Combat Arms 2869 Grove Way Castro Valley, California 94546-6709 Telephone (415) 538-6544 BBS: (415) 537-1777 ò€€€€€€€€€€€€€€€€€€€€€€€€€€À Ò Ò Ò Information About Gold Ò Ò Ò Ë€€€€€€€€€€€€€€€€€€€€€€€€€€ô The following material is extracted from "The Complete Metalsmith" by Tim McCreight, published in 1982 by Davis Publications Inc., Worcester, Massachusetts 01608, ISBN 0-87192-135-9. €€€€€€€€€€€€€€€€€€€€€€€€€€€€€€€€€€€€€€€€€€€€€€€€€€€€€€€€€€€€€€€€€€€€€€ GENERAL A. Gold was probably the second metal to be worked by early man, being discovered after copper. Quality gold jewelry has been manufactured as long ago as 3000 BC. B. If all the gold ever found in the world (about 120,000 tons) were cast into a single cube, this ingot would be 69 feet on each side. C. It's been found that one ounce of gold can be flattened to a sheet that will cover almost 100 square feet or be drawn to a wire nearly a mile long. D. Gold is so malleable that it can be made into a foil that is less than five millionths of an inch thick (0.0000033" or 0.00025 mm.). At this minimal thickness the sheet of gold will be semi- transparent. E. Gold dissolves in aqua regia (one part nitric acid to three parts of hydrochloric or sulfuric acid) and solutions of chlorine, potassium cyanide or sodium cyanide. TECHNICAL DATA Chemical Symbol Au Melting point 1063¿ Celsius (1945¿ Fahrenheit) Hardness 2.0 to 2.5 Specific gravity 19.23 (cast) 19.29 - 19.34 (worked) Atomic weight 197.2 THE QUALITY OF GOLD Since pure gold (correctly termed "fine" gold) is too soft for most uses it is alloyed with other metals to achieve a desired hardness, color and melting point. Silver and copper are the two most common additives but many other metals can be used. The relative amount of gold in an alloy is called the "karat". This signifies proportion and should not be confused with carat, which describes the weight of jewelry stones. KARAT % OF GOLD 6 25.00% 10 41.67% 14 58.33% 16 66.67% 18 75.00% 20 83.33% 22 91.67% 24 100.00% COMMON ALLOYS OF GOLD Percentages of Ò S C Ò i o O Ò G l p Z t Ò o v p i h Ò l e e n e Melting Ò d r r c r Point Specific €€€€€€€€€€ô Symbol: Au Ag Cu Zn ¿C ¿F Gravity ALLOY NAME 24 karat - fine 100 1063 1945 19.3 22 karat yellow 92 4 4 977 1790 17.3 22 karat (coins) 90 10 940 1724 17.2 18 karat yellow 75 15 10 882 1620 15.5 18 karat yellow 75 12.5 12.5 904 1660 15.5 18 karat green 75 25 966 1770 15.6 18 karat rose 75 5 20 932 1710 15.5 18 karat white 75 25 Pd 904 1660 15.7 14 karat yellow 58 27 17 802 1476 13.4 14 karat green 58 35 7 835 1535 13.6 14 karat rose 58 10 32 827 1520 13.4 14 karat white 58 42 Pd 927 1700 13.7 10 karat yellow 42 12 41 5 786 1447 11.6 10 karat yellow 42 7 48 3 876 1609 11.6 10 karat green 42 58 804 1480 11.7 10 karat rose 42 10 48 810 1490 11.6 10 karat white 42 58 Pd 927 1760 11.8 Note: Pd = Palladium VOLUNTARY PRODUCT STANDARD 70-76 This is an industry standard that has set the legal tolerances since 1976. The standard allows the variation of 3 parts per 1,000 (which amounts to 0.072 karats or K) on unsoldered goods and 7 parts per 1,000 (0.168K) on soldered objects. This is called "plumb" gold, meaning accurate gold. Jewelry manufacturers were given until 1981 to dispose of all of their old merchandise made at lower standards. TESTING In order to be scientifically accurate, a sample MUST be assayed in a testing laboratory. However, the following two tests have been used for years and, with a little experience on your part, will be accurate enough for the average crafts person or trader. A. Determining if the substance is actually gold. 1. With a small file, make a scratch in an inconspicuous spot. 2. Using a wooden, glass or plastic stick, apply a drop of 70% nitric acid to this spot and observe the reaction. a. No reaction It is gold. b. Bright green bubbling all over It is brass. c. Green only in scratch Gold layer over brass. d. Milky in scratch Gold layer over silver. 3. When the test is completed, rinse the substance AND the stick well in running water. B. Determining the karat of the gold substance. 1. Karat determination requires you to have a "kit" of the following items: a. 70% nitric acid. b. Aqua regia (see earlier description of this). c. Samples of known karat quality of gold (usually in a needle form). d. Touchstone of slate or ceramic. 2. The object to be tested is rubbed on the stone ("touched") in order to leave a streak on the touchstone. 3. A parallel line is then made using on of your known karat test needles. 4. Wash the 70% nitric acid over the two marks if the substance is known to be low karat gold and use the aqua regia if the substance is thought to perhaps be of a higher karat rating. 5. Observe the reactions. When the sample changes color at the same RATE as the "test" streak, a match has been made. This would require you to have test needles for several karat levels of quality. MISCELLANEOUS INFORMATION A mixture of roughly equal parts of gold and silver is called "electrum." The maximum hardness of electrum is reached at the 50:50 ratio. The hardness alloy of gold, silver and copper is reached at 12 karat. This is 50% gold, 25% silver and 25% copper. It is called 12 karat yellow. An increase in the copper content to the gold content in an alloy will lower the melting point up to 18% copper. At 18%, the melting point will be 880¿C or 1642¿F. If you wish to continue lowering the melting point beyond this (such as when making a special solder), add silver. White gold usually has 10% to 20% nickel in it and can contain copper, manganese, tin and zinc. It has NO silver in it! When pouring a gold ingot, sprinkle an even mixture of powdered charcoal and ammonium chloride (called "sal ammoniac") over the metal during melting. It will yield a bright, tough ingot that will withstand rolling. If iron (Fe) or steel are known or suspected to be present, use a flux of 1 part potassium nitrate and 2 parts potassium carbonate. Melt this mixture over the molten gold to purify the gold. Then cool the gold and remelt it. During the second melting, sprinkle ammonium chloride over the molten gold. Finally, pour the molten gold into a warm (not hot) mold.