MATICS. ) The making of gold from base metals by means of the "philoso pher's stone" and discovery of the elixir of life were the chief aims of the alchemists of the middle ages and many of the advances in early chemistry were the direct outcome of such ex periments, and even to-day the transmutation of base metals into gold is not regarded as scientifically impossible. Gold is found very widely distributed in nature, but usually occurs in such small quantities as not to repay extraction. It is generally found in the native or uncombined state but is almost invariably associated with variable proportions of silver. It exists in two forms—reef gold, in which the metal is embedded in a solid matrix, and allu vial gold, which has been formed by the weathering of auriferous rocks. Gold also occurs in sea water; assays of sea water from various sources vary from 5 to 267 parts of gold per ioo,000,000 of water. These minimal amounts represent in the aggregate an enormous quantity, which has been calculated at r0,000,000,000 tons (Mendeleyev, The Principles of Chemistry). Many proc esses, fraudulent or otherwise, have been proposed for extracting gold from this source, but none has proved commercially possible.
The principal elements with which gold is admixed in nature are silver, tellurium, copper, iron, bismuth, mercury, palladium and rhodium. The native gold-silver alloys are known as electrum, and have a colour range from pale yellow to pure white, depending on the amount of silver present.
Gold is found combined with tellurium in the minerals calave rite, in which generally the gold is partly replaced by a certain amount of silver ; sylvanite, ; nagyagite, which contains lead, copper, antimony and sulphur; and other tellurium ores to which local names have frequently been given. Gold mercury ores are known as gold amalgam. The other metals mentioned above are usually present in small amounts only and are not of great importance.
Gold is generally present to a small extent in iron pyrites, and much alluvial gold is regarded as having been produced by the weathering of this mineral; in addition, galena, PbS, which often contains silver, is generally found to include appreciable amounts of gold, so that the silver obtained from lead by cupel lation is usually auriferous. The common occurrence of gold to gether with silver is well shown in the fact that, even at the present time, several of the smaller countries in North, Central, and South America possess a silver coinage which, being of the intrinsic silver value of their contents and not mere tokens, as in Great Britain, renders the extraction of gold from them an economical process. This condition existed in Germany prior to the establishment of the German Empire in 1871, when each small German State possessed its own silver currency. These coins were recalled by the Imperial Government and refined by a company specially formed for this purpose (Deutsche Gold and Silber Scheide Anstalt of Frankfurt a/M) with the result that after several years' work, gold to the value of about £3,000,000 was recovered.
Gold in the massive state possesses a characteristic yellow colour which by multiple reflection becomes orange or even red. This colour can be remarkably affected by alloying the gold with other metals. Small quantities of silver reduce the depth of yellow (vide supra), and when the amount of silver is increased to 30-40% a distinctly greenish tint results. Copper, on the other hand, deepens the yellow shade, and British standard gold coinage of 22-carat gold, or 91.67% Au to 8.33% Cu, is noticeably redder than the pure metal.
The effect of small quantities of the platinum metals on the colour of gold is very marked; thus, less than 25% platinum gives a pure white alloy—the white gold of the jeweller—which, as it can be made to contain 75% of gold, can be and is hall-marked as 18-carat gold. Palladium has a still greater whitening power than platinum and about 12% is sufficient to produce a perfectly white alloy. A remarkable colour effect is produced by alloying gold with aluminium. Roberts-Austen produced a fine purple alloy (containing about 8o% Au and 20% Al), but unfortunately too brittle to be made into jewellery.
In a finely divided state the colour of gold is very variable, depending upon the size of the particles. The usual colour of precipitated gold is brown, but black, purple, blue and pink shades are also known. In very thin sheet or leaf, gold is translu cent and transmits a greenish light. Gold when pure is the most malleable and ductile of all the metals; it can be beaten to not more than o•0000r mm. in thickness (see GOLD-BEATING), and a single gram has been drawn into a wire 2 miles long. Traces of other metals reduce considerably the malleability and ductility, lead being especially injurious in this respect. Cadmium, tin, bismuth, antimony, arsenic, tellurium and zinc act in like manner.
Gold is one of the softest metals, its hardness varying accord ing to treatment and being somewhere between that of zinc and tin ; it is considerably softer than silver under all conditions. Dental gold is really a gold sponge produced by electrolytic meth ods ; it is so soft as to be capable of welding at ordinary temper atures.
The specific gravity of gold also depends upon its previous treatment. Cast gold is always somewhat lighter than gold that has been rolled or drawn. Different observers give the specific gravity of cast gold as from 19.23 to 19.29 and for worked gold as from 1 9. 29 to 19-34. Precipitated gold, however, has a greater density and varies with the precipitant employed and the temper ature of precipitation. Ferrous sulphate appears to give the densest precipitate which has been found by G. Rose to be as high as 20.72. For practical purposes the density of pure gold may be taken as 19.3.
The melting point of gold has been determined by many observers with varying results but the mean of recent observations is 1,063° C. Gold is comparatively easily volatilized at high tem peratures; at its melting point the loss is insignificant, but be comes appreciable at higher temperatures, and at 1,250° C it is 2.6 parts per thousand per hour (T. K. Rose). In all mints and gold refineries the flues are carefully swept periodically and con siderable quantities of the metal are thus recovered. In the presence of other metals, volatilization is greater than with pure gold, tellurium and selenium being most active in this respect and zinc and mercury less so. Some of the recently reported transmu tations of mercury into gold have been traced to the mercury containing a minute quantity of gold which had not been removed by simple distillation. The boiling point of pure gold is about 2,50o° C.
The electrical conductivity of gold is greatly influenced by traces of impurities, so reported values are very variable. At ordi nary temperatures the conductivity of gold is about 75% of that of pure silver, which has the greatest conductivity of any metal. The electrical resistance, which is the converse of conductivity, steadily diminishes with a lowering of the temperature, and at the boiling point of helium in vacuo (i.e., below 5° Absolute) it has practically disappeared (H. K. Onnes), or, in other words, gold is then a perfect conductor of electricity.
The mean specific heat of gold is 0.03, a number which agrees well with the law of Dulong and Petit. Its coefficient of linear expansion is about 0.000014 for r ° C.
The spark spectrum of gold is very complicated ; the most prominent lines in the visible spectrum lie at 6278 and 5957 in the orange and red, 5837 and 5656 in the yellow, 5065 in the green, 4793 and 4437 in the blue, and 4065 and 3898 in the violet.
Gold is permanent in air or water under all conditions of tern perature. It is insoluble in nitric, hydrochloric or sulphuric acids, but soluble in hot selenic acid forming gold selenate. Hot telluric acid likewise dissolves it. The usual solvent for gold is aqua regia (q.v.)—a mixture of 3 volumes of strong hydrochloric acid with one volume of strong nitric acid which in practice is always diluted with a considerable volume of water. The nitric and hydrochloric acids interact producing nitrosyl chloride (NOC1) together with free chlorine, which attacks the metal. The other halogen elements, fluorine, bromine and iodine, also attack gold freely, producing the corresponding halogen compounds. Gold is also soluble in aqueous solutions of alkaline sulphides and thio sulphates. Alkali cyanides, even in very dilute solution, attack gold readily, especially in the presence of air or oxygen (see below) .
Gold and Oxygen.—Gold and oxygen do not combine directly under any conditions; hence all oxides and hydroxides have to be made by indirect methods. Two well-determined oxides of gold are known, namely aurous oxide, and auric oxide, The oxides Au202, and have been described, but their individuality is doubtful.
Aurous hydroxide, AuOH, is best prepared either by treating a neutral aqueous solution of auric chloride, with mercurous nitrate, or by decomposing aurous chloride, AuCl, or bromide, AuBr, with cold dilute caustic potash or soda (not ammonia). It is also obtained by boiling an aqueous solution of auric chlo ride with the alkaline salt of an organic acid such as potassium acetate. It is a violet-black powder which, on heating to about 200° C loses water giving violet-brown aurous oxide, which at 25o° C decomposes into gold and oxygen. The oxide and hydroxide have feebly basic properties and are capable of forming salts with halogen acids.
Auric hydroxide is produced by precipitating a solution of auric chloride or of aurichloric acid, with a limited amount of caustic alkali. The hydroxide thus prepared cannot be entirely freed from alkali by washing, and the precipitation is preferably effected with magnesia or zinc oxide, excess of the precipitant being removed with dilute nitric acid. Auric hydroxide is a brownish-black powder which, on drying over phosphoric oxide, forms a brown powder of auryl hydroxide, Au0 (OH) , dehydrated at 14o° C to trioxide, and this oxide on further heating to 17o° is said to lose oxygen and form the oxide Auric oxide is capable either of forming salts with haloid acids or of acting as an acidic anhydride by combining with strong bases to form curates. Potassium aurate, is a yellow crystalline compound ; Ba 2 is a yellow precipitate.
Halogen Compounds.—Fluorine does not act on gold in the cold but only at a dull red heat, when a yellowish deposit is formed.
Two chlorides of gold are known with certainty, aurous chloride, AuCl, and auric chloride, The identity of an intermediate chloride, is doubtful. Aurous chloride is almost always formed by heating auric chloride. The optimum temperature is about 175° C, and several days are required to complete the reaction. If a higher temperature is used complete decom position occurs into gold and chlorine. This decomposition of auric into aurous chloride takes place to some extent even in hot aqueous solution.
Aurous chloride is a yellowish-white solid insoluble in cold water but undergoing slow decomposition into gold and soluble auric chloride.
Auric chloride can be obtained either by heating aurichloric acid to 200° in a stream of chlorine, or by dissolving gold in chlorine water, preferably in darkness. It is obtained as a reddish brown powder or as ruby-red crystals ; it gives a neutral solution and can be sublimed unchanged in a stream of chlorine. The auric chloride of commerce is really aurichloric or chloroauric acid, a brown deliquescent substance very soluble in water or ether.
If gold is dissolved in aqua regia and the resulting solution freed from nitric acid by evaporation with further quantities of hydrochloric acid to near the crystallizing point, the dissolved gold compound corresponds to the formula but on allowing this solution to crystallize, brownish-yellow crystals of aurichloric acid are formed, having a strongly acid reaction. These crystals always contain a small amount of aurous chloride unless chlorine has been passed through the solution during evaporation. They are also frequently contaminated with small amounts of silver chloride as this substance is soluble in strong solutions of auric chloride and is only precipitated therefrom by considerable dilution with water.
Aurichloric acid forms a series of salts called aurichlorides or chloroaurates, having the general formula MAuC14. These salts may be obtained either by neutralizing the acid with the metallic base or by treating the acid with the equivalent amount of the metallic chloride. The aurichlorides of lithium, potassium and sodium are very soluble in water; those of rubidium and especially caesium are much less soluble. The sodium salt, NaAuC14,2H20, separating in yellowish-red prisms, is an article of commerce under the name of "sodio-gold chloride" ; it has the advantage over aurichloric acid of being non-deliquescent. Aurichloric acid com bines with the chlorides of many organic bases to form well defined crystalline aurichlorides, frequently used in identifying and purifying such bases.
On mixing aqueous solutions of potassium iodide and or some auric iodide, is produced, but being some what unstable, it decomposes to a large extent into aurous iodide, AuI, and free iodine. The latter reaction is complete on warming. Although unstable by itself, yet in combination with alkali and alkaline-earth iodides auric iodide forms a stable series of complex auriodides. The potassium salt, crystallizes in black, lustrous prisms. Iodine in aqueous, or preferably aqueous-alco holic, solution combines with metallic gold to produce aurous iodide, AuI, a white or lemon-yellow powder insoluble in water.
Gold Cyanides.—In the presence of air gold dissolves in aqueous solutions of potassium or sodium cyanide to form potas sium or sodium aurocyanide, KAu (CN) 2 or NaAu (CN) and on precipitating this solution with dilute hydrochloric acid, aurous cyanide, AuCN, is deposited in yellow, insoluble, microscopic, hexagonal plates. Auric cyanide, has not been isolated with certainty, but stable complex salts are known with alkali and other cyanides. Potassium auricyanide, forms colourless efflorescent crystals. The silver salt, is formed by precipitating a solution of with silver nitrate. From this salt auricyanic acid, is obtained by removing the silver with hydrochloric acid and crystallizing the solution.
Fulminating Gold.—When auric oxide or a gold solution is treated with strong ammonia, a black powder is formed called fulminating gold When dry it is a very powerful explosive, as it detonates either by friction or on heating to about 145° ; it should always be handled with great caution.
Purple of Castius.—When a solution of auric chloride is precipitated with a solution of stannous chloride a reddish or purplish precipitate is produced containing both metallic gold and tin hydroxide. The composition of this precipitate is as vari able as is its colour. This product is mainly used in the preparation of ruby glass.
Liquid Gold.—A preparation known as "liquid gold" (German, Glanzgold) is very largely used in the decoration of pottery and earthenware. It consists essentially of a sulpho-resinate of gold dissolved in various essential oils, together with small quantities of bismuth, rhodium and sometimes other metals. The liquid gold is applied by suitable means to the surface of the glaze of the ware : it is then allowed to dry and fired at about 7oo-800° C. A brilliant film of metallic gold is thus left on the surface of the ware. (F. E. M.)