Hard solders are distinguished as brass, German silver, copper, gold, silver, etc., according to the alloys used (see Brass Solders, Copper Solders, etc., for other hard solders).

The designation "hard solder" is used to distinguish it from the easily running and softer solder used by tinsmiths, and it applies solely to a composition that will not flow under a red heat. For the purposes of the jeweler solder may be classified according to its composition and purpose, into gold or silver solder, which means a solder consisting of an alloy of gold with silver, copper, tin, or zinc-like metal or an alloy of silver with copper, tin, or zinclike metal. According to the uses, the solder is made hard or soft; thus in gold solders there is added a greater amount of silver, whereas for silver solders there is added more tin or zinc-like metal.

In the production of solder for the enameler's use, that is for combining gold with gold, gold with silver, or gold with copper, which must be enameled afterwards, it is necessary always to keep in mind that no solder can be used effectually that contains any tin, zinc, zinc alloys, or tin or zinc-like metals in any great quantities, since it is these very metals that contribute to the cracking of the enamel. Yet it is not possible to do without such an addition entirely, otherwise the solder would not flow under the melting point of the precious metals themselves and we should be unable to effect a union of the parts. It is therefore absolutely necessary to confine these additions to the lowest possible percentage, so that only a trace is apparent. Moreover, care must be taken to use for enameling purposes no base alloy, because the tenacity or durability of the compound will be affected thereby; in other words, it must come up to the standard.

In hard soldering with borax, direct, several obstacles are encountered that make the process somewhat difficult. In the first place the salt forms great bubbles in contact with the soldering iron, and easily scales away from the surface of the parts to be soldered. Besides this, the parts must be carefully cleaned each time prior to applying the salt. All these difficulties vanish if instead of borax we use its component parts, boric acid and sodium carbonate. The heat of the soldering iron acting on these causes them to combine in such a way as to produce an excellent flux, free from the difficulties mentioned.

Composition of Various Hard Solders

Yellow solders for brass, bronze, copper, and iron:

I

Sheet-brass chips, 5 parts, and zinc, 3 to 5 parts, easily fusible.

II

Sheet brass chips, 3 parts, and zinc, 1 part; refractory.

III

Sheet-brass chips, 7 parts, and zinc, 1 part; very refractory and firm.

Semi-white solders, containing tin and consequently harder:

I

Sheet brass, 12 parts; zinc, 4 to 7 parts, and tin, 1 part.

II

Copper, 16 parts; zinc, 16 parts, and tin, 1 part.

III

Yellow solder, 20 to 30 parts, and tin, 1 part.

White solders:

I

Sheet brass, 20 parts; zinc, 1 part, and tin, 4 parts.

II

Copper, 3 parts; zinc, 1 part, and tin, 1 part.

To Hard-Solder Parts Formerly Soldered with Tin Solder

To repair gold or silver articles which have been spoiled with tin solder proceed as follows: Heating the object carefully by means a of small spirit lamp, brush the tin off as much as possible with a chalk brush; place the article in a diluted solution of hydrochloric acid for about 8 to 10 hours, as required. If much tin remains, perhaps 12 hours may be necessary. Next withdraw it, rinse off and dry; whereupon it is carefully annealed and finally put in a pickle of dilute sulphuric acid, to remove the annealing film. When the article has been dipped, it may be hard soldered again.