Probably the first metal known to man, gold is a malleable, ductile and soft metal. It is one of only two metals that are not grey-silver in color, the other being copper. Gold's chemical symbol is Au, its atomic number is 79 and it is found free in nature or in ores containing other metals such as copper. It is an excellent conductor of electricity and heat and in its pure state is unaffected by many chemicals and acids. The freezing, or solidification temperature of gold is used as a calibration point for the International Temperature scale. Gold melts at 1063 degrees Celsius (1945 degrees Fahrenheit) and has a density of 19.3 grams per cubic centimeter.
Gold is one of the most ductile metals, a single ounce can be drawn into a fine wire nearly five miles long. It is also one of the most malleable metals, an ounce can be hammered into a thin sheet covering 100 square feet. It is one of the most reflective metals, a surface coated with a thin layer of gold will reflect 99% of incoming infrared radiation.
Brief History
Gold was probably first found in nugget form on the ground and used by prehistoric cultures as a tool. More sophisticated gold art objects were discovered in what is now southern Iraq in the Royal Tombs at Ur, dating to 3000 BC.
Timeline
3000 BC: Egyptians master the art of beating gold into gold leaf and alloying it with other metals.
1500 BC: The Shekel (two-thirds gold) used as a standard unit of measure throughout the Middle East.
1091 BC: Squares of gold are legalized in China as a form of money.
58 BC: Julius Caesar seizes enough gold in Gaul (France) to repay Rome's debts.
1511 AD: King Ferdinand of Spain sends explorers to the Western Hemisphere with the command to "get gold."
1717 AD: Isaac Newton, Master of the London Mint, sets price of gold that lasts for 200 years.
1787 AD: First US gold coin is struck by Ephraim Brasher, a goldsmith.
1803 AD: North Carolina site of first US gold rush. The state supplies all the domestic gold coined for currency by the US Mint in Philadelphia until 1828.
1848 AD: California gold rush begins when James Marshall finds specks of gold in tailrace of John Sutter's sawmill near the junction of the American and Sacramento Rivers.
1887 AD: Glasgow doctors, Robert and William Forrest, and chemist John S. MacArthur patent the process for extracting gold from ore using cyanide.
1896 AD: Two prospectors discover gold while fishing in the Klondike River in northern Canada, richer finds were rumored farther south in Alaska's Yukon, spawning the Alaska Gold Rush in 1898 -- the last gold rush of the century.
1900 AD: US adopts the gold standard for its currency.
1922 AD: King Tutankhamen's tomb (1352 BC) opened to reveal a 2,448 lb. gold coffin and hundreds of gold and gold-leafed objects
1933 AD: President Franklin D. Roosevelt bans the export of gold, halts the convertibility of dollar bills into gold, orders US citizens to hand in all the gold they possess and establishes a daily price for gold.
1934 AD: Roosevelt fixes price of gold at $35 per ounce.
1947 AD: The first transistor, the building block for electronics, is assembled at AT&T Bell Laboratories. The device uses gold contacts pressed into a germanium surface.
1968 AD: On March 15, central banks give up fixed price of gold at $35 per troy ounce and let it free float.
1969 AD: Gold coated visors protect astronauts' eye from searing sunlight on the moon (Apollo 11 moon landing).
1970 AD: Charged coupled device invented, using gold to collect electrons generated by light, eventually used in hundreds of military and civilian devices, including video cameras.
1971 AD: The colloidal gold marker system is introduced by Amersham Corporation of Illinois. Tiny spheres of gold are used in health research laboratories worldwide to mark or tag specific proteins to reveal their function in the human body for the treatment of disease.
1974 AD: On December 31, US government ends its ban on individual ownership of gold.
1980 AD: Gold reaches intra-day historic high price of $870 on January 21 in New York.
Other Uses of Gold
Gold is used in almost all industries, from computers to healthcare. Gold's ability to conduct electricity and heat and not tarnish makes it ideal for contacts and wires in electrical and electronic components, its high density and reflectivity make colloidal gold useful for medical diagnosis. Gold is non-reactive in the human body, making it useful for tooth repair and other medical implants.
It is used as a reflective coating on optics ranging from lasers and mirrors to compact disc coatings and jet canopies.
Common Gold terminology:
Karat: a ratio of the amount of pure gold contained in a piece
10 Karat = 10/24 gold or 41.66% gold, 58.33% other metals
14 Karat = 14/24 gold or 58.33% gold, 41.66% other metals
18 Karat = 18/24 gold or 75% gold, 25% other metals
Gold Filled: A layer of gold alloy permanently bonded to a base metal. This resulting "sandwich" is then rolled to desired thickness. The pure gold content of the finished item must be at least 10% of the total weight.
Rolled Gold Plate: Similar to Gold Filled, but the pure gold content of the finished item may be less than 5% of the total weight.
Vermeil: Gold plating, at least 15 micro inches thick, on sterling silver.
Gold Leaf: Sheets of hand-hammered pure gold, usually about 3-5 micro inches thick, applied to other surfaces.
Gold Alloys
Pure gold is too soft to be used for most jewelry. To make gold more durable and strong, it is alloyed with other metals. The most common yellow gold found in jewelry contain mixtures of gold, copper, silver and zinc. The color of the finished alloy can be altered by changing the proportions of the alloying metals. More silver will make the gold take a greenish color and more copper lends a reddish hue. Zinc is used in casting alloys to reduce oxidation in the melt, but tends to make the finished alloy difficult to roll or form into wire.
White gold became popular during the early 20th century when platinum was declared a strategic metal to be used in the war. Gold was alloyed with nickel or palladium to create a precious white metal to replace platinum. The nickel-gold alloy age hardens and is succeptable to cracking, and it some people react to the nickel content with rashes or blackening of the skin.
Gold can also be alloyed with other metals to make different and interesting colors. Blue, black and purple gold can be created using iron or aluminum, but these alloys are not malleable or ductile, are brittle and must be cast under vacuum or in an inert atmosphere.
Working with gold
Gold has been the metal of choice for jewelry for thousands of years. It melts at a relatively low temperature, easily mixes with other metals to lend strength and durability to a finished piece, and is easy to work. In its pure form, gold does not discolor when heated and will not react to most chemicals. Gold alloys will discolor when heated, so a mixture of boric acid and alcohol is applied to the surface to reduce the oxidation. This coating is removed and any discoloration that has occurred can be eliminated by pickling in an acid solution.
Gold conducts heat very well and great care must be taken not to melt previous solders when welding or soldering to an assembled piece.
When working the metal, it will work-harden, that is, it will become harder and more springy as it is hammered or rolled. This is useful to create snaps, catches and other components. After work-hardening, if required, the metal can be annealed to return it to its softer state.
A fine and lustrous polish is easily attained, but too much polishing will reduce precise corners to rounded edges. Inexperienced or careless polishers may actually polish away metal in critical places such as the tips of the prongs that secure gemstones.