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: