Plumber's wiping solder, for use with the ladle and the soldering cloth, is made up by melting together pure lead and block tin in the proportion of 2 pounds of lead to 1 pound of tin. Plumber's fine solder is made of about equal parts of those two metals. Strip solder - used with the copper-bit - is made in the proportion of 2 pounds of tin to 3 pounds of lead. Gas-fitter's solder may be made in the proportion of 8 pounds of tin to 9 pounds of lead, tinsmith's copper-bit solder is 1 pound of lead to 1 pound of tin. The proportion of lead and tin may vary within certain limits without apparent effort on the solder.
Plumber's wiping solder, when in a bar, should have a clean grey appearance, and not be dirty-looking. The ends of the bar should be bright, and show several tin spots mottled over their surfaces. In use, the solder should work smooth, and not granular. The tin should not separate from the lead on the lower part of the joints. One test for the quality of solder is to melt it and then pour on to a cold but dry stone about the size of a dollar, and take note of the color and size and also the number and sizes of the spots that appear, but the only reliable test is to make a joint and note the ease with which it can be worked. For making joints on lead pipes copper-bit solder made in thin strips is generally used. This is the kind used also for soldering zinc. Some plumbers prefer solder finer, others coarser than the usual average which is given above.
The usual method of making solder is as follows: An iron pot is suspended over a coke fire, to which enough broken coke is added to bank up all round the pot. Sheet-lead cuttings and scraps of clean pipe are put into the pot until it is rather more than half full. Preference is given to pig-lead over sheet, and to new cuttings over pipe, because the lead rolled into sheets is generally purer than that used for pipe. Some pipe is made of old metals which contain lead, tin, antimony, arsenic, and zinc, it is inadvisable to put such material in the solder-pot. The effect would be to raise the melting point of the solder, and in applying it to the joint to be soldered it would in all probability partially melt the lead. Moreover, the metals named do not alloy perfectly, but partake more of the nature of a mixture which partially separates when making a joint, some metals, especially zinc, show as small bright lumps on the surface. Joints made with such solder, which usually is called poisoned metal, are difficult to form, and they usually leak when in water pipes. The appearance of such joints is a dirty grey, instead of bright and clean as when pure solder is used. From this it is clear that in making solder great care must be taken to exclude zinc from the pot. Zinc, lead, and tin do not alloy well, lead will unite with only 1.6 per cent of zinc, and above that proportion the metals are only mixed when melted, and on cooling partially separate.
Sufficient lead having been melted in the pot, about ½ pound of lump sulphur, broken into pieces about the size of hickory nuts, is added, and the whole well stirred with a ladle, the sulphur unites with zinc and other impurities. The resultant sulphides are skimmed off in the form of a cake, more sulphur being added so long as sulphides continue to form. The bowl of the ladle, in the intervals of stirring, should be laid on the fire, to burn off any adherent sulphur. When sulphide ceases to be formed, a handful of resin is thrown into the pot, and the lead stirred. When the resin has burned, the lead is again skimmed, and a piece of tallow about the size of a hen's egg is put into the pot, the lead being again stirred and skimmed. In stirring the lead it is lifted up and poured back by the ladleful, a larger amount of lead being thus exposed to the action of the cleaning material.
Best block tin is now added in the required proportion, and after the molten mass has been well stirred a little of the mixture should be run on to a stone to test its fineness. If it appears too coarse more tin is added, if too fine, more sheet-lead. Finally, a little resin and tallow having been added, the solder is skimmed and is then ready for use or for pouring into moulds. When plumber's solder is heated in an open pot, the surface exposed to the air combines with oxygen, and on heating to redness, the combination takes place more readily. The tin melts at a lower temperature than lead, and so its specific gravity is lighter, floats. when melted, and so the solder becomes poorer when too highly heated, owing to the tin's oxidation. If the dross is melted with a flux, or with powdered charcoal, which will combine with the oxygen, the solder will again become fit for use, but it is sometimes necessary to add a little more tin.
Burning the solder must be carefully avoided. A pot of solder after it has been red-hot has always a quantity of dross or dirt collected on the top. This is principally oxide of tin and oxide of lead, the tin and lead having united with the oxygen in the atmosphere to form oxides of these metals. Lead being roughly 50 per cent heavier than tin, the tendency is for the tin in the molten mixture to form the upper layer of the solder - the part most exposed to the action of the atmosphere. When the solder becomes red-hot, there is therefore more tin burned than lead. Hence the solder becomes too coarse, and more tin must be added. Zinc is the greatest trouble to the solder pot. Great care has to be taken to exclude it, or to get it out. It may get into the solder from a piece of zinc, having been put into the pot by mistake for lead, but more commonly brass, which is an alloy of copper and zinc, is the source of the zinc that poisons the pot, into which brass filings find their way whilst brass is being prepared for tinning. If the filing is done at the same bench as the wiping, splashes of metal may fall on the filings, which will adhere, and thus get into the pot. Solder that is poisoned by arsenic or antimony is beyond the plumber's skill to clean, but zinc can be extracted by stirring in powdered sulphur when the solder is in a semi-molten condition, and then melting the whole, when the combined sulphur and zinc will rise to the surface, and can be taken off in the form of a cake, the solder being left in good condition for use.