Tuesday, 30 July 2013

Synthetic Corundum

Synthetic Corundum
Aluminium oxide. This is the most common synthetic product used as a substitute for natural gems, and it has been widely employed for technical purposes as well; for example, for the "jewels" in watches.

Crystal system

Trigonal like the corresponding mineral, In most cases, the synthetic product does not have well-developed crystallographic faces.


The vast majority of synthetic corundums produced by the Verneuil flame fusion method are pear-or sausage-shaped, thinning down into a short peduncle at one end. Specimens produced by other methods of fusion (mainly the Czochralsky process, also known as "pulling from the melt'') are uncommon and have a squat, cylindrical shape with horizontal striations, terminating in a cone at one end, or a very long, cylindrical, rod shape. But the most recent (costly and therefore little used) flux melt and hydrothermal processes produce crystals, singly or in groups, which are very similar to the natural ones. A complete range of colors, as well as colorless, can be obtained. Colors may be red to pink, orange, yellow, green, blue violet and gray-green turning to violet-red. Physical properties Identical to those of natural corundum: hardness 9; density about 4.0 g/cm3


This is concentrated in certain countries with highly developed chemical industries, notably Switzerland, France, Germany, Italy, Czechoslovakia the Soviet Union, Japan, and the United States.

Synthetic ruby

This was the first synthetic gem to be manufactured on an industrial scale and quantities have steadily increased to the present day, the Verneuil method being the most widely used.


Synthetic ruby is usually bright red, differing very little from the natural stone, the physical proper-ties of which are also faithfully reproduced. Ills given the same oval, round or pear-shaped mixed faceted cuts or is made into cabochons. But it is also cut into special shapes, often weighing 5-15 carats; for instance, rectangular with a smooth, convex upper surface and faceted lower surface, or with the top part convex, but consisting of numerous, juxtaposed square facets and the bottorn part faceted; or oval, again with a smooth, convex upper surface and faceted lower surface. These cuts are characteristic of synthetic rubies. They are found in large stones that are highly transparent, being completely free of inclusions, and are often used for large, old-fashioned rings for men or set into religious objects. Beads 2-3 to 7-8 millimeters in diameter are also typical; of a perfectly even, bright red color and uniform diameter, they are made into necklaces and brace-lets. These pieces of jewelry could not possibly be made of natural rubies. On the rare occasions when natural rubies

are used for this purpose, they are generally of mediocre color, full of inclusions, and of graduated diameters. Fine quality natural ruby is much too valuable to be treated in this way.

Distinctive features

Synthetic ruby These are almost exclusively internal: generally speaking, this material is quite limpid. Provided one can find the right direction, thin (noncrystal lographically oriented) curved lines, characteristic of growth by deposition of successive layers of molten material, will be visible under a lens, or better still a microscope, and sometimes, small gas bubbles and "swarms" of minute, opaque foreign bodies (unmelted alumina powder) can be seen. In the past, a network of internal cracks was sometimes produced by a sudden change of temperature to compensate for the suspicious absence of inclusions. The resulting stones appeared vaguely similar to some natural rubies with numerous inclusions.


Extremely low, bearing no relation to that of their natural counterparts; in fact, most of the cost of ordinary synthetic rubies is in the cutting. 'The rare synthetic rubies produced by the flux melt or hydrothermal processes are much more expensive, costing little less than the better secondary gems. Hence, manufacture of these is not normally economical. But given their resemblance to natural stones and the possibility of some of them being sold as "good," there is a market for them. While sale of these stones invariably starts out perfectly above board, it some-times ends in a highly profitable fraud after a few changes of hands, because of the high value of natural rubies of similar characteristics.

Synthetic sapphire

Production of this synthetic gem started a few years later than ruby, greater difficulties being encountered in reproducing the color. Appearance The color of medium-light faceted stones often looks darker at the edges, due to an optical effect. It may also be colder and grayer than natural sapphires. It is cut into all the shapes used for the natural stones, both faceted and cabochon. The cabochon cut is, in fact, the one that best suits it. Synthetic sapphires cut en cabochon are the most convincing and hardest to distinguish from natural stones.

Distinctive features

The color, sometimes with an unusual shade and color zoning, and the absence of blue-green pleochroism may distinguish synthetic sapphire from some, but not all, natural sapphires, given their variability. Here, too, the main distinctive features are internal and only visible under a lens or microscope: broad bands representing curved growth lines emphasized by a different depth of color are much more clearly visible than those of synthetic rubies if the stone is examined against the light. Sometimes gas bubbles and minute foreign bodies either separately or in ' 'swarms" follow the growth curves. Cost As with synthetic ruby, the cost is very low and mainly accounted for by the cutting process.

Synthetic star ruby and sapphire

Synthetic ruby
This variety of corundum has also been manufactured synthetically for a few decades. Titanium oxide is added to the alumina. It is then precipitated out as tiny crystals along the corundum crystal lattice by a process of very slow cooling, giving rise to the star effect.


These stones are invariably cut into round or oval cabochons, which generally have a star with six very obvious, if thin, rays. They normally have very limited transparency. The usual colors are bright red (ruby), or equally bright blue (sapphire), although blue-gray and dullish red stones have been produced.

Distinctive features

Synthetic CorundumThe star is very obvious and clear-cut, more so than in most natural gems. The curved growth lines characteristic of synthetic corundum are nearly al-ways clearly visible. Sometimes, these are faintly visible in the form of concentric circles on the unpolished lower surface of the stone as well. One of the leading manufacturers also used to engrave the company's initial, a squat letter L," on the underside of cabochons.


Low, although a bit higher than that of normal synthetic rubies and sapphires, as the method of producing them is more complex.

Colorless and other varieties of synthetic corundum

Early attempts with ruby proving relatively straightforward synthetic corundum soon began to be produced in many different colors, not so much to imitate other types of natural corundum (some of these colors do not occur naturally), as to provide a highly effective, inexpensive ornamental material or even to imitate gems of quite a different mineralogical nature.


Colorless and other varieties of synthetic corundum The varieties most often seen are colorless, pink, various shades of yellow including brown or orange-yellow, and violet. More rare are gray-green stones that turn reddish in artificial light. The colorless variety was used in the past to imitate diamond; the pink is a good imitation of pink sapphire; the yellows have mainly been used to imitate topaz, although they are not very similar; and the amethyst violet variety is normally for some inexplicable reason called synthetic alexandrite, despite the fact that it looks quite different from alexandrite chrysoberyl The variety which changes color is intended to be an imitation of alexandrite, but it is not a very convincing one. These stones are given more or less all the types of cut used for colored stones, particularly those they are designed to imitate, but the round, mixed cut is more often seen than with natural stones. These stones are often quite large (easily 5-15 carats or more), except for the colorless variety, which generally appears in small stones, which are harder to distinguish from diamond. All the varieties of synthetic corundum have the characteristic fine luster of natural corundum, although this is not always shown to advantage in poorly cut stones.

Distinctive features

These synthetic corundums, which are all produced by the Verneuil method, usually display characteristic growth curves, although these may be barely visible, if at all, in the yellow and orange-yellow varieties. They lack, of course, the typical inclusions of their natural counterparts. Where they are used to imitate a gem of a different mineral type, this can immediately be detected by measurement of the physical properties.


Very low even for fine specimens.

Writer - Curzio Cipriani & Alessandro Borelli

Monday, 29 July 2013

Underground Hazards of gold

Fire at underground mineUnlike coal and iron mines; where the greatest perils are explosions caused by dreaded firedamp, the worst hazards in South African gold mines are cave-ins at great depths.

Every gallery that is driven into the ground causes some disruption of the geological structure in its immediate surroundings. In the deepest South African mines, it is possible to encounter traction and pressure more than twenty-five times greater than the stress factor of ferroconcrete. It would be reasonable to expect that the greatest pressure would be on the gallery roof. But that is not generally true. The greatest stress tends to be within the rock, between ten and thirty feet above the roof.

 In a rock burst, events can happen with the speed in of an explosion. Whole galleries have been known to w collapse. However, a gallery usually comes down in re bursts of about eight inches at a time. The collapse and shattering noise that accompany it are merely re the visible and audible elements of a developing cataclysm. A cave-in is much like an earthquake, although the mechanical cause is different. There is never total quiet in a mine, even when it is not worked. There le is always some sound of cracking, sometimes muffled, at sometimes very sharp. Miners say "the mine is talking". It is the stress in the rock that they hear. For years, the South African mining authorities have financed a comprehensive program of research to study the causes of cave-ins and to develop countermeasures. The results of this research are similar to those of Y, seismologists.

It seems that in high pressure zones, microscopic ally fine alterations in the rock take place when mining disturbs the rock structure. These disturbances n can be relieved by the cutting of a gallery. But, such disturbances can also result in the accumulation of pressure within the rock mass. When the forces involved exceed the limits of elasticity of the rock, micro fissures develop which increase the volume of the rock. These add to the build-up of pressure. The forces finally become so powerful that they can only be relieved by movement of the entire surrounding re mass of rock. The gallery roof sinks. (In an earthquake, it is the lithosphere, the earth's outer crust, which at rises as faults are formed. However, in the case of earthquakes, it can take months or years, or even enormously greater spans of time, before the increase. of pressure within the rock produces visible consequences.)

The causes and courses of terrestrial upheaval have by now been so thoroughly researched that geologists are able to forecast the frequency and nature of micro fissures in rock with considerable accuracy. There are three main determining factors: depth, size of gallery, and capacity of gallery roof to absorb movement. These factors can be combined in a single equation which covers the amount of energy released by working the subterranean rock.

The South African Chamber of Mines has had mathematical models of these processes developed and programmed for computers so that mining engineers need only feed plans for work programs into computers to receive, within minutes, analysis of the safety factors involved in their projects. It has, for example, been calculated that when dealing with reefs more than a foot-and-a-half thick, at least fifteen percent of the reef must be left intact to support the network of galleries.

Such measures reduce the possibility of cave-ins, but they do not altogether eliminate the danger. Preventive measures are still required to protect miners from cave-ins which, despite precautions, take place from time to time at locations where pressure has, unnoticed, accumulated gradually. The answer to these persisting hazards seemed simple propping up the galleries. But, in the early 1960's, when South African mines began, to use quantities of hydraulic pit props, employed so widely in European mines, they experienced an unpeasant surprise. Pressure either made the props collapse or simply punctured the gallery roofs.

It took years to develop a high performance, rapid yielding hydraulic prop to do the job. It is remarkably similar to the hydraulically sprung undercarriage of an airplane. Today, over twenty thousand such props are in use in mines. They are set up in galleries at intervals of about three feet to offer miners protection from cave-ins and rock falls. Since these props were introduced, accidents due to falling rock have been reduced by ninety percent.

Automation Underground

The technology of gold mining continues to be steadily improved. Efforts these days concentrate principally on mechanization and improved management techniques above all the use of electronic data processing.

Mechanical improvements are principally concentrated on efforts to improve efficiency at the rock face. South African mining is very labor intensive. But machines have been developed which require less manning and which can operate in smaller spaces. There are now drilling and loading machines which are able even to continue functioning during blasting. Such contraptions have in places made it possible for the work force to be cut by half, with increased profitability, corresponding to a production increase of four hundred percent. Such developments are, however, not without social and political ramifications.

The shallower the mine, the more profitable it tends to be, even when the difference is only a matter of inches. This is because the extraction of non-gold rock is just as expensive as the extraction of gold-bearing ore. Wherever it has proved possible I to alter the ore-to-rock ratio in favor of ore, there has been a marked increase in return over investment.

Aside from isolated experiments, all South African mines have used labor intensive methods involving blasting. This is because at such great depths the rock is extremely hard. Moreover, the method is eminently suitable for work at such levels. At the same time, conditions rule out the use of the kind of new machinery employed in American and' European mines, with their large, relatively shallow galleries. But much effort is being devoted by the South Africans to designing equipment capable of operating productively in subterranean confined spaces.

There are considerable gold losses during blasting and loading of ore onto conveyors. The pressure of the explosion separates some of the gold particles from the ore and transforms them into fine dust which escapes into the air. After blasting, the entire area is carefully scored with hand brushes, but much of the gold dust is lost forever in rock cracks and fissures.

Mine Safety and Rescue
Some gold dust affixes itself to particles of rust, found in all mines, and is also lost. Methods still in the experimental stage are being developed for retrieving whatever gold dust now tends to be overlooked. In addition, new mining machines are designed to release no dust at all. The whole thrust of new techniques is to lose none of the precious metal in the process of acquiring it.

If a mine is to operate with maximum profitability, there must be an efficient, comprehensive plan for its overall operation. Development of the mine, excavating, transportation in shafts and galleries and all other aspects of getting the gold from the ground must not only be calculated to produce as much as possible, but also to produce it in steady quantities. The extremely expensive chemical and mechanical equipment used in processing the metal has limited capacities. If more ore is extracted than the plant can handle at any given time, expensive storage space must be provided. But underproduction is also expensive because the plant does not then operate at full capacity. 

Planning departments of the mining industry construct models to study such problems. These models simulate the extraordinarily complex processes involved in mine operations. Appropriate computer programs give engineers an opportunity to test their operations with their models and examine all possible consequences and alternatives. They are thus able to pinpoint weak spots and bottlenecks and to develop procedures for overcoming them. 

Saturday, 27 July 2013

Gold from the World's Deepest Mine

Gold egg
The richest known gold fields in the world are located in a comparatively small area on the southern tip of the African continent marked by layers of auriferous reefs in the Witwatersrand area near Johannesburg. They are worked these days at levels from 1,600 to 12,000 feet below the surface. Some of the forty-one South African gold mines are among the deepest in the world and the technology of deep mining is more highly developed in South Africa than anywhere else.

South African mining technology, rather than geophysical peculiarities, can be credited with making it possible for miners to work at all at such great depths in relatively tolerable temperatures. In most parts of the world, temperatures rise one degree centigrade for every 107 feet of descent. This means that at the deepest levels of the mines in South Africa, temperatures should reach boiling point, and beyond. But in the Witwatersrand area, the temperature increases much more slowly at the rate of one degree centigrade per 390 feet. Thus, at the deepest levels, temperatures are kept down to circa 50 degrees centigrade. Effective ventilation techniques can bring this down to forty degrees centigrade.

Gold jewelleryGold mining techniques do not differ extensively from those used in mining other substances, such as coal and iron. The big difference is the extra-ordinary depth at which gold ore has to be mined. To reach it, miners must cut through rock that is often harder than granite.

The Mine at East Driefontein

South African mining techniques are best illustrated by reference to the East Driefontein mine which was begun relatively recently. It is part of the mining complex near the town of Carletonville, one of seven such fields which form a 300-mile arc across central South Africa.

From 1963 to 1967, test 'borings were taken to determine the pattern of gold-bearing reefs in the area and to analyze the rock: The tests indicated that there was some seventy-seven million tons of gold-bearing ore in the region, with a yield of about eighteen grams of gold per ton. The area .boasted three reefs the Ventersdorp Contact Reef which circles practically the entire area; below it, the Main Reef which covers some sixty percent of the area; and, still further down, the Carbon Leader Reef which is almost the same size.

Gold smart carThere are three geological problems complicating access to the reefs a huge water-bearing dolomite sheet, a hard layer of lava above the reefs, and the depths of the reefs, some of which are about 13,000 feet.

Shortly after work began on the East Driefontein mine on October 26, 1968, there was serious flooding from the dolomite layer. Both the East and neigh-boring West Driefontein mines were flooded. Elaborate pumping systems had to be set up for both mines and it took more than a year to pump them dry. In the early stages, tens of millions of gallons of water were pumped to the surface daily'.

Both main shafts of the East Driefontein mine were deliberately sunk in the water resistant area of the Pretoria rock formation. It had been hoped that this would help avoid the danger of flooding and the necessary depth of 5,000 feet would therefore be easily attainable. That would have meant that the reef, which descended from 3,000 feet at an angle of twenty-two degrees, could be worked by means of galleries from the two main shafts. But, during preliminary stages, it was decided to sink a shaft to a depth of 6,500 feet. Each shaft had a diameter of twenty-four feet. Where they went through the dolomite layer, the shafts were reinforced with concrete.

Gold pieceFrom the bottom of the two main shafts, the two sub-vertical shafts were sunk, as a second phase, to 9,500 feet. The winding engines of the main shafts were above ground. Those serving the sub-vertical shafts were set up in subterranean caverns. The largest of these, in the hard Ventersdorp lava of East Driefontein mine, is the size of a church nave 110 feet long, 45 feet wide, and 48 feet high.

The roof prop method is employed to excavate these caverns. This entails gouging out a. gallery at the level of the cavern's eventual ceiling, with the ensuing excavation taking place on both sides and below the gallery. Particularly hard rock requires special blasting techniques which produce especially smooth walls. A great many blast holes are bored close together along the line of advance. Charges are all detonated simultaneously and the pressure of the explosion takes the line of least resistance, usually sideways rather than forward.

Smooth walls and gallery ceilings reduce the danger of subsequent rock falls. At various depths, horizontal galleries from the main shafts, often extending more than a half mile, connect to gold-bearing reefs. The heights of these reefs vary considerably, from a few millimeters up to, in exceptional cases, nineteen feet. The conglomerate is easily recognized as a yellow ocher band woven through the dark Ventersdorp lava.

Deep Mining

Extracting ore at great depths poses very special mining problems because pressure is considerable in deep mines. The process leaves comparatively shallow hollows, sometimes of substantial length. These are subject to the pressure of the entire rock mass above them. From horizontal galleries, other galleries are driven to follow the line of the reef. Numerous other galleries are cut at right angles to them. The height of the working faces, known as stopes, is kept to a minimum because the economics of mining have much to do with the relationship between the amount of rock that has to be extracted and the usable ore it provides. The galleries in which it was necessary to work doubled over or horizontally were once supported by timber pit props. These days, special hydraulic props are used. Once side galleries have been pierced, further galleries, parallel to the main gallery, are cut so that a rectangular network is formed. This permits the conglomerate to be removed in such a way that only thin support pillars need be left within the pattern of galleries. Areas that have been worked dry are filled in with barren rock. Thus, only a small proportion of such rock has to be brought to the surface.

Getting the Ore Out

Gold coinA large gold mine depends as much on an effective transport system as it does on efficient extracting processes. Mechanical loading devices are situated at the work face in modern mines to transfer the ore to conveyor belts after each blasting. The loading machines are designed to fit into shallow galleries.

Conveyor belts, which are constructed in sections, take the ore through the working galleries to the tracks of an underground railway in the main gallery. From there, the ore is transported in trucks to the hoisting shaft where it is emptied into buckets and raised to the surface.

The underground railway is generally the most extensive part of the mine's transportation system. It has been reckoned that the amount of track laid in South Africa's mines slightly exceeds that of its surface railroad system. 

Friday, 26 July 2013

From Gold Fever to Gold Rush

From Gold Fever to Gold Rush
In 1848, word that gold had been found in California on John Sutter's property triggered the first gold rush of modern times. In the stream of Sutter's saw mill near Sacramento now California's state capital gold nuggets were discovered. Thousands of adventurers and fortune hunters from all over the world converged on California, each seeking his own personal Eldorado.

In those days, prospecting techniques were still promitive. Usually trusting to frivolous luck, the prospectors tended at first to scour the beds of shallow rivers and streams, washing the sand in pans, whirling them in a circular motion. If they were lucky, a glimmer of gold dust, perhaps even a few larger grains, could be seen in the pan when everything else had been washed away. When a substantial deposit was located away from waterways, the first consideration was to obtain a water supply. Gold bearing sand and gravel was then shoveled into a narrow channel with built in wooden slats and the water was played over it. The gold was retained by the slats as the water carried other substances away. Through this laborious method, California's annual gold production amounted to ninety five tons two years after the beginning of the gold rush. That was half the total world production at the time. By 1863, prospectors with financial backing had introduced more sophisticated methods to get at the gold hydraulic techniques in which the rock was sluiced away by high pressure jets of water.

goldIn the 1850's, some of the California prospectors made their way to Australia, to join the gold rush there. They brought with them the experience and methods of their California days. Australia soon was to become, for a short time, the greatest gold producer in the world, and then gold was also discovered in New Zealand. Much of it was found in river deltas from which it was extracted with the help of dredges.

Rich gold deposits had also been unearthed in Russia. In the 1830s, the newly invented centrifuge was widely used there to separate gold from virgin rock. And, as the nineteenth century drew to a close, gold was also found in the inhospitable region straddling the Alaska-Canada border. This led to the most dramatic gold rush in history. In 1896, during a serious economic depression, thousands braved the harsh elements to seek elusive gold in the Far North.

But the richest yields of all were to be found elsewhere. In 1886, in Witwatersrand, near present-day Johannesburg in South Africa, gold was found in exhilaratingly large quantities. The gold-bearing lodes were on the surface of a forty-mile-wide bowl of countryside. But, for the most part, relatively thin layers of conglomerate extended underground and reached straight down to depths of up to 10,000 feet. These conglomerates were probably the result of the collapse of a mountain range some two billion years ago. It is likely that what had been surface rock was pressed together over millions of years and squeezed down into the depths of the Earth. The gold is in the form of fine particles pressed into the rock. The gold-laden "reefs" extend underground for more than seventy miles.

Row gold
By the end of the nineteenth century, some fourteen percent of the world's gold production came from South Africa. By 1916, that figure had been raised to forty-two percent of the world's gold production, and reached about fifty percent by 1930. During the 1960s, South Africa produced seventy percent of the world's gold. Even when new rich gold sources were found, the country managed to produce a large portion of the world's total. In 1974, for example, 1338 tons of gold were produced worldwide. About fifty-four percent (729 tons) came from South Africa. The country's total production to date amounts to about 20,000 tons. The best know gold fields Of South Africa, Southwest Africa (Namibia) and Rhodesia are believed to contain some 18,000 tons of gold. That is considered to be about sixty percent of the total recoverable gold resources in the world.

New Methods for Gold Extraction

South Africa's spectacular success as a gold producer was intimately linked with the introduction of the cyanide separation process for treating ore and with generally improved prospecting techniques.

The new separation method was first used in South Africa in 1890. Finely ground gold ore was treated in a solution of sodium cyanide into which air had been blown. Gold entered into a complex compound with the sodium cyanide and was later separated by precipitation with another metal, such as zinc. This technique proved very efficient. Even the finest gold particles could be extracted from the ore. In South Africa, the method is used either alone or in tandem with the amalgamation process. South African gold tends to be very pure.

During the early years of the Witwatersrand fields, yields of twenty grams of gold per ton were the order of the day. Now, however, the yield in some mines has sunk to five, and even two, grams per ton. But when the price of gold soared, after it was allowed to float in 1968, a new inducement for technological advance was provided. Research into new and more effective mining and processing techniques was encouraged, with the higher gold price justifying increased development costs.

Until about fifty years ago, the search for new deposits was conducted without the full benefit of scientific method and equipment. Finds were usually made by chance, sometimes the result of the persistence of a lucky prospector proceeding merely on a hunch. Only in a few isolated instances was gold discovered through the efforts and experience of trained professional prospectors.

Not until the 1940's and 1950's were truly modern scientific prospecting techniques and mechanisms developed. These included instruments for measuring the force of gravity on the earth's surface, highly sensitive meters which indicate gravity variations. Analysis of these variations allows for an assessment of the structure of the earth's crust which helps t determine the kind and density of rock layers.

Various other magnetic and electric techniques are also now employed. Magnetic measurements determine local variations in the earth's magnetic field. This method is used to indicate the presence of iron sulphur compounds which sometimes contain gold. Airplanes equipped with the proper instruments can carry out magnetic investigation over regions which would otherwise be, for practical pre discovery .purposes, inaccessible.

Other devices are used to measure electrical conductivity on the earth's surface, with reference to both direct and alternating current. Telltale differences permit conclusions to be drawn about the mineral structure of the surface.

With these and other techniques, large areas of South Africa were meticulously surveyed. Various surveys indicated that an area about 150 miles south-west of Johannesburg might contain substantial gold deposits. The mining rights were acquired by the Anglo-American Corporation, South Africa's largest gold mining company, which had conducted the survey. A borehole was sunk on a farm near the town of Welkom and gold-bearing quartz was located at a depth of about 4,500 feet. Further drillings indicated that one of the richest lodes in the world had been discovered. The company now operates several mines in the area. South African gold is often found coupled with uranium oxide and prospectors often use Geiger counters in their search. The method has a drawback, however. Test drillings are required to make room for the meters which measure radioactive presence.

A more recent prospecting technique is a spin-off of space technology. Pictures taken by earth-circling satellites are scanned for hints of geological structures similar to those in which gold has already been found. So far this method has had limited success.

The second most productive gold fields in the world are those of the Soviet Union, discovered by geologists in the Soviet Republic of Uzbekistan, in the Kyzylkum Desert. This is the only large opencast gold mine so far discovered. Workable gold-arsenic beds were recently discovered in Sweden.

Gold of the Chemists

In his professional capacity, a chemist thinks of gold simply as a metal, like other metals. It has a place in the periodic table of elements. Its atomic number is seventy-nine. Its atomic nucleus has seventy-nine positively charged particles protons with a similar number of negatively charged electrons surrounding them. The atomic weight of gold, in relation to carbon 12, is 196.967. In chemical compounds, gold appears with both univalent and trivalent auro compounds, the latter being more stable.

goldThere is only one natural gold isotope gold 197. Its atomic nucleus contains 79 protons and 118 neutrons. However, by using nuclear reactors, atomic physicists have succeeded in producing twenty-six artificial gold isotopes. They are classified according to the number of their nuclear particles. Apart from Au 180, Au 182 and Au 184, various isotopes between Au 177 and Au 204 can be created.

Precious metals are remarkably stable chemically. Neither hydrogen, nor oxygen, nor nitrogen can be dissolved in gold. Hot sulphur vapor will not corrode it. Oxygen cannot form spontaneous oxides with gold at high temperatures. Auro- and aurio-oxides can, however, be produced indirectly. Gold is resistant to water and most acid bases. Only aqua regia, a particularly potent mixture of hydrochloric and nitric acids, and other acids which produce nascent chlorine, can corrode gold (producing gold chloride.) Thus gold, particularly in powder form, dissolves in hydrochloric acid in its gaseous state. Gold may also be dissolved in liquid alkaline cyanide with which it forms complex salt compounds. In addition, gold joins with liquid mercury. This chemical response can be used to separate the metal from surrounding rock.

Gold is not easy to produce in its chemically pure form. However resistant it may be to chemical compounds, it frequently produces mixed crystals with other metals. The density of gold has so far not been established with total accuracy because such impurities are dispersed only with the greatest difficulty. The most reliable values are 19.369 g/cm3 at zero degrees centigrade and 19.297 g/cm3 at twenty degrees centigrade. Both values were established by X-ray crystallography. Accordingly, gold is the fifth heaviest of the natural chemical elements a kilogram of gold corresponds to a sphere with a diameter of no more than forty-two millimeters.

The precise melting point of gold has been accurately measured: it holds the highest value on the international temperature scale used for calibration of temperature measuring instruments 1063 degrees centigrade or, on the absolute scale, 1336.16 degrees Kelvin. Like most other metals, gold shrinks as it cools to room temperature by about two percent. That means when cast, gold is easily freed from its mold. At 2966 degrees centigrade, molten gold vaporizes.

With regard to electricity and heat conductivity, gold trails silver by about thirty percent. In these respects, silver ranks highest among metals. 

Wednesday, 24 July 2013

The Special Product of Gold

Gold Car
As early master goldsmiths, the Etruscans attempted another, more personal ornamental and decorative gold working skill using the metal in dentistry. Broad bands of gold were attached to the Etruscan patient's existing teeth to help set in place artificial teeth carved from bone or ivory. Teeth in gold or I simply calves' teeth were used to fill gaps. Dentists of the period also secured loose teeth with bands of gold wire.

The Greeks, for all their substantial intellectual contributions to civilization, contributed little to the technology of gold mining and gold working. The Romans, however, were more concerned with practical matters. They developed various mining techniques. Pliny the Elder (AD 23-79) was, for a short period, governor of the conquered provinces of Spain which dispatched considerable amounts of gold to Rome. He left behind a vivid account of gold mining in Asturias, Galicia and Lusitania.

The hunger for gold, particularly among the rich and powerful, never ceased to grow. Known sources of gold were, however, exhausted over comparatively brief periods. It was therefore not surprising that, in the early Middle Ages, the idea of deriving gold from "baser metals" was widely promoted. It was to produce unexpected results.

Gold HomeThere was nothing new to the idea of making gold from other substances. Old papyri tell of Egyptian foundry men who produced an alloy of copper, tin and zinc which proved to be similar to gold in color. There was also malachite, the beautiful green veined stone which, when heated to high tempera tures, produced a lump of copper. Why then should t there not be a stone which could be made into gold? The belief that gold could be made from "lesser" substances can be traced back to ancient Egypt; so can the word for the process alchemy.

The European alchemists of the middle Ages were under strong pressure from their feudal patrons to produce gold. They failed. It was their latter day colleagues the atomic physicists who would succeed, although the cost of the process they would use would far exceed the value of the metal produced. However, the dream of making gold in a laboratory produced a rich harvest of 'non golden chance results over the course of time. Those medieval alchemists, with their steaming brews and bubbling phials, achieved an insight into the relationship between materials which they might not have attained had they not been fruitlessly seeking gold. Without the efforts of t those luckless experimenters, the birth of modern chemistry might have been considerably delayed.

Gold Scorpion
An unsuccessful attempt to make gold is said to have led the fourteenth century German Franciscan monk Berthold Schwarz to the invention of gunpowder, the use of which revolutionized warfare in the late middle Ages. Johann Kunckel (1630-1703) was awarded a barony and the aristocratic name of von Lowenstern because he accidentally dropped a piece of gold wire into a chemical solution, thereby producing, not gold, but deep ruby red glass. It earned a fortune for his patron, a Saxon duke.

Johann Friedrich Bottcher was less lucky. In 1708 when his abortive effort to produce gold led instead to the first European commercial production of porcelain (it had been made in China long before), he was punished rather than rewarded. Fearing he might betray the secret of this profitable discovery to others, his patron, another Saxon duke, took the precaution of having him locked away in prison.

The long adventure and dream of the alchemists was finally brought to an end when Dmitri Ivanovich Mendeleyev (1834-1907), professor of chemistry at St. Petersburg University, formulated the periodic table of elements. For the first time, a coherent, reasonably comprehensive system was devised to classify all the known elements according to their distinctive properties. No longer could serious persons suggest that one element could be transformed into another. (Renewal of such suggestions was to await the dawning of the nuclear age.)

Geographic Ramifications of the Gold Quest

Chemistry was not the only science in which new in ground was broken in the search for gold. Geography also made spectacular advances. Christopher Columbus and his fragile flotilla set sail to find a short route to a land of gold to the cast and he discovered America by mistake. The Spanish conquistadors who followed in his wake were motivated less by a hankering for new horizons than by an insatiable hunger for gold. It was a hunger which resulted in the sacrifice, in the name of their God and their Christian rulers, of many thousands of the Indians they encountered in Mexico and in Central and South America.

It was also an epoch which saw considerable technological advances in gold mining and processing. Bartholomaus and Anton Weiser, wealthy merchants and financiers from the German town of Augsburg, received from the Spanish throne authorization to dig for gold in the colony of Venezuela. The Weiser brothers wanted to employ a new technique for the separation of gold from rock, the technique of amalgamation. Some historians suggest that this sixteenth century invention was really rediscovery and that the Romans had already used it as early as the first century AD.

Gold GunThe Weiser expedition ended in bankruptcy. No gold was found in Venezuela. The journeys the brothers made between 1528 and 1546 proved to be in vain and the mining rights reverted to the Spanish throne. Gold was, however, discovered in adjacent Colombia. Between the middle of the sixteenth century and the beginning of the nineteenth century, Colombia could be credited with about a quarter of the world's gold production.

The Welsers had been unlucky, but some of their ideas like the amalgamation process proved revolutionary. Amalgamation involves extracting gold from its ore in the form of an alloy or amalgam with the use of mercury. The lighter rock moves to the surface of the ore and can be removed without further processing. Most of the gold can be removed from the amalgam simply by pressing it out. The remaining metal alloy can be distilled away at temperatures of about 360 degrees centigrade. The mercury evaporates and gold is left. Amalgamation eventually became the most important gold processing technique in use until the end of the nineteenth century. The method, so much simpler and more effective than anything known before, made possible a substantial increase in gold production.

Extraction of Gold from the Ground

How Much Gold Is There in the World?

Gold Materials
There can only be an estimate of the total amount of gold on Earth. Those who have been bold enough to venture a figure suggest there may be about twenty billion tons of the precious metal in the outer crust of our planet, and perhaps another eight billion tons in solution in the oceans of the world. But that would not be all there is. In the Earth's mantle, beneath the outer crust, there are probably enormous quantities of gold in pure form and in tellurium and selenium compounds. Compared to such estimates, the amount of gold extracted from the Earth so far is extremely modest. only about 80,000 tons. Most of it, about 60,000 tons, has been mined during the twentieth century.

Dealing with such figures can be confusing. Even if the total amount of gold believed to be woven through the fabric of the Earth is enormous compared with the amount extracted so far, gold remains a rare metal. Known workable gold sources have a total estimated yield of some 30,000 tons. If gold extraction continues at the present rate, those sources are likely to be exhausted by the turn of the century. The implicit contradiction between enormous sources of gold some twenty eight billion tons in the Earth's crust and oceans and an extractable total of some 30,000 tons, can be misleading. Gold can really be found in just about all parts of the world, but usually only in very minute amounts. For example, gold equivalent in size only to about a speck of dust is contained in a cubic yard of sea water. Gold-bearing rock is deemed workable when it contains a minimum of five grams of gold per ton. Such a disproportion would seem to make extraction barely worthwhile. But even before the recent sharp rise in the price of gold, its value was sufficiently substantial for men to work every possible source. In California and Alaska, where the appropriate equipment is available, it is even profitable to work sand and gravel with a gold content of barely fifty milligrams to the ton, a hundred times less than the yield that used to be thought the very minimum for commercial consideration.

gold brickPerhaps the strangest gold extraction project was the one conducted after the First World War by the Nobel prize winning German chemist Fritz Haber, the man who .discovered the process for synthesizing ammonia. Haber set out to improve methods for extracting gold from sea water. His motive was not it personal gain; it was patriotism. He hoped to make a substantial contribution to paying off German war debts (following the Nazi rise to power in Germany in 1933, Haber fled to England). His elaborate calculations indicated that much more gold could be derived from the sea than had previously been possible. However, to his profound disappointment and considerable chagrin, his breakthrough foundered on a misplaced decimal point which had disguised the fact that the Haber method would have made costs infinitely higher than potential profits. The enterprise was abandoned and even now there is no satisfactory method to extract from the oceans of the world even a meager fraction of the huge amounts of gold they contain. Whether modern technology will long be inhibited by past and present limitations is, of course, for the future to decide.

Where Does Gold Come From?

Cosmologists, geologists and others who explore the origins of the Earth agree that billions of years ago, this planet was a rotating ball of hot gases and interstellar dust. Gradually that ball cooled and grew more dense. This stabilization process reached a crucial points some four and a half billion years ago at which time most of the chemical elements including gold were formed. Some scientists suggest that during that formative period, the Earth was subjected to radioactive fallout from the planet Uranus. That fallout, they say, created lead isotopes which are still in evidence. Traces of gold discovered in meteorites and on the moon probably date back to the same period.

The heavier elements including gold settled deeper than the lighter ones into the Earth's still molten interior during the period of rock formation. At the same time, centrifugal forces, produced by the Earth's rotation, pulled lighter elements toward the surface, and in many places carried other substances upward as well. The cooling of the magma, and the enormous pressures produced by that process, were responsible for the creation of the Earth's rock formations in their earliest forms. To say that the most ancient known surface rock formations on Earth date back billions of years is to conjure up numbers so immense as to be almost beyond comprehension.

The movement of the Earth's crust following the cooling process also lifted various rock formations to the surface. Included in this uplifting process were veins of precious ore. (It is, of course, well known that the face of the Earth has never ceased to change. Volcanoes, earthquakes, land subsidence and other upheavals and shifts are part of an endless transformation. Observance of terrestrial developments from space has, in recent years, expanded our understanding of the extent of the alterations continually taking place on the surface of our planet.)

Gold generally retained its chemical identity, unlike some other metals which became chemically interwoven with rock. The exceptions were gold and tellurium and gold and selenium compounds, and a few complex gold salts. However, gold is often found in minute amounts in rock. It can be found in copper pyrites and often in quartz deposits. Gold is usually Si found mixed with other elements, including silver, a copper, platinum, iron and lead and occasionally with mercury and bismuth. These mixtures however, arc not chemical compounds.

Gold mining procedures distinguish between primary it deposits so called mountain gold, with the mineral often found in quartz veins and secondary sources in alluvial and other surface deposits. The latter o consists mostly of grains, flakes or dust carried down erodes. A portion of this gold is always carried down into gravel or river beds in the surrounding rock 1 to the sea.

The German scientists Rabenau and Rau recently o formulated a new theory of the origin of .gold o which they confirmed through controlled experimentation. They established that gold, which is normally h not readily soluble, dissolves in certain liquids raised ct to very high temperatures. If there is a temperature difference within a hot bath of, say, between 480 and n 500 degrees centigrade, the gold in the solution will it shift from the colder to the hotter zone.

The experiment demonstrated that ideal solvents were iodocholoride or hydrobromic acid together with an oxidizing agent, such as oxygen. At the lower temperatures, gold combined with the iodine chlorine or bromine to form a relatively stable compound that shifted into the higher temperature zone. There the compound gradually broke up again and released the gold. The gold molecules could attach themselves to precious metal crystals already present and these crystals grew.

If the temperature differential was accompanied by high pressure, the experiment also worked in weak solutions of cooking salt. It would seem that such high temperature solutions have played an important part in nature. At the appropriate temperature, gold dissolves and moves into the warmer zone to crystallize there. The German scientists dissolved ten grams of gold wire and were able within a few days to grow gold crystals measuring a centimeter across. It is now believed that this process explains the origin of nuggets that are to be found in great quantity in many parts of the world.

From Rock to Gold Ornaments

No one knows when, in the vast regions of prehistory, man first took an interest in gold. Probably gold was first worked during the fifth or sixth millennium BC. The earliest evidence of gold working comes from ancient Egypt. Hieroglyphics from the period between 4100 to 3900 BC tell of shining, yellow "workable rock" which was apparently used for ornamentation.

Gold is known to have been extracted from alluvial deposits in the' Nile around 3500 BC. Large pieces were hammered out in metal or wooden molds to form elegant artifacts. These were fairly pale in color, since Nilotic gold had a pronounced silver content.

It was probably around the same time that the Egyptians discovered the smelting process. At first T they applied this skill to copper. But goldsmiths were soon concocting alloys by mixing various metals in a smelter. No doubt they were surprised to discover h that various gold alloys had a much higher melting e point than gold itself.

High temperatures were produced by the first metallurgist’s wit h the use of blowpipes, not much different than those still employed by tradition oriented gold-smiths. From that breakthrough in the evolution of the technology of gold processing, it was a simple step to join together pieces of pure gold with high melting points by using alloys with lower melting points. Thus did the Egyptians invent the art of soldering. Their skills developed to such a high level that when gold ornaments are examined even today, the joins arc I often invisible to the naked eye.

It seems likely that the Egyptians soon discovered how to do their smelting in crucibles fire resistant containers to melt down gold dust and flakes to produce larger pieces of pure gold. However, they never cast jewelry; they molded their gold by working it over a sturdy underlay, made of clay, tar or other such substances. They then used a small, blunt chisel for embossing.

The First Gold Mines

gold brickProspectors in ancient Egypt systematically washed the sands of the Nile. They are said to have extracted about eighty grams of gold from a ton of sand, a remarkably high yield. They must have noticed that this alluvial treasure was often to be found with quartz, so plentiful in the mountains skirting the Nile Valley. About 3000 BC, Egyptians drove adits up to thirty feet deep into those mountains, to tap the often twisting quartz veins to be found there. These were the first gold mines in history.

The ancient prospectors and mine engineers were resourceful and thorough. No source of gold has ever been unearthed in Egypt which had not at least been investigated by them. But they were highly selective in the sources they exploited. They only tapped workings in which the yield ranged from one hundred to five hundred grams a ton.

The process was arduous. The hard, quartz bearing rock was first made brittle through heating with fire. It was then broken away layer by layer from gallery walls by slaves. Then the flakes of gold embedded in the quartz had to be extracted. The ore was pulverized manually in mortars and the resulting powder was sprinkled over inclined boards to be washed down. The process was repeated until all the light rock particles had been washed away and only pure, heavier gold dust remained.

The invention of the bellows, around 2000 BC, helped the Egyptians remove undesirable impurities from gold. With the use of bellows, operated by slaves in shifts, a temperature of 1000 degrees centigrade, required for the smelting process, could be maintained. Raw material was placed in a Crucible between layers of porous stone or slate dust, mixed with rock salt and iron sulphate. The crucible was then heated until it was red hot, just short of the 1063 degrees centigrade melting point of gold. The process, continued over a period of days, eliminated impurities and alloyed metals, leaving virtually pure gold behind.

Gold Working from Antiquity to the middle Ages

Gold Shoes
The first known stone molds for gold bars and rings date back to the twelfth century BC. This is surprisingly recent, considering that casting techniques for copper, bronze and, later, iron were known as early as the sixth millenium BC. In the eleventh century BC, a way of testing gold was discovered that is still sometimes used today: the touchstone method. The touchstone is a flint like black stone. When gold is rubbed across it, it leaves a telltale streak. The color can reveal how much silver or copper the gold contains. Silver colors the gold white; copper colors it red. A drinking song dating back to 550 BC says: "Gold is tested by the touchstone and can be clearly recognized. In time, a man's character, good or evil, will be recognized too."

The Egyptians devised a method for covering less valuable metals with gold gilding with fire. Across the Mediterranean, in Italy, the Etruscans developed this technique to perfection in the following way:  one portion of gold and one of lead were reduced to finest dust and made into a thin solution by means of gum arabic dissolved in water. The metal to be gilded, perhaps copper, was dipped in the solution and then heated in a fire. After several repetitions of this procedure, the copper began to acquire an increasingly pronounced golden color. The heat dispersed the lead in the solution, but not the gold.

The Etruscans also mastered the techniques of granulation and filigree. For granulation, the molten metal was quenched the Etruscans let it fall drop by drop into water and the resulting granules were soldered to a gold surface. For filigree, wire was hammered flat and was then bent into intricate patterns and soldered together.