with the fusing points of the component parts of the alloys. It is necessary to bear in mind the fact that all metals, and indeed all matter, are composed of minute particles or molecules, and that there is nothing existing that is a strictly solid uniform mass. It is also acknowledged that the molecules of different substances always assume a distinctive shape, and when metallic matter is crystallized, as it is said to be when it becomes solid by the action of cold, these particles are attracted to each other by a force of more or less power according to the nature of the metal, whether it is said to be hard or soft.
Now the force by which these aggregations of minute particles are held together is what is called cohesive attraction, and the power of this force to hold the particles together depends to a very great extent upon the particular shape which these extremely small particles assume, and the amount of surface which they present to each other. It is very easy to conceive that if a number of bodies have mutual attraction for each other, the larger the surface that comes in contact the more force is there exerted one with the other. If, for instance, the particles take the form of spheres like a number of marbles, the surface in actual contact is comparativelv very small indeed, the same would be the case if they were very irregular in form. But if each particle took the form of a cube, or some other regular body, the attraction would be greatly increased, as each of the particles approached and fitted into its proper place. It is not contended that the molecules are actually attracted into absolutely close contact, because, as a matter of fact, they are not. In every substance, however hard and solid it might appear to be, there are certain interstices between the particles which are called pores, the capacities of which vary according to peculiar conformation of the particles, and the degree of affinity which one set of particles may have for others in the same mass. It follows then that as a rule the hardness or softness of any substance depends, according to the theory of cohesive attraction, upon the close and compact nature of the molecules, and the large or small spaces or Interstices between them, that is, so far as the action of heat is concerned. If it is required to make a hard substance soft and pliable, some power is necessary to exert a reactionary influence upon the attractive force which causes the particles to cohere. Now the only powers that will effectually produce this result is heat, when heat is applied to nearly all metallic substances, the first thing it does is to enlarge the bulk by the almost irresistible force of expansion. The effect that heat has on a solid is to cause the particles to be thrown farther apart from each other by a repulsive force, overcoming to a certain extent the force of cohesive attraction. This repulsive action continues to increase as the temperature is raised, until the attractive force has to give way to the force of gravity.
The result is the particles will no longer cohere in a mass, but fall away from each other and become in a state of fluid, and if they are not kept together in a vessel of some kind dur-ing their high temperature they will run in any direction by the influence of gravity like ordinary liquids. When a metal is in such a condition it is said to be melted or fused. There are some metals, zinc for instance, the particles of which are separated to a much greater extent than is the case with fusion only. For if the heat is applied so that the temperature is raised above fusing point, - evaporation takes place, and the molecules are driven off in the form of vapor.
When two distinct metals are mixed together, such as tin and lead, the cohesive attraction is modified to a large extent, because the molecules of one have a comparatively small affinity for the other. Of course tin has a certain amount of affinity for lead, in fact, if there were no affinity between the two, solders would be useless on lead, because tinning could not be effected if such were the case. But what seems certain is, when the two metals are alloyed, the molecules are not held together by the same attractive force that is exerted when a metal is not alloyed, that is, the particles of one metal do not, by reason of their difference of construction or conformation, have the same affinity for each other as they do when they are not intermixed with other particles of a different nature.
Consequently, when such combinations of metals are subjected to the action of heat, the particles mutually assist each other to separate, and gravitate like liquids to a level surface, with a much lower degree of temperature than is required to obtain the same effect when the metals are melted separately.
Then with regard to wiping solder, it retains its fluid and plastic state for a much longer time than lead or tin would before they are mixed, showing that the particles, probably for the same reason, do not solidify so quickly as they would in a separate state. If they did, joint-wiping would, of course, be impossible, for on the peculiar power that solder has to retain its heat, or rather the effects of heat, depends the success of the most important parts of plumbing work. An alloy of lead and tin contracts considerably in cooling, the result of this can be seen when a solder pot is placed on the fire. Before the bulk of the solder melts, but as soon as that part which is near the hottest part of the fire begins to fuse, the molten metal forces its way up to the top, between the sides of the mass of solder and the sides of the pot, this often continues until the top of the unmelted mass is covered with a melted layer which has forced its way there, showing that when the solder cooled it contracted into a smaller space than it occupied when it was in a fluid state. Consequently, when the lower part of the solder is melted first, the expansion that takes place forces it of necessity to the top, because there is not room for the increased bulk in the space it was reduced to during the process of cooling. But if antimony, the fusing point of which is 840 degrees Fahrenheit, is added to lead and tin, the result is just the reverse, for on cooling this alloy expands. The latter alloy is generally used for casting types for printing, the proportions of which are two of lead, one of antimony, and one of tin, although a more expansive alloy is made of nine of lead, two of antimony, and one of bismuth. Then with regard to the hardness of metals, it is not always that the hardest metals require the highest temperature to fuse them. Tin, for instance, is much harder than lead, yet it fuses at a temperature nearly 200 degrees Fahrenheit lower than lead.