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.