A few crystals of saltpetre, say one or two ounces, were then dropped into the centre of the metallic surface, and as they melted, their spreading out over the whole surface was aided by the concentric motion of the bottom of a small crucible. The moment the visible oxidising action began to slacken, the melter skimmed off, by a small blacklead dipping crucible, the fluxed matter as rapidly as was consistent with the care necessary to avoid taking up metal. The remainder in the melting pot was the toughened metal.

§ 145. Gold Plating. This consists of joining a bar of gold and gilding or other metal together by sweating or soldering. A bar of gold of a desired quality and a bar of base metal are first made perfectly flat under a stamp or press; then the surfaces to be joined are filed or scraped clean; borax is next prepared and well rubbed over the surfaces. The two bars of metal are firmly secured together by iron wire, placed in a muffle, and the temperature raised nearly to the point of fusion when the metals unite into one compact bar. This is termed joining by "sweating."

Another method, which is more generally adopted, is to join the two bars by soldering. The two bars are prepared as in the former case. The metal-bar, being larger than the gold-bar, supports the pieces of solder, which are placed along one side, and half-way along each end. The whole is then strongly heated in a muffle until the solder melts, and joins them together. These compound bars may be rolled, stamped, spun, or otherwise manipulated, as though they had been melted to form one homogeneous mass. The gold follows the reduction of the base metal during the processes of rolling, etc., and retains the relative proportion of thickness between them.

HARD SOLDERS FOR GOLD WORK

§ 146. With respect to hard solders, it may be taken as a general guide that the more nearly the solder approximates in composition and properties to that of the metal to

be soldered, the more perfect will the union of the two parts be, and the stronger the point of juncture. On the other hand, the greater the difference between the melting points of the solder and the metal to be soldered the easier will the operation be. In all cases it is necessary that the solder should have a lower melting point than the metal to be soldered.

Solder is said to be "hard" when it has a high melting point, i.e. at or above a red heat, and is literally hard with regard to its power of resisting the pressure of a cutting or abrading tool. As the melting point of the solder alloy is lowered by the addition of an easily fusible metal, or by the addition of a greater amount of the more fusible constituents of the mixture, the solder is said to be " easy" or softer.

Gold solders are made in a great variety of degrees of softness and hardness by the addition of different proportions of a more fusible metal, such as silver, to the gold. Thus, in 18-carat gold, for example, the solder may be made from the 18-carat alloy by adding a certain proportion of silver and copper, so as not to materially alter the colour. If the solder is made too poor in quality, the articles will not colour properly. Coloured gold solders contain 1 part of silver to 4, 5, or 6 parts of alloyed gold according to the degree of fusibility desired. Gee gives the following tables for coloured-gold solders :

Description.

1

These solders may be rolled thin, and cut with the shears and pressed into suitable pieces termed "pallions," or filed into dust, according to the needs of the workman.

1 Goldsmiths' Handbook, pp. 136, 217. 1881.

oz. dwt. gr. oz. dwt. gr. oz. dwt. gr. oz. dwt. gr. £. s. d.

The following table gives the composition of various solders for alloys of different standards :-

§ 147. Colouring of Gold. This operation consists of imparting a colour to gold articles after every other process has been completed. Its object is to give to alloyed gold all the appearance of fine gold itself by dissolving out the base metal from the surface of the articles and leaving a facing of gold of a deep rich colour. Two distinct modes of colouring are adopted by jewellers, termed respectively dry colouring and wet colouring. The latter is most frequently practised, as the former cannot well be applied to gold inferior to 18 carat.

Wet Colouring. The ingredients of the mixture employed in this process have a powerfully solvent action on the base metal with which the gold is alloyed, and a weaker action on the gold itself, so that the article loses weight in direct ratio to the length of time it is submitted to the colouring process, and this loss is greater as the gold is lower in quality. 1 Gee states that the colouring is hastened and the loss in weight reduced to a minimum by using old colouring liquid, and he assumes that the dissolved gold is, to some extent, deposited again on the article, because the loss in weight of some common qualities of gold was found to be very little, and the amount of gold recovered from the spent colouring liquid very small indeed. This statement is in accord with the well-known fact that in any liquid in which a metal, say copper, is electro positive to the metal in solution, say gold, the latter is deposited on the former.

Many different mixtures are used for colouring gold, some of which will be afterwards given in tabular form. The following has been supplied to the author by an experienced Birmingham jeweller, which he has found to be effective :

The nitrate and salt are pounded to a fine powder and placed in a previously warmed plumbago crucible about 8 inches by 7 inches, then stirred with a wooden spoon for a minute or two. The acid is then added with about 1 ounce of boiling water, and the mass constantly stirred until it boils up to the top of the pot. The work, which has been previously cleansed in hot potash or soda solution, is then suspended in the colouring liquid by means of a silver or platinum wire for about one minute, then well swilled in boiling water. A little more water is added to the colourpot, and when the liquid boils up the work is again 1 Goldsmiths' Handbook, p. 161.

immersed for another minute, and swilled in boiling water as before. This operation of dipping and swilling is repeated several times, the colouring liquid being weakened by adding water before each immersion, until the desired appearance is attained. The work is finally well washed in hot water and dried in boxwood saw-dust. The whole process takes five to seven minutes.

The coloured work is next scratch-brushed, on a lathe, with a revolving brush made of very fine brass wire and having stale beer dropping on it. If the colouring has been properly conducted, a beautiful rich and dead colour will be produced.

Dry Colouring. This term is applied to the colouring process when no liquids are used as constituents of the mixture. The ingredients used are—

These substances are ground to a fine powder, well mixed and placed in a previously heated blacklead "colour "-pot, of the same dimensions as that described for use in wet colouring, but the same pot must not be employed for dry colouring as has been used for the wet process. It is well to get the pot nearly red-hot before placing the "colour" in it. The mixture must then be constantly stirred with an iron rod. It will first boil up as a greenish liquid, then solidify, and afterwards boil up a second time and become thoroughly fused, having a brownish-yellow colour. At this stage the work, which has been previously annealed and dipped in dilute aquafortis, is dipped in the "colour," being suspended on a silver or platinum wire, the latter being preferred, and kept in motion for about a minute and half, then immersed in boiling water containing a little aquafortis. The immersion and swilling are again repeated, when the articles possess a beautiful colour. They are then washed in hot water containing a little potash, and finally dried in warm boxwood saw-dust.