(silver, iridium, gold, osmium, palladium, platinum, rhodium, ruthenium)
White and shiny metal element, symbol Ag and atomic number 47, belonging to group IB (or 11) of the periodic table and to the transition elements series.
Known since antiquity, it was used by some peoples to manufacture ornamental objects and coins. Alchemists called it Moon or Diana, in the goddess Moon's honour, and attributed the symbol of the waxing moon to it.
Silver is an excellent electricity conductor and, apart from gold, it's the most ductile and malleable metal. Its hardness goes from 2.5 to 2.7 it is therefore harder than gold but more tender than copper. It melts at about 962 °C, boils at about 2212 °C, and has a relative density par to 10.5 and atomic weight par to 107.868.
Silver isn't very reactive; it's insoluble with dissolved acids or bases, but it can be easily dissolved in concentrated nitric or sulphuric acids; at room temperatures it doesn't react with oxygen or water. If exposed to the air it doesn't oxidize but its easily etched by the sulphur and sulphides contained in small amounts in the atmosphere; the characteristic blackening is due to the formation of silver sulphide on the surface of the metal. Black silver sulphide, Ag2S, is one of the less solvable in aqueous solutions; this feature is utilized to set apart silver ions from other cations.
Silver is relatively abundant in nature but it's often found combined with other elements and nearly always impure: the richest prospects are in Peru and Norway, where mines have been exploited for hundreds of years. It's often found together with gold and can be collected in considerable quantities during the digging out and working of this metal. It can also be found combined with other elements in several rocks and in numerous minerals, like cerargyrite (also called silver chloride or more often horn silver), pirargyrite, sylvanite, and argentite. The metal can also be obtained as by-product of lead, copper and zinc working. Nearly all the silver produced in Europe derives from the working of lead sulphide or galena. The majority of the silver mined in the world comes from Mexico, Peru, Canada, United States and Australia.
During the mining processing, silver minerals are bought to high temperatures in special furnaces to allow the transformation of sulphides in sulphates; metallic silver is collected from these compounds through chemical precipitation. Several metallurgical processing is used to extract silver from the argentiferous minerals of lead, copper and zinc. To make an example in the amalgamation process the grounded minerals are added a small amount of liquid mercury that, put together with silver makes a particular amalgam; this material is separated from residues and then distilled in order to provide, at the end of the process, pure silver. In the leaching methods, the silver melts in a salt solution (usually sodium cyanide), they then make it precipitate by brining the solution in contact with metallic zinc or aluminium. The process used to separate silver from copper is called Parker, it's commonly used also for the extraction of precious metals from lead. Impure silver produced metallurgically is refined with electrolytic methods or through cupellation, a process that removes eventual impurities through vaporisation or absorption.
The use of silver is popular in jewellery, silverware production, silver plate and coin minting. The most valuable silver, generally used for silver services, is constituted for the 92.5 per cent by pure metal and for the remaining 7.5 per cent from copper. The silver used for industrial applications instead is usually a silver alloy with good mechanical features such as hardness and wear resistance. Silver used to be utilized for the production of mirrors and glass coverings, but nowadays in these fields it's been substituted almost completely by aluminium. Currently high quantities of this metal are used for the production of electronic equipment or electric circuits. Furthermore, solutions solved in silver nitrate (Ag NO3) and other unsolvable compounds like potassium, are used in medicine as antiseptics and bactericides.
The halide silver salts (silver bromide, chloride and iodide), which darken if exposed in the light, are used in preparing emulsions for photographic films. These salts are solvable in sodium triphosphate, which is a compound normally used for photographic fixing processes.
White metal, extremely hard, flaky, symbol Ir and atomic number 77; belongs to the transition elements series of the periodic table. It was discovered in 1804 by the British chemist Smithson Tennant and it was named after the iridescence of some of its compounds.
Iridium is chemically totally inert, it resists to aqua regia but melted salts etch it; in its compounds it reaches oxidation stages 3 and 4. Its atomic weight is 192.22; it melts at approximately 2454°C, boils at approximately 5300 and its relative density is par to 22.4, the highest among all the elements.
It's an extremely rare metal; you can find it in alluviums in alloys together with platinum, under shape of platiniridium and with osmium as osmiridium. It's mainly used in alloys with platinum to build precision instruments, surgical instruments, fountain pens, weights and lengths.
Metallic element with symbol Au (aurum in Latin) and atomic number 79, belonging to the family of the transition elements of the periodic table.
Pure gold is shiny yellow and is the most ductile and malleable metal: it can be reduced in laminas 0.0001 mm thick and pulled in very fine and long strings (29 grams of gold can be transformed in a string 100 Km long). Its hardness goes from 2.5 until 3, it is therefore one of the most tender metals; it's an excellent heat and electricity conductor. Gold is a black powder when finely subdivided, in colloidal suspension the solution becomes of a colour between ruby red and purple red.
From a chemical point of view it's a very low-reactive metal: it isn't etched by air, humidity, or heat, but it solves in solutions containing chloride, bromides, iodide, in oxidizing mixes, alkaline cyanides and aqua regia, a mixture of chloride and nitric acid. Its most important compounds are chlorides and cyanides. It melts at 1064°C, boils at 2808°C; its relative density is 19.3 and atomic weight par to 196.967.
It isn't rare to find precious metals like gold and platinum at a pure stage in a sac or a vein inside a sedimentary or igneous rock mixed together with other minerals. They are geologic veins, formed by the sediments settled by undercurrents during the past millenniums, or due to magma injections in igneous rock fissures. Gold particularly, is often found in quartz veins.
Even if it's very rare in nature, gold is homogenously distributed on the earth's crust, usually in quartziferous rocks and alluviums, at its native stage or in alloys with silver. Together with tellurium and silver it constitutes minerals like calaverite and sylvenite. It can be in gold amalgam together with mercury and in small quantities in pyrites; traces of it can be found in galena. It is also contained in seawater within a concentration between 5 and 250 parts against 1000 parts of water in weight. Altogether the quantity of gold in seawater is believed to amount to approximately 9 billions of tons, but the recovery costs are extremely high.
Gold was known since ancient times, not just for its beauty and corrosion resistance, but also because the extraction and working of it were easier than the ones of any other metal.
It's used in jewellery and coins since antiquity; first in its native condition, then in alloys with other materials that give it the necessary hardness; the amount of gold contained in an alloy is expressed in carats.
For gilding and inscriptions it's used under shape of very fine laminas (leafs); Cassius' purple is a colloidal solution of finely divided gold and tin hydroxide, formed through the reaction of gold chloride with tin chloride and used to colour glass. The tetrachloride auric acid is used in photography, while the potassium dicyanaurate is used for gold plating. Gold is also used in odontology, instead its radioisotopies are used in biological researches and in the diagnosis of cancers (isotopic tracers).
Gold is extracted from rivers by using dredges, machines provided of a series of excavators that scrape the bottom pouring their content in a special screen. The recovered material gets mixed and washed with water; so that the auriferous sand passes through the holes of the screen and deposits it's self on the oscillating tables. This method can be also used in dry riverbeds as long as there is water under the soil: they dig a well, then the pump water from it in to the riverbed, after that the dredge can be put to navigation.
During the extracting process of gold from rocks, the auriferous minerals are collected and crumbled with special machinery; an amalgam is made on copper leafs covered with mercury (amalgam process); gold is then obtained through distillation, while the mercury is recycled. Gold can also be recovered by cyanidation, which consists in treating the amalgam with a sodium cyanide solution, letting the gold precipitate. In both methods gold has then have to be purified and separated from silver through electrolysis. Some metals, especially those where gold is combined with tellurium have to be calcined, i.e. warmed at high temperatures before extraction.
The rarest form of gold to be found is the nugget; the biggest nugget that has ever been found, the Welcome Stranger, weights 71 Kg and was casually discovered just under the earth's surface in Victoria, Australia during 1869.
Gold was already in use in ancient times when the antique populations of the Mediterranean used to obtain it from alluvium sands and from gravel. The same method was used in India, Central Asia and the west regions of the Mediterranean. Following the evolution of the extraction techniques, the auriferous veins were exploited already during the first years of Christ's era; further progress in gold's extraction and production were made beginning from the Medieval times.
Currently the main countries producing gold are South Africa, United States, Russia, Australia, Canada, China and Brazil.
Fragile metallic element, white-azure colour, symbols Os and atomic number 76; belongs to the family of the transition elements of the periodic table. Discovered in 1803 from chemist Smithson Tennant, osmium has hardness equal to 7, melts at 2700 °C, has a very high relative density, par to 22.61, and atomic weight 190.2. It resists to the action of acids, but it's solvable in aqua regia or smoking nitric acid; it forms salts were its oxidation stage goes from +1 to +8.
The metal is found in platinum minerals and under shape of alloy together with iridium. Alloys with iridium and platinum are used for the definition of measure and weight standards.
Relatively rare metallic element, white-silverish colour, symbol Pd and atomic number 46; it belongs to the transition family of the periodic table.
Palladium was discovered in 1804 from the British chemist William Hyde Wollaston. It's ductile, malleable, corrosion resistant, and has almost the same physical features of platinum, but it melts more easily and can be welded easily. If finely grinded, palladium can absorb different gasses on its surface; if it's bought to the temperature of approximately 100 °C, it absorbs quantities of hydrogen and acetylene par to 1000 and up to 3000 times its volume. It's etched by concentrated acids and rapidly corroded by aqua regia. It forms divalent and tetravalent compounds and has the same chemical behaviour of platinum. It melts at approximately 1554 °C, boils at about 2970 °C, has relative density par to 12,02, and atomic weight 106.4. Quite rare on the earth's surfaces, it can be found, under pure stage, in the platinum minerals and combined with other elements in nickel minerals.
It is mainly used in the communication fields, to coat electric contacts in automatic switches. Its also used in odontology; non-magnetic springs manufacturing for watches and clocks; as coating for some kinds of mirrors. In jewellery, in gold alloys, it forms white gold.
Very rare metallic element, symbol Pt and atomic weight 78; it belongs to the VIIIB (or 10) group of the periodic table; it's therefore a transition element.
It's the most important among the platinoids group, which also includes ruthenium, rhodium, palladium, osmium, and iridium. These elements were already used in alloys in ancient Greece and Romans' times, but they are only mentioned in European literature beginning from the sixteenth century. The distinction between platinum and others platinoids dates back to the beginning of the nineteenth century.
Platinum is a grey metal with 4.3 hardness; it has e very high melting point, it's ductile and malleable, it dilates very little and has particularly high electric resistance. Chemically it's relatively inert and it resists to air, water, acids and normal reagents etchings; it solves slowly in aqua regia forming hexchloroplatinic acid (VI), formula H2PtCl6; halogens etch it and it reacts by heating with hydroxide, nitrate and sodium cyanide. It melts at 1772 °C, boils at 3827 °C; its relative density is equal to 21.45 and its atomic weight par to 195.08.
In nature platinum can be found in its elementary stage, but it's also in some not common minerals, some of them are sperrylites, copperites and braggites. Spheres weighting about 9 Kg were found.
The scars chemical reactivity and the high melting point make platinum ideal for the production of instruments like melting pots, pliers, funnelsands and capsules. Usually it's added small quantities of iridium that give it much better hardness and resistance. It is also used to make electrical contacts for instruments that take measures in high temperatures; under shape of sponge or platinum black, it's used as catalyser in chemical industry. Considerable quantities of Platinum are also used in jewellery, often in alloys with gold, and in odontology.
Silvery metallic element, symbol Rh and atomic number 45; it's a transition element of the periodic table. Discovered in 1803 from chemist William Hyde Wollaston, it's a very resistant metal, hardness par to 4, un solvable in acids; melts at 1996°C, boils at 3727 °C, relative density equal to 12.4 and atomic weight par to 102.906.
When part of a compound rhodium manifests all the oxidation stages from +1 to +6. It's name, which rises from the Greek rhodon, "pink", alludes to the colour of the solution of many of its salts. Rhodium is found in alloys inside platinum, osmium-iridium and gold rhodium (rhodites) minerals.
Rhodium is mainly used together with platinum: the resulting alloy is mainly used in thermocouples for high temperature measurements; it combines the excellent chemical resistance of platinum with the hardness and mechanical resistance of rhodium. In its pure condition, instead, this metal is used for the manufacturing of mirror glasses and in jewellery for plating and silverware. Rhodium's black, a finely subdivided metal containing oxides and hydrides, is utilized as a catalyser and as pigment for china production.
White-grey metallic element, chemically inert, symbol Ru and atomic number 44; it belongs to the transition elements of the periodic table.
It was discovered in 1844 by the Russian chemist Karl Karlovič Klaus and its name alludes to the Ruthenian region, nowadays part of Ukraine. It's quite rare on earth's surface and can be found in platinum minerals in its metallic stage. Ruthenium melts at approximately 2250 °C, boils at 3900 °C; its relative density is equal to 12.3 and its atomic weight to 101,07.
Together with platinum and palladium it forms high hardness alloys, very wear-resistant and widely used in jewellery, for metal-china dental prosthesis, for fountain pens nibs and for non-magnetic pins. The ruthenium-molybdenum alloy, at temperatures lower than -263 °C, behaves as a superconductor. The metal in its pure condition is extremely resistant to acid and aqua regia etches.