| MadSci Network: Physics |
John, That probably was a gold alloy dagger and a work of art. All you want to know about gold can be found in the American Society for Metals “Metals Handbook” 9th edition, volume 2. Gold does work harden. Annealed[300 C] wrought gold has a tensile of 130M Pa[mega pascals] and an elongation of 45%. At 60% reduction the tensile increases to 220 M Pa and the elongation drops to 4%. The hardness about doubles but it is still a soft metal. To change the hardness, color, workability, corrosion resistance, cost, and other properties of gold, gold is alloyed with silver, copper, nickel, zinc, palladium, platinum, and probably other metals and in various combinations of the above. There are many combinations: silver-copper mixes are most common for yellow gold, while nickel, zinc and palladium are found in white gold. Platinum is used in some electrical alloys where hardness and superb corrosion resistance is needed. Some platinum gold alloys reach tensile over 900 M Pa. Gold is sold by weight; 24 carat gold is pure gold. Common carat weights are 22,18, 14, and 10. 18 carat gold is 75 % gold by weight but only 50- 60 atom % gold. Its properties are still largely determined by the properties of gold. The lower carat weights are less than 50 atomic % gold and the properties are very dependent on the alloying metals. Many of the alloys respond to heat treating and tempering to vary hardness. The lower the gold content the more susceptible the alloy is to corrosion and the greater the color variations. If you have a working interest in goldsmithing, which seems to be an interesting artisan’s field, I would suggest visiting a local working artist and ask some polite questions. They would probably be all too happy to talk to someone with a genuine interest in the field and might have a specific answer. Or the next time we are in Cairo.... Regards JimG
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