En Zarif Metal "Gold"

History of Gold and Its Use in Antiquity

Gold ranks foremost among metals whose mining began earliest in history. Because it generally does not combine naturally with other elements, it can be obtained without any processing. Elegant and refined objects have been fashioned from this easily worked beautiful metal. Craftsmen of ancient civilizations used gold generously to decorate tombs and temples. Gold objects made more than 5,000 years before Christ have been found in Egypt. Among the earliest though not the oldest, the most notable are the gold items found in Tutankamon's tomb by Howard Carter and Lord Carnarvon in 1922. This young pharaoh ruled Egypt in the 14th century B.C. The traveling exhibition consisting of these items, called the "Tutankhamun Treasures," drew more than six million visitors to six cities in the United States in 1977-79. At the ancient Mycenae fortress near Nauplion, discovered by Heinrich Schliemann in 1876 in Greece, the tombs of nobles contained a great variety of gold figurines, masks, cups, diadems and jewelry, along with hundreds of decorated beads and buttons. These elegant works of art were created more than 3,500 years ago by skilled craftsmen.

Ancient Gold Sources

Ancient civilizations obtained gold mostly from sources in the Middle East. Mines found in the Upper Nile and Nubian Desert regions near the Red Sea supplied most of the gold used by Egyptian pharaohs. When these mines became exhausted, gold mines likely found elsewhere in Yemen and South Africa were utilized. Merchants in Mesopotamia and Palestine procured gold from Egypt and Arabia. Recent research conducted at the Mahd adh Dhahab mine ("Cradle of Gold") in present-day Saudi Arabia has revealed that gold, silver and copper were extracted from this region during the reign of King Solomon (961-922 B.C.). Particularly the gold in the Aztec and Inca treasures of South America was plundered by Conquistadors between the 15th and 19th centuries. Conquistador is the name given to Spanish soldiers, explorers and adventurers who invaded large portions of the Americas and brought them under Spanish colonial rule, and means conqueror. During this process, namely the discovery of new lands, the treasures of civilizations found there were seized by Conquistadors and the invaluable works of these cultures were melted down and cast into coins and ingots.  

Gold as Currency

Today, nations of the world use gold as a medium of exchange in monetary transactions. A large portion of the United States' gold reserves are held in the vaults of the Fort Knox Bullion Depository. The depository is located adjacent to a military facility, approximately 30 miles southwest of Louisville, Kentucky. The bars in the depository are sized similar to a standard brick, approximately seven inches long, three and a half inches wide and one and a quarter inches thick. Each of these gold bars contains approximately 400 ounces of pure gold. The gold in the depository is stored in vault compartments without packaging. Beyond the financial sector, gold is used in jewelry and similar products, electrical-electronic applications, dentistry, aircraft and aviation industry, art, medicine and chemical sectors.  

Properties of Gold

Gold is called a "noble" metal (an alchemical term) because it does not oxidize under normal conditions. Its chemical symbol is Au, derived from the Latin word "aurum." In pure form, gold metal has a metallic luster and is sun-yellow, but mixtures of gold with other metals such as silver, copper, nickel, platinum, palladium, tellurium and iron produce various color tones ranging from silver-white to green and orange-red. Pure gold is relatively soft and easily malleable. The specific gravity of pure gold is 19.3, for mercury 14.0 and for lead 11.4. Pure gold, as commonly occurs in sediments, has a density of 16 to 18, while the associated waste rock (gang) has approximately a density of 2.5. The difference in density allows gold to be easily separated from clay, silt, sand and gravel using various shaking and collecting devices such as pans, spirals and sluice boxes.

Gold Amalgam

Mercury (quicksilver) has a chemical affinity for gold. When mercury is added to material containing gold, the two metals form an amalgam. Mercury is subsequently separated from the amalgam. Extraction of gold and other precious metals from ore by treatment with mercury is called mercury amalgamation. Gold dissolves in aqua regia, also known as Royal Water, which is a mixture of hydrochloric and nitric acids, and in sodium or potassium cyanide. This latter solvent is the basis of the cyanide process used until reaching the process of gold plating. Gold plating is an electrochemical process in which a thin layer of gold is deposited on the surface of another metal. An electric current draws positively charged gold ions through a gold bath solution via a negatively charged metal piece, allowing them to adhere. Gold plating is commonly applied to older pieces.

Gold plating stages:

• Preparation: The surface to be plated must be free of dirt or oil. Stripping, polishing, tumbling, sandblasting and other operations can prepare the surface. • Cleaning and rinsing: The surface should be subjected to electrical cleaning, steaming or ultrasonic cleaning. This is a deeper cleaning stage that ensures the surface is ready for the next step. Subsequently, the piece is rinsed with water to remove cleaning agent residues. • Strike layer: A buffer layer, usually made of nickel, is then applied to the metal to be plated. This develops the bond between the plating material and the substrate. • Additional rinsing: After the strike layer is applied, the piece is rinsed again. • Base plating: Additional plating layers will be added to the base material. A given piece typically has several plating layers (usually copper, nickel and gold). • Final plating: Under carefully controlled temperature and voltage, the piece is immersed in the plating solution to draw ions onto the surface. Different temperatures and voltages are required for different metal types. • Final rinsing: The piece is rinsed with water and left to dry after final plating. If not satisfied with the result of the gold plating process, it may be necessary to repeat the procedure. Since the late 1800s, the process of laying a thin layer of gold over another metal for greater beauty and durability has been used commercially. Besides the appeal of displaying gold details on a piece or the appearance of solid gold, gold is plated for industrial purposes and is important for use in circuit boards. There are two main electrochemical methods: tank and brush. Both involve electric current, electrodes (anode and cathode) and an electrolyte solution or a preparation containing gold.

Chemicals Used in Gold Plating Cleaners

The object or areas to be plated must be absolutely clean for plating to proceed properly. A combination of different processes is used, including acid cleaners, alkaline cleaners, abrasives and solvents, to remove both organic and inorganic matter as well as sand and soil.

Pretreatment

Depending on the type of metal being plated, processing may be required to deposit an intermediate plating metal or to smooth the surface layer for gold deposition. For example, in gold plating over a copper alloy, nickel is plated first, then gold. Sometimes other finishes such as chromium must be removed using a chemical stripper.

Electrolyte Solutions

To obtain an electrolyte, the metal must be in a condition where it can dissociate and form ions. Gold is a stable metal and requires strong chemicals to achieve this. Although techniques using sulfites and thiosulfates exist, gold is typically mixed with cyanide, called cyanoate. Many special formulas exist for these solutions. In tank electroplating, cyanoate is dissolved in an acidic bath that receives the electrodes. In brush electroplating, an applicator with a stainless steel core applies the cyanoate as a gel. Electric current passes from the steel applicator through the gel to the metal object being plated as the gel advances.

Acids

For tank electroplating, the pH of the electroplating solution must be adjusted to prevent the formation of hydrogen cyanide, a lethal gas at pH values above eight. However, when pH falls below three, cyanoate precipitates from solution. Both inorganic and organic acids, including phosphoric acid, sulfuric acid and citric acid, are used to adjust pH in the workable range.

Other Additives

Brighteners are metal salts of transition metals such as cobalt, nickel and iron. They give gold better wear resistance and brighter colors. Certain organic compounds are added to increase the density of gold plating. Some of these organic additives are polyethylenimine, pyridine sulfonic acid, quinoline sulfonic acid, picoline sulfonic acid and substituted pyridine compounds. Buffering agents such as citrate/oxalate buffer and wetting agents may also be added to maintain pH in the proper range. Prepared by: B. Serhat Cengiz

Sources • geology.com • sciencing.com • goldplating.com