The process of brazing is conducted as follows : -
Granulated spelter and borax, ground together in water, are spread over the carefully cleaned surfaces of the joint, and exposed gradually to the heat of a clean open fire; the borax fuses first, and then the solder.
With silver solder the joint is covered with borax and water, or dry powdered borax, and the solder, cut into little square plates, is laid along the joint.
Tin makes the solder fusible, but as it is more expensive than lead, only so much tin should be included in a solder as will make it fit for the purpose for which it is intended.
The addition of a little bismuth makes the solder still more fusible.
The more fusible solders are known in the trade as fine, and those containing less tin as coarse solders.
"Any zinc getting into plumber's solder will ruin it, by making it too brittle to work.
"Solder may be purified of any foreign matter, such as zinc, if only present in small quantity, by burning it out on the fire, letting the pot get red hot till it goes off in vapour and scum, which can be skimmed off the top." 1
"In making solder the proportions of the metals can be judged of from the appearance of the alloy. When it contains a little more than one-third of its weight of tin, its surface on cooling exhibits circular spots due to a partial separation of the metals; but these disappear when the alloy contains two-thirds its weight of tin." 2
"It is never advisable to buy ready-made solder, as you cannot depend upon the alloy; too much lead and too little tin, which is the dearer of the two, is almost sure to be put into plumber's solder; besides which there is always plenty of scrap lead about, which can be used for the purpose.
"When a good deal of soldering is to be done, the plumber will often start with a little excess of tin in his solder, as by degrees it will pick up lead from the lead work on which it is used, which will decrease its fusibility."1