Gold in Industry
International statistics indicate that three industries absorb most of that part of the world's gold production which is neither hoarded nor used for the manufacture of currency. In 1974, the jewelry industry used about 214.1 tons of pure gold, the electronics industry used 98.9 tons, and dentistry used 65.6 tons. The 74.2 tons remaining for industrial use was largely employed in surface-coating various products.
Thus, in 1974, a total of 452.8 tons sixty one percent of total gold production for that year was used in manufacturing. Almost without exception, industry and commerce used one form of gold alloy or another. These were produced in a wide range of combinations. In every major industrial country, highly specialized enterprises have sprung up to meet the need for gold alloys. In some cases, such plants are state-owned but usually acquire their supplies of "raw" gold and old gold through the agency of private firms. Where necessary, the old gold is separated metallurgically into its constituent elements for recycling. Enterprises processing precious metals are, incidentally, among the few businesses that have always practiced almost total recycling procedures. They tend to make almost complete use of waste and scrap. This is not for ecological reasons but because of the value of the precious metals involved.
Metallurgy has now reached so advanced a state of development that alloys can virtually be "made to measure" for almost every requirement. They can be produced according to an enormous variety of specifications: color, chemical and physical characteristics, malleability and temperability. When necessary, combinations of qualities can be achieved.
Gold content is of special significance in the manufacture of jewelry. It is measured in carats. Pure gold is said to be twenty-four carats fine. A gold alloy only seventy-five percent pure is said to be eighteen carat gold. Measurement is also made in parts of pure gold per thousand. If ten percent of a gold alloy is other metals, it is said to be 900 gold. Fourteen carat gold is 585 pure, and so on.
The various kinds of gold solder used in the jewelry and dental industries make very specific demands on the metallurgist. The solder has to be of the same color and gold content as the surrounding substances, particularly in jewelry. But in order to guarantee an effective connection, the solder must have a lower melting point than the pieces it connects. Solder with a particular gold content is sometimes known as control solder. It is defined according to a regulated color scale and a scale of melting points and is divided by name and quality into various categories. It makes multiple soldering possible without damaging existing joints.
Fine, Finer, Finest
In the past, goldsmiths working by hand produced their own alloys for sheet, wire and casting purposes. An intricate series of closely guarded secrets and techniques were involved. But the quality of the finished product and its gold content had to be taken on trust by the customer who had no choice but to rely on the craftsman whose work he commissioned a Nowadays standards of quality are so high that it is necessary to have modern smelting and heating equipment, a well-furnished laboratory and a highly qualified all team of specialists to produce the finished product.
The raw material of all gold alloys should be the purest possible gold. Financial institutions, which are C very much concerned with material value, find so-called "Good Delivery" gold with its fineness of 995. A perfectly acceptable. This gold is in fact further refined T before it is processed metallurgically. Methods include crefinement by electrolysis. This is based on the principle that metal ions, under the influence of an electric current in a conducting solution, change their polarity from plus to minus.
"Good Delivery" gold is poured around thick anode plates suspended in a gold chloride bath. The cathodes consist of refined sheets of pure gold. With the correct electrolytic stirring and current, the gold moves to the cathode from the anode and separates there into dark yellow crystals. Silver, combined with insoluble silver chloride, sinks to the bottom of the bath while platinum metals iridium, osmium, palladium, rhodium and ruthenium remain suspended in the solution and may be extracted chemically. The gold which forms around the cathode is more than 999.9 percent pure.
In the past, the purity of gold could only be determined approximately with the use of a touchstone. Later, the Archimedean principle which Archimedes allegedly discovered in 250 BC while bathing was also used. Archimedes had been commissioned to determine the gold content of a crown without damaging it. While in the bathtub, he established that the amount of water displaced by a body corresponds to the volume of that body, and perceived that the specific gravity of a body could be deduced from its weight and volume.
These days the gold content of a test piece is usually analyzed. Another method employs calorimetric measurement the light absorption of a test specimen is compared with the standard reaction of known gold alloys.
Gold Alloys -The Supreme Metallurgical Skill
Alloys can also be described as solid state mixtures. They are small crystalline agglomerates, not chemical compounds. That is why they have no clearly defined melting point at which the substance liquefies. Alloys melt in temperature band liquid islands of crystals with a lower melting point form first to be followed by those with higher melting points. The crystalline structure alters during the heating process before the melting point is reached. Quenching makes for stabilization of changes in many alloys in a process known as tempering.
Color, Hardness, Chemical Stability
Alloys used in jewelry making must meet specific color and gold content requirements. Colors tend to range from red to yellowish-green. They generally consist of gold-copper-silver alloys in which the copper and silver components vary in proportion. Other characteristics are also varied. The amount of silver in the alloy does not affect its hardness, but copper does. The hardest alloys are twelve and fourteen carat in which copper and silver are dominant.
White gold alloys tend to be very popular. It is possible to produce soft white gold, with palladium content, or hard gold-nickel alloys. Platinum gold alloys are much in demand in dental technology because they are capable of withstanding the chemically aggressive environment of dental cavities remarkably well. They also have the hardness required in dental work, in view of intense pressure often produced in the chewing process.
Gold alloys are melted in small quantities. Fine gold and alloy metals are usually provided in granular form because they can thus be mixed with greater accuracy. During the melting process, impurities including oxygen in the air must be carefully avoided. Smelting only works successfully in a vacuum or with a protective gas.
Before alloy bars are processed into such semi-finished products as sheet metal and wire, mechanical means are employed to remove burrs and porous surfaces. The bars arc then placed in a roughing mill through which they pass several times. This alters their crystalline structure to the extent that they must be homogenized with a first interim heat treatment. Through various stages of the milling process, constantly interspersed with additional heat treatment, sheets and rods gradually attain their finished appearance. The final stage for sheets involves passing them through rollers which produce a high surface finish. Wire goes through a drawing bank where polished diamond and hard metal draw stones produce the desired affect.
Seamless tubes are produced by means of punched-out metal discs. They are formed in the course of several deep draw stages on great presses and shaped into tubes. The final stage takes place on special drawing banks which create correct lengths and cross-sections.
Modern Casting and Traditional Technology
One of the oldest of all techniques is used for casting gold alloys for making jewelry and for dental technology the so-called cire perdue (lost wax) process. While in jewelry manufacture the rough casting is only an approximation of .the form and dimensions of thing final products, dental casting, however, requires also lute precision for surfaces which cannot be furtha ,Id worked. Tolerances in dental casting consist of tenths of millimeters.
In jewelry-making, models often made of tin are putting through an intermediary stage to produce equivalents rig in wax. For this purpose, they are bedded down in silicon. After the silicon hardens, the models are re-moved, leaving an outline, a negative form, which is then filled with wax to produce a precise facsimile of the entire model. If several identical pieces of jewelry are is required, it is a simple matter to cast the appropriate number of wax models; the negative can be used time I and time again. To cast the series in a single operation, individual models are laid out in a cluster. This is sometimes called a "wax tree".
A hollow form is needed for casting the metal. The wax tree is placed in a steel casting cylinder with the crown pointing downward. It is surrounded with a plaster-like fire-resistant substance which is packed! tightly. Only the stem of the tree appears above the casting. The cylinder is heated in a furnace to about 250 degrees centigrade; the wax melts and can be poured or centrifuged out, while the cast bakes to become a "muffle".
During preliminary heating, the appropriate proportions of alloy have been brought to melting point. The liquid alloy is now poured into the cavity left by the stem of the tree. It flows through its branches to the appropriate molds. When the cast is full, there is a momentary pause before the centrifuge is started. The delay is needed to permit air and gas holes to be swallowed in the still molten metal.
Centrifugal force gradually presses the setting metal into all corners and angles of the form, producing complete and accurate castings. To free them, the muffle in which they were formed must be destroyed, a hence the name "lost form." The final stages include r carving, lapping and polishing.
The process employed in dental technology is similar, except that dental work is always "made to measure" and each item is individually produced to size. Since only one piece is cast each time, the casting cylinders arc notably smaller. The molds, which often are very complex, require particular attention because only a casting that fits properly is of any use. As metal sets, it loses volume.
As it cools, it shrinks. When a casting has very varied cross-sections, as is often the case in dentistry, intense stress can be produced in the broader areas so that cavities are formed in the metal. This may be prevented by casting channels set above the broad areas, provided with spherical bulges known as "lost heads". When cooling, the metal sets in these lost heads after setting in the casting, because of its greater volume. This has the effect of drawing metal out of the lost heads so that cavities form there with no resulting damage.
Writer – Micbael Globig