Properties of Gold & Alloys

From very early times the ancients were attracted by the beautiful colour, the brilliant lustre, and the indestructibility of gold, and spared no pains in the endeavour to acquire it. In the code of Menes, who reigned in Egypt in 3600 B.C. or about 2000 years before Moses, the ratio of value between gold and silver is mentioned, one part of gold being declared equal in value to two and a half parts of silver, and it is, therefore, clear that the extraction of both metals from the deposits containing them must have been carried on before that time. It is, indeed, probable that gold was the first metal observed and collected, since it occurs in fragments of all sizes in loose sand, and the operations of collecting the larger pieces and melting them together are so simple. Among the rock carvings of Upper Egypt there are several illustrative of the art of washing auriferous sands by stirring and working them up by the hand in hollowed-out stone basins, and subsequently melting the gold in simple furnaces with the aid of mouth blow-pipes. The earliest of these carvings is supposed to date back to about 2500 B.C. However, in ancient times gold appears to have been mainly derived from India, and that country continued to supply most of the gold used in Europe until the discovery of America by Columbus.

In order to collect alluvial gold, the sands were washed down over smooth sloping rocks by means of running water, and the particles of gold, sinking to the bottom of the stream by reason of their high density, were entangled and caught in the hair of raw hides spread on the rocks. Among the hides used were sheepskins, and hence originated the form of the legend of the Golden Fleece. Stripped of its heroic dress, this legend merely describes a successful piratical expedition about 1200 B.C. to win gold, which was being laboriously obtained from streams with the help of sheepskins or goatskins by the inhabitants of what is now Armenia. Similar expeditions have not been unknown in much later times, and the method of obtaining gold by washing river sand is still practised, with improvements in matters of detail, in many parts of the world. Metallurgists are almost proverbially conservative in their methods. Hides are even now occasionally employed to catch the gold, but sheep’s wool, when used, is generally in the form of blankets.

At the present day, however, when auriferous sands are washed, the aid also is invoked of what Baron Born called in 1786 the “elective affinity” of mercury for gold when mixed with impurities. The ease with which gold-amalgam can be collected, in spite of its being less dense than gold itself, is due to the fact that it is wetted by mercury.

In the history of gold, it is also of interest to the metallurgist to remember that the earliest dawn of the science of chemistry was heralded by the study of the properties of gold, and by the efforts which were made to invest other matters with these properties. From the fourth to the fifteenth century, chemistry, which was first called “chemia” (xnµeia), and then “alchemy,” was defined as the art of transmuting base metals into gold and silver, almost all the labours of philosophers being intended to aid directly or indirectly in solving this problem. At the end of this period, while Paracelsus was giving to chemistry a new aim—that of investigating the composition of drugs, and their effect on the human body—Agricola was reducing to order the numerous empirical facts which together made up the art of metallurgy, and although alchemy died hard, its era of usefulness may be said to have ended here. Gold has doubtless been the cause of many of the wars and marauding expeditions from which the world has suffered, but on the other hand it has been instrumental, in a far greater degree than most other commodities, in promoting the growth of civilisation, the efforts of the alchemists having laid the foundations of the science of chemistry, and those of the gold-seekers having resulted in the discovery of new countries, and in the spread of knowledge of all kinds.

The Colours of Gold

.—The lustre and fine colour of gold have given rise to most of the words which are used to denote it in different languages. The word “gold” is probably connected with the Sanscrit word “jvalita” which is derived from the verb “jval,” to shine. It is the only metal which has a yellow colour when in mass and in a state of purity. Impurities greatly modify this colour, small quantities of silver lowering the tint, while copper raises it. In a finely divided state, when prepared by volatilisation or precipitation, gold assumes various colours, such as deep violet, ruby and reddish-purple, the tint varying to brownish-purple and thence to dark brown and black. This purple colour has been supposed by some experimenters (viz., Guyton de Morveau, Buchner, Desmarest, Creuzbourg and Berzelius) to be due to the formation of a coloured oxide of gold of unknown composition, but Buisson, Proust, Figuier and, more recently, Kruss have shown that no oxygen can be obtained from this coloured material, and that it probably consists of metallic gold. Similar colours are seen in purple of Cassius, and in Roberts-Austen’s purple alloy of aluminium and gold, the colour in each case being probably due to a particular form of finely divided gold. “ Faraday’s gold,” a ruby-coloured liquid prepared by the action of phosphorus and ether, or of formaldehyde on a solution of chloride of gold, is a solution of colloidal metallic gold in water. Finely divided gold gives a faint blue tinge to light transmitted through the liquid in which it is suspended. Gold precipitated from its solution as bromide is in a different molecular condition from that formed from chloride, giving out 3.2 calories in passing into the latter state. The surface colour of small particles of native gold is often apparently reddened by being coated with translucent films of oxides of iron. Very thin plates of gold are translucent, and appear green by transmitted light, while remaining yellow by reflected light. On heating, the green colour changes to ruby red, but is restored by the pressure of a hard substance by which the state of aggregation is again altered (Faraday). Molten gold is green, and its vapour is also probably greenish.

Malleability and Ductility of Gold

Malleability . and ductility are possessed by gold at all temperatures to a far higher degree than by any other metal. A single grain of gold can be drawn out into a wire over 500 feet long, and leaves of not more than of an inch in thickness can be obtained by beating. Faraday has shown that the thickness of these leaves may be still further reduced by floating them in a dilute solution of potassium cyanide by which they are partly dissolved.

What is the Hardness of Gold

Gold is softer than silver and harder than tin. Its hardness corresponds to the number 979 in Bottone’s scale, in which the diamond is 3,010.

How Tough is Gold

The purest gold obtainable has a tenacity of 7 tons per square inch and an elongation of 30.8 per cent., but the presence of as little as 1/2000 of other elements, especially bismuth, tellurium, lead, and other metals with high atomic volumes, greatly lowers these constants, as well as the malleability and ductility of the metal, while its hardness is increased. See also the section on alloys of gold.

Specific Gravity of Gold

The specific gravity of gold when precipitated from solution by oxalic acid is 1949 (G. Rose); when cast it varies from 19.29 to 19.37, but this can be raised by compression to over 19.48. Henry Louis has shown that the specific gravity of unannealed “parted” gold (i.e., the residue left after boiling silver-gold alloys in nitric acid) is 20.3, its density being lowered by the process of annealing. When precipitated by ferrous sulphate, its density may be as high as 20.72 (G. Rose).

Taking the density of pure gold at 19.3, then 1 c.c. of pure gold weighs 19.3 grammes or 0.6205 oz. troy. The weight of 1 cubic inch is 316.25 grammes or 10.168 ozs. troy. The weight of 1 cubic foot is 546.485 kilogrammes or 17569.9 ozs. troy. The volume of 1 kilogramme of gold is 51.81 c.c., or 3.162 cubic inches, and the volume of 100 ozs. troy is 161.16 c.c., or 9.835 cubic inches.

Cohesion of Gold

On heating, gold can be welded like iron below the point of fusion, and finely divided gold agglomerates on heating without being subjected to pressure. Pressure alone is also sufficient to make gold dust cohere, while a true flow of the particles of gold can be induced in the case of the pure metal and some of its alloys.

Specific Heat of Gold

The specific heat of gold is 0.0324 (Regnault) or 0.0316 (Violle).

What is the Fusibility of Gold

Gold fuses, after passing through a pasty stage, at a clear cherry-red heat, just below the fusing point of copper and much above that of silver. The metal expands considerably on fusing, and contracts again on solidifying. The freezing point was given by Heycock & Neville at 1061.7°, and more recently by Berthelot as 1064°.

What is the Spectrum of Gold

In the gold spectrum Huggins saw 23 lines, the wavelengths of the most important ones being 523.1, 583.5, and 627.6 respectively.

Latent Heat.—The latent heat of fusion of gold is 16.3, and the normal lowering of the freezing point for 1 atom of impurity in 100 atoms of gold is 10.6°, but the presence of a few per cent, of silver causes practically no alteration in the freezing point.

Is Gold Magnetic

Gold is diamagnetic, its specific magnetism being 3.47 (Becquerel), if that of iron is taken as 100.

Conductivity and Expansion.—Its electrical conductivity is 76.7 and its thermal conductivity 53.2 (Wiedemann and Franz; or, according to Despretz, 103), that of silver being 100 in each case. Its coefficient of linear expansion is 0.0000144 between 0° and 100° (Fizeau).

What is the Atomic Weight and Volume of Gold

Its atomic weight, formerly believed to be about 196.2, has been more recently given by Kruss as 196.64, by Thorpe and Laurie as 196.85, and by Mallet as 196.79. The atomic volume of gold is 10.2.

Volatilisation of Gold

The boiling point of pure gold has not been determined; calculated according to Wiebe’s formula it would be about 2,240°, or nearly 500° above the melting point of platinum. However, contrary to the belief of the older experimenters (Gasto Claveus, and others), it is sensibly volatile in air at far lower temperatures. Robert Boyle was unaware of this fact, but Homburg gilded a silver plate in 1709 by holding it over gold strongly heated in the focus of a burning mirror (Encyclopedia Britannica, 1778, and Gmelin’s Handbuch), and St. Claire Deville volatilised and again condensed gold when melting it with platinum. The rapid volatilisation of gold, when heated by an ordinary blowpipe, was first proved in 1802 by Dr. Robert Hare, of Philadelphia (Tilloch’s Magazine), a purple stain being thus produced on bone-ash in a few seconds. Lastly, a discharge of high-tension electricity from gold points causes its volatilisation, and if the discharge is sent through a fine gold wire stretched on paper, it converts it into a purple streak of finely divided condensed particles of the metal. The rapid distillation of gold caused by heating it in a current of air of considerable velocity, such as that furnished by a blowpipe, by which the liquid is thrown into waves, may be shown at any time by heating a fragment of the precious metal of the size of a pin’s head on a bone-ash cupel in the oxidising flame of a good mouth blowpipe. Almost immediately after the fusion is complete, a purple stain of condensed gold begins to form on the outer margin of the cupel. The author has found that a piece of gold ; weighing 0.5 gramme loses half its weight in an hour, if heated on a cupel by a foot blowpipe (the temperature attained being probably less than 1,300°), and only a few minute beads are observable, detached from the main button. Alloys of copper and gold disappear much more rapidly. No doubt most of the gold passes off as spray, but perhaps part of the loss may be due to rapid volatilisation, and could not be correctly described as mechanical loss.

The volatility of gold, both when pure and when alloyed with silver and copper, was investigated by Napier, who found that an alloy of 100 parts gold to 12 parts of copper, if kept for six hours at a temperature just high enough to keep it melted, lost 0.234 per cent, of its gold contents, and at the highest temperature attainable in an assay muffle, it lost 0.8 per cent, in six hours. An increase in the amount of copper present caused an increase in the loss of gold. In the simple operation of pouring about 30 lbs. of a gold-copper alloy from a graphite crucible into moulds, fumes were given off, of which the part condensed in a wet glass beaker held above the crucible contained 4.5 grains of gold. Napier also found that gold does not appear to volatilise so readily when alloyed with silver only, as when copper is also present.

Makins found that gold volatilises sensibly along with silver and lead, when melted with these metals in a muffle in an ordinary bullion assay. The loss of gold by volatilisation on melting its copper alloy is the common experience of mints. At the Sydney mint, Leibius found that the sweepings from the top coping-stone of a chimney 70 feet high contained 1.46 per cent, of gold and 6.06 per cent, of silver. Similar results have been obtained at some other mints.

According to experiments made by the author, the loss of gold on heating the pure metal rises with the temperature, being four times as great at 1,250° as at 1,100°, whilst it is insignificant at 1,075°, and probably nil at the melting point, 1,061°. The nature of the atmosphere has also an effect on the rate of volatilisation, the loss in carbon monoxide being double that in air, and six times that in coal gas. A protective layer of charcoal would, therefore, increase the loss by volatilisation. The volatilisation of gold is also increased by the presence of any metallic impurity, even by the non-volatile metals, such as platinum. Lead and platinum have a very slight effect in increasing the volatility of gold; copper and zinc a more marked effect, while 5 per cent, of antimony or mercury causes losses amounting to about 2 parts per 1,000 of gold per hour at 1,245°. The metals which have most effect in reducing the surface tension of the liquid gold appear to increase its volatility in the greatest degree.

It may be added that Hellot stated that if an alloy of 1 part of gold and 7 parts of zinc is heated in air, the whole of the gold comes off in the fumes, but recent experiments show that if mechanical loss due to the violent boiling of zinc is avoided, the amount of gold carried off by zinc fumes is insignificant.

It has also been shown by the author that tellurium does not cause volatilisation of gold at temperatures below 1,100°. Samples of an alloy of 78 per cent, of gold and 22 per cent, of tellurium were heated in a porcelain boat inclosed in a porcelain tube, through which a glass tube was passed. A current of water through the glass tube kept it cool. The alloys were heated for various lengths of time up to one hour at temperatures between 500° and 1,100°, in currents of different gases, air, carbon monoxide, hydrogen, and water gas (carbon monoxide and hydrogen in about equal volumes) being used in successive experiments.

In each case the whole or a part of the tellurium was sublimed and condensed on the cold tube, but the sublimates in only one case contained a trace of gold. In the other cases the whole of the gold was found still to remain in the boat. The exception was when a current of air was passed, the oxide of tellurium condensed on the cold tube in that case being found to contain 0.03 per cent, of the total gold originally present, while 99.96 per cent, of the gold was found in the boat.

A second series of experiments on a telluride ore from Western Australia, containing over 1,000 ozs. of gold per ton, gave similar results.

The losses incurred in roasting gold tellurides are probably due to fine dust being carried away mechanically, or to the absorption by the furnace bottoms of the very fusible mixtures which are formed.

Crystallisation of Gold

Little is known of the Allotropic Forms of Gold. The marked influence of traces of other metals on the properties of gold has already been touched on; from this and from the variations in colour and other properties the existence of several allotropic modifications of gold might be inferred. In alloys containing appreciable quantities of other metals, evidences of allotropy are not met with so frequently. The potassium alloy, however, containing 10 per cent, of gold, on being attacked by water, leaves a black finely-divided gold powder, and there is reason to believe that this combines with water to form a hydrate.

Wilm states that if gold is dissolved in dilute sodium amalgam under water, the aqueous liquid becomes dark violet, and when this is acidulated with hydrochloric acid, a black precipitate of pure gold is obtained. The black gold differs from the ordinary modifications in its extreme lightness; moreover, it is soluble in alkaline solutions, and does not amalgamate with mercury or with sodium amalgam. When heated, it yields the ordinary modification as a violet red powder. This form of gold appears, from Wilm’s account, to resemble the black precipitate obtained on digesting certain aluminium-gold alloys with hydrochloric acid.

ALLOYS OF GOLD