This section is from the book "Spons' Mechanics' Own Book: A Manual For Handicraftsmen And Amateurs", by Edward Spon. Also available from Amazon: Spons' Mechanics' Own Book.
For large and heavy pieces of iron and steel, copper or brass is used. The surfaces to be united are first filed off, in order that they may be clean. Then they are bound together with steel, and upon the joint a thin strip of sheet copper or brass is laid, or, if necessary, fastened to it with a wire. The part to be soldered is covered with a paste of clay, free from sand, to the thickness of 1 in., the coating being applied to the width of a hand on each side of the piece. It is then laid near a fire, so that the clay may dry slowly. The part to be soldered is held before the blast, and heated to whiteness, whereby the clay vitrifies. If iron is soldered to iron, the piece must be cooled off in water. In soldering steel to steel, however, the piece is allowed to cool slowly. The semi-vitrified clay is then knocked off, and the surface is cleaned in a proper manner. By following the hints given, it will be found that a durable and clean soldering is obtained. If brass, instead of copper, is used, it is not necessary to heat so strongly; the former recommends itself, therefore, for steel. Articles of iron and steel of medium size are best united with hard or soft brass solder.
In both cases the seams are cleanly filed and spread over with solder and borax, when the soldering seam is heated. Hard brass solder is prepared by melting in a crucible 8 parts brass, and adding 1 of previously heated zinc. The crucible is covered and exposed to a glowing heat for a few minutes, then emptied into a pail with cold water, the water being strongly agitated with a broom. Thus the metal is obtained in small grains or granules. Soft brass solder is obtained by melting together 6 parts brass, 1 of zinc and 1 of tin. The granulation is carried out as indicated above. Small articles are best soldered with hard silver solder or soft solder. The former is obtained by alloying equal parts of fine silver and soft brass. In fusing, the mass is covered with borax, and when cold, the metal is beaten out to a thin sheet, of which a sufficiently large and previously annealed piece is placed with borax upon the seams to be united and heated. Soft silver solder differs from hard silver solder only in that the former contains 1/16 of tin, which is added to it during fusion. Very fine articles of iron and steel are soldered with gold, viz. either with pure gold or hard gold solder. The latter can be obtained by fusion of 1 part gold, 2 of silver, and 3 of copper.
Fine steel wire can also be soldered with tin, but the work is not very durable. Hard and soft brass solders are used for uniting copper and brass to iron and steel, silver solder for silver, hard gold solder for gold.