Soldering is the process of uniting the edges or surfaces of similar or dissimilar metals and alloys by partial fusion. In general, alloys or solders of various and greater degrees of fusibility than the metals to be joined, are placed between them, and the solder when fused unites the three parts into a solid mass; less frequently the surfaces or edges are simply melted together with an additional portion of the same metal.
The chemical circumstances to be considered in respect to soldering, are for the most part set forth in the section on the fusibility of alloys, page 300 to 304, to which the reader is referred. It is there explained, that the solders must be necessarily somewhat more fusible than the metals to be united; and that it is of primary importance, that the metallic oxides and any foreign matters be carefully removed, for which purpose the edges of the metals are made chemically clean, or quite bright, before the application of the solders and heat; and as during this period their affinity for oxygen is violent, they are covered with some flux which defends them from the air, as with a varnish, and tends to reduce any portion of oxide accidentally existing.
The solders are broadly distinguished as hard-solders, and soft-solders; the former only fuse at the red heat, and are consequently suitable alone to metals and alloys which will endure that temperature; the soft-solders melt at very low degrees of heat, and may be used for nearly all the metals.
The attachment is in every case the stronger, the more nearly the metals and solders respectively agree in hardness and malleability. Thus if two pieces of brass or copper, or one of each, are brazed together, or united with spelter-solder, an alloy nearly as tough as the brass, the work may be hammered, bent and rolled, almost as freely as the same metals when not soldered, because of the nearly equal cohesive strength of the three parts.
Load, tin, or pewter, united with soft solder, are also malleable, from the near agreement of these substances, whereas when copper, brass and iron arc soft-soldered, a blow of the hammer or any accidental violence almost certain to break the joint asundrr, so long as the joint is weaker than the metal generally; and therefore the joint is only safe when the surrounding metal from its thinnest is no stronger than the solder, so that the two may yield in common to any disturbing cause.
The forms of soldered joints in the thin metals have been figured and explained in pages 391 to 394; and soldered joints in thicker works resemble the several attachments employed in construction generally. When the spaces between the works to be joined are wide and coarse, the fluid solder will probably fall out, simply from the effect of gravity; but when the crevices are fine and close, the solder will be as it were sucked up by capillary attraction. All soldered works should be kept under motionless restraint for a period, as any movement of the parts during the transition of the solder from the fluid to the solid state, disturbs its crystallization and the strict unity of the several parts.
In hard-soldering, it is frequently necessary to bind the works together in their respective positions; this is done with soft iron binding-wire, which for delicate jewelry work is exceedingly fine, and for stronger works is the twentieth or thirtieth of an inch in diameter; it is passed around the work in loops, the ends of which are twisted together with the pliers. The Asiatics seldom use binding-wire, see note AF, Appendix. Vol. II., page 977.
In soft-soldering, the binding-wire is scarcely ever used, as from the moderate and local application of the heat, the hands may in general be freely used in retaining most thin works in position during the process. Thick works are handled with pliers or tongs whilst being soft-soldered, and they are often treated much like glue joints, if we conceive the wood to be replaced by metal, and the glue by solder, (see page 57,) as the two surfaces arc frequently coated or tinned whilst separated, and then rubbed together to distribute and exclude the greater part of the solder.
The succeeding "Tabular View of the Processes of Soldering" may be considered as the index to the entire chapter; which refers to the ordinary methods of soldering most metals. The chapter is arranged under three divisions, illustrated in distinct sections, preceded by one section on the modes of applying heat.
Note. - To avoid continual repetition, references are made to the pages of this volume which illustrate the respective subjects, and also to the lists on the opposite page, in which some of the solders, fluxes, and modes of applying heat are enumerated.