Best solder: Fine gold, 12-1/2 parts; fine silver, 4-1/2 parts; copper, 3 parts.
Medium: Fine gold, 10 parts; fine silver, 6 parts; copper, 4 parts.
Common: Fine gold, 8-1/2 parts; fine silver, 6-1/2 parts; copper, 5 parts.
The gold solder is cast in long ingots, rolled thin and flat, and cut up or filed into dust, and thus applied to the cleaned joints, using borax as a flux. After the joint has been closed under a blowpipe flame, the whole ring is annealed on an annealing plate to a dull red heat, then cooled, pickled in acid, and polished. The film of grease left by the polishing process is washed off in hot soda water, and the ring dried in hot sawdust. Hard-soldered rings may be coloured with a film of electro-deposited gold.
If the gold is of common quality, under 12-carat, to remove any excess solder make a mixture by reducing to powder 1 oz. of green copperas and 1/2 oz. of saltpetre and boiling in 5 oz. of water. This will crystallise when cool. Redissolve the crystals in eight times their bulk of muriatic acid. For use, add boiling water, and place the gold in the hot mixture. For gold of 12-carat or over, nitric acid and water (1 part of acid to 2 of water) will dissolve the solder without injuring the gold.
Gold solders used on gold articles are made from gold of the quality of the article--say, 18- or 15-carats--to which is added 1/12th or more of silver and 1/24th or more of copper. The quality of the solder is always a trifle inferior to the metal on which it is used, so that the solder may melt at a lower heat than the article. The melting point of 18-carat gold is 1995° F., of 15-carat 1992°, and 9-carat 1979°, while easy silver solder melts at about 1802° F. This shows that although 9- or 15-carat gold could be used to solder 18-carat, it is not possible to use 18-carat to solder 15-carat. The same principle applies to silver and brass; and the quality of the solder has to be known before any attempt should be made to carry out the actual soldering of an article. Another important point is that thin gold articles, like brooches, will not bear so hard a solder as the same quality of gold will do when made up solid, as in the case of a bangle ring. Solder for 18-carat and 15-carat is made thus: Take 1 dwt. of the gold, and add 2 gr. fine silver and 1 gr. fine copper; melt well together, and roll out thin. For 12-carat, the addition of 3 gr. fine silver and 1 of fine copper to the dwt. is advisable; while for 9-carat the most useful solder is made from 1 part fine gold, 1 part fine copper, and 2 parts fine silver.