Two methods of soldering are in common use among jewellers and silversmiths. Soft-soldering is done with fine solder (1 of lead and 2 of tin), and is used for articles that will not bear much heating. The metal is filed or scraped clean and bright where the solder is wanted to run, killed spirit is applied, and a little solder is run on the surfaces by applying gentle heat. Having thus been “tinned,” the parts are placed together and heat applied until they unite, a spirit lamp or a blowpipe being used. This is the sweating process, already referred to a number of times.

For soldering catches and joints to cheap metal brooches that have been silver-plated or gilt, the same solder is used as in the above. Both catches and joints can be cheaply purchased, hard-soldered on to small plates, square, oval, or crescent shaped, to suit all kinds of brooches. Take one of these and hold it with an old pair of soldering tweezers in the flame of a spirit lamp, and give it a coating of solder on its under side. First wet it with the killed spirit, and then place a small portion of solder on it, and hold it in the flame until it flows all over the plate. It can be assisted to flow evenly by a copper wire, which is also useful to apply the acid flux. Having “tinned” the catch, clean (by scraping bright) the brooch, and place the catch in position. Direct a gentle blowpipe flame to it until it is seen to settle down and the solder flows. Then wash it immediately in warm water to remove the acid and dry in sawdust, kept in a warm place. Use as little solder as possible, and only clean the brooch where the solder is required to run. Attention to these points will ensure a neat job.

Hard-soldering on jewellery, etc., is done with silver or gold solder, and requires the articles to be heated to a bright red. The parts are cleaned, and a paste of borax and water is applied as a flux. A small piece of the solder is also dipped in the borax paste and laid over the join. Gentle heat is first applied to bake the borax hard, then by the use of a blowpipe the parts are raised to a red heat until the solder runs. The instructions given in the preceding chapter apply generally.

With regard to gold-cased jewellery, it is useless to attempt to hard-solder gold that has the least trace of soft solder or lead on it; the heat causes the lead to heat into and rot the gold, and the articles will tumble to pieces. The only way to mend, say, a gold-cased lead ring is by soft-soldering a tin band or plate over it, applying the heat very gently to avoid melting the lead inside.

For hard-soldering a gold ring without discolouring it, use solders containing gold, the precious metal in the solder being afterwards laid bare by a process of annealing and pickling. The solders are prepared to suit the quality of the gold to be soldered, so that they may “colour” well and thus hide the joint. The following is a list of coloured solders:

Best solder: Fine gold, 12¹/₂ parts; fine silver, 4¹/₂ parts; copper, 3 parts.

Medium: Fine gold, 10 parts; fine silver, 6 parts; copper, 4 parts.

Common: Fine gold, 8¹/₂ parts; fine silver, 6¹/₂ 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.

Great care must be exercised in hard-soldering gem jewellery, as the stones are likely to be injured. Diamonds are the only stones that it is safe to heat to redness in soldering. Fancy coloured stones, such as rubies, emeralds, sapphires, topazes, amethysts, garnets aquamarines, or pastes must not be made hot. A ring with any of these stones may be hard-soldered at the back if the stones are covered up with a pad of wet tissue paper to keep them cool; but if the soldering has to be done anywhere near the stones, they must all be taken out by un-setting.

Articles set with pearls, turquoises, opals, or cat’s-eyes (these things are not really “stones”) will bear no heat whatever, and must all be taken out before soldering.

In cases where it is very desirable to leave the stones in place, in order to prevent their bursting when heat is applied to the jewellery, cut a juicy potato into halves and make a hollow in both portions, in which the part of the ring having jewels may fit exactly. Wrap the jewelled portion in soft paper, place it in the hollow, and bind up the closed potato with binding wire. Now solder with easy-flowing gold solder, the potato being held in the hand. Another method is to fill a small crucible with wet sand, bury the jewelled portion in the sand, and solder in the usual way.

To restore the colour of gold or silver after soldering, dip the articles while hot into pickling acid (1 part of sulphuric acid to 10 parts of water); or put them in a little acid in a pan and boil them in it. Here, again, diamonds are the only stones that may be dipped hot, and there is a slight risk even with them. Pearls, etc., must not touch the acid, either hot or cold.

After heating and pickling, all gold is of a pale colour, and the commoner the gold the paler. However, 18-carat gold may be restored to its original colour in a few moments by buffing with rottenstone and oil on a leather buff or on a brush, and following with rouge in the same way; or it may be burnished if the nature of the article permits this. Poorer qualities of gold may be restored to their bright colour by the same means, but this takes longer. Most 9-carat articles are gilt to improve their colour, and after soldering must be re-gilt to restore their original appearance.