The assay buttons are flattened by a hammer weighing about 7 lbs., with a convex face, on an anvil kept quite bright and clean and used for this purpose only. A heavy blow is first delivered on the centre of the button, the diameter of which is thereby increased to nearly that of a threepenny piece. Two lighter blows are then given on opposite sides of the disc so as to elongate it, giving it the form shown at b, Fig. 102.
After being annealed in the iron tray, f Fig. 101, which is placed in the muffle by the tool, g, and left until it is red hot, the flattened buttons are passed in succession through a pair of jeweller’s adjusting rolls which are used for this purpose only, and are kept clean and bright. The oil is removed as completely as possible from the rolls before they are used, as otherwise the first fillets come out thinner than the remainder. The rolls are adjusted so that one passage through them reduces the buttons to the required thickness, which is about 0.25 millimetre, or 0.01 inch, or about the thickness of an ordinary visiting card. The “ fillets ” (c, Fig. 102) thus obtained should all be of uniform size and thickness, with “wire edges,” as ragged edges expose them to loss during the boiling. After being rolled they are replaced in the tray, f, and annealed at a dull red heat. The buttons and fillets are often annealed by a blowpipe on charcoal instead of in the furnace. They must not be made too hot, as that entails a loss of gold in the parting acid. It is best to raise them to a low red heat. The object of the first annealing is to soften the buttons and facilitate their passage through the rolls, while that of the second is to enable the fillets to be rolled into “cornets” or spirals, d, between the finger and thumb, or round a glass rod. Care is taken to leave that which was formerly the lower side of the button outside, for a reason given below. This face is easily recognised, as it is less brilliant than the other.
This was formerly effected by boiling with nitric acid in glass “parting flasks.” Platinum boiling trays save time, and are now used whenever possible. The silver is dissolved by the acid, which must be free from chlorine in any form, sulphuric and sulphurous acids, or sulphides from which sulphuric acid may be formed. These substances dissolve gold in the presence of boiling nitric acid. A small quantity of silver is kept in solution, in the stock of acid, so that chlorine or any other haloid, if present, would instantly be detected. It is sometimes stated that the acid must be free from nitrous fumes, but this is not necessary, as silver protects gold from the action of nitric and nitrous acids, and as soon as parting begins great quantities of nitrous fumes are generated.
When the flasks are used, 2 ozs. of nitric acid of specific gravity 1.2 are put into each flask and raised to boiling point. A cornet is then introduced and boiling continued for fifteen or twenty minutes (i.e., for about ten minutes after nitrous fumes cease to be given off). Hot distilled water is then added, the solution of nitrate of silver decanted off, and the flask washed by filling with hot water, and decanting. Two ounces of hot nitric acid of specific gravity 1.3 is now poured in and the boiling continued for fifteen or twenty minutes, a piece of fireclay or capillary glass tube being added to prevent bumping ; after which decantation and washing is twice performed. Another boiling with acid of specific gravity 1.3 is recommended by Chaudet with the object of dissolving out the last traces of silver and leaving the gold quite pure. This practice has been adopted by many assayers, but is useless and causes loss of gold. If any small particles of gold have become detached from the cornet, time must be allowed for them to settle before each decantation. After the last decantation the flask is filled with hot water, the top covered by a small porous crucible, and the whole is carefully inverted; the pure gold, which is of a dark brown colour and exceedingly fragile, falls through the liquid and rests in the crucible, the water which enters with it being afterwards poured off. The crucible is dried and then annealed at a red heat over gas or in the muffle, when the gold shrinks greatly, though still preserving its shape, hardens and regains its ordinary pale yellow colour. It can now be weighed.
When a platinum boiler is used the cornets are put on platinum pins, as at the Mint, or more usually into platinum cups, one of which is shown in Fig. 103. These cups are supported in a platinum tray (which holds 144 cups at the Mint) and the whole lowered by a platinum hook into a platinum vessel containing about 60 ozs. of hot nitric acid. Some attention must be paid to the temperature of the acid. At the Royal Mint acid of specific gravity 1.26 (in which, however, a small quantity of silver is already dissolved) is used, and the temperature at the moment of introduction of the tray is from 90° to 100° C. At higher temperatures the action is so vigorous that the vessel may boil over. Formerly, when the ratio of 2.75 parts of silver to 1 part of gold was used, great care was necessary in putting the assay pieces into the acid. The temperature of 90° could not be departed from widely. If the acid was colder than this the cornets tended to break up, some pieces being usually detached in the boiling of 144 cornets, even if the temperature was only 1° or 2° lower than the correct point. If the temperature of the acid is as low as 85°, the cornets are not immediately attacked ; they turn a coppery hue, and then bubbles of gas form slowly on them. In such cases (where the action does not begin within one or two seconds after entrance into the acid) cornets containing 2.75 parts of silver to 1 of gold all break up in acid of SG 1.26, and the assays are spoilt. Cornets containing 2 parts of silver to 1 of gold are less delicate. They can be put into acid of SG 1.26, even if it is cold, or at any temperature up to the boiling point, without showing any signs of breaking up. They can also be put into hot acid of SG 1.32 without injury, but are not safe in cold acid of the same strength. They break up in acid of SG 1.42, whether it is hot or cold. Cornets containing larger proportions of silver may not break up if the first acid is weaker, say of SG 1.2.
Boiling is kept up gently for about thirty minutes, and the tray is then withdrawn, drained, washed by dipping vertically in and out of a vessel of hot distilled water, drained again, and placed in a second platinum boiler filled with boiling nitric acid of specific gravity 1.32 (also containing a little nitrate of silver). In this the cornets remain for a period of about thirty minutes, when they are drained and washed as before, and are then ready for annealing.
The platinum tray is dried on a hot plate, and annealed over gas or in a muffle kept specially for the purpose, and free from lead fume, particles of bone-ash. The temperature should be as high as possible, consistent with the safety of the cornets. They fuse at 1,064° C. (having the same melting point as pure gold), at which temperature the muffle appears orange-red. If annealed at a low temperature, the cornets are rough in texture, dull and fragile, being crushed easily between the finger and thumb. In this condition they adhere to the platinum, and, in detaching them, fragments are often left sticking fast inside the cups. If annealed properly the cornets are smooth, lustrous, and hard, showing signs of incipient fusion under a magnifying glass, and only yielding to considerable force exercised by the finger and thumb. Under these conditions they can always be detached from the platinum entire, and do not readily absorb moisture and gases from the air. By being annealed the cornets, which after boiling are very soft and fragile, and dark brown in colour, shrink, and harden, and regain the ordinary yellow colour of gold (e, Fig. 102).
Relative advantages of Parting in Flasks and in Platinum Boilers.—The use of platinum trays and boilers effects a great saving of time in decanting and washing, as one operation takes the place of as many as 144. If the standard of an alloy is unknown, so that it is not certain that the cornet will remain entire in the acid, it must, of course, be boiled separately, as, if one cornet in the tray breaks up, fragments may adhere to a number of others. The manipulation of the platinum tray is easier than that of parting flasks, and, in addition, the treatment of the cornets is more uniform, so that the correction afforded by the use of checks is more trustworthy. On the other hand, if platinum or palladium is present in an individual cornet, it imparts a straw-yellow or orange colour to the acid ; but where a number of cornets are boiled together, it is obviously impossible to say from which the colour is derived, so that less information on the subject is obtained.