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.
The best solder for general purposes, to be employed in soldering silver, consists of 19 parts (by weight) silver, 10 of brass, and 1 of copper, carefully melted together, and well incorporated. To use this for fine work, it should be reduced to powder by filing; the borax should be rubbed up on a slate with water, to the consistency of a cream. This cream should then be applied with a fine brush to the surfaces intended to be joined, between which the powdered solder (or wire) is placed, and the whole is supported on a small block of charcoal to concentrate the heat. In the hands of a skilful workman, the work can be done with such accuracy, as to require no scraping or filing, it being only needful to remove the borax when the soldering is complete, by immersion in "pickle."
Silver soldering as applied to silversmiths' work, is an art which requires great care and practice to perform it neatly and properly. The solder should in every way be well suited to the particular metal to which it is to be applied, and should possess a powerful chemical affinity to it; if this is not the case, strong, clean, and invisible connections cannot be effected, and that is partly the cause of roughness in goods, and not, as may more frequently be supposed, from the want of skill on the part of the workman. The best connections are made when the metal and solder agree as nearly as possible in uniformity as regards fusibility, hardness, and malleability. Soldering is more perl and more tenacious as the point of fusion of the solder rises. Thus tin, which greatly increases the fusibility of its alloys, should not bo used excepting when a very easy running solder is wanted, as in soldering silver which has been alloyed with zinc. Solders made with tin are not so malleable and tenacious as those prepared without it. Solders made from silver and copper only are, as a rule, too infusible to be applied to the general run of silver goods.
Solders are manufactured of all degrees of hardness, the hardest being an alloy of silver and copper; the next silver, copper, and zinc; the most fusible, silver, copper, and tin, or silver, brass, and tin. Arsenic is sometimes used to promote fusion but its poisonous vapours render its use inadmissible. In applying solder, of whatever composition, it is of the utmost importance that the edges, or parts to be united, should be chemically clean; and for the purpose of protecting these parts from the action of the air and oxidation during the soldering process they are covered with a flux, always borax which not only effects the objects just pointed out, but greatly facilitates the flow of the solder to the required places. Silver may be soldered with silver of a lower quality, but easy running solder may be made of 13 dwt. fine silver, 6 dwt. brass; the composition of brass being so uncertain, it is best to fuse zinc and copper with the silver, and the following proportions make a very easy running solder: 12 dwt. fine silver, 6 dwt. pure copper, 1 dwt. zinc. Brass sometimes contains lead, which burns away in soldering and must be carefully guarded against. Solder for filigree-work is prepared by reducing easy flowing solder filings and mixing it with burnt borax powdered fine.
In this state it is sprinkled over the work to be soldered, or the parts to be soldered are painted with wet borax, and the solder filings are sifted on and adhere to the borax. The flux which adheres to the work after soldering is removed by boiling the article in a pickle of sulphuric acid and water, 1 part to 30.