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Pure gold, the most malleable of all metals, is incredibly workable. A single grain (1/480 of a troy ounce) can be pounded from a cube measuring 1.497 mm along each edge into six square feet of ultra-thin gold leaf. It is also the most ductile: Theoretically, a grain of pure gold can be drawn into a wire 1 1/2 miles long.
Because pure gold is so soft and pliable, jewelry made of it would too easily get bent and distorted in the course of wear. To get around this problem, jewelers use an alloyed form known as jewelers use an alloyed form known as karat gold (this has nothing to do with the metric carat used to measure diamond weight). Alloying increases its hardness and provides a variety of different colors.
We specify the gold content of karat gold by the familiar code 14K, 18K, etc. The K (karat) number tells us how many parts, by weight, of pure gold are contained in 24 parts of the alloy. Thus:
10K = 10/24 = 41.67% pure gold
12K = 12/24 = 50.00% pure gold
14K = 14/24 = 58.33% pure gold
18K = 18/24 = 75.00% pure gold
And of course, 24K means, essentially, 100 percent pure (or fine) gold. At one time, 14 karat could actually bne only 13.5 karat by weight; the FTC put a stop to this practice in 1970. Today, the term “plumb gold” refers to the new standard; 14 karat gold is now a full 14/24.
While American manufacturers customarily use the K numbers in their quality stampings, foreign manufacturers often stamp their products witha three-digit number which gives the gold content as parts per thousand. A 750 inside, say, and Italian-made ring indicates 18 karat gold; stampings of 583 or 585 correspond to 14 karat gold.
As far as the American jewelry trade is concerned, the FTC prohibits the use of the word “gold” for any alloy below 10K, in the US, this minimum karatage is commonly used for class rings and the popular, modestly priced lines of rings that feature initials, synthetic birthstones, and inexpensive colored stones. The gold mountings used for diamond jewelry are generally 14K or 18K. Gold jewelry made in many places i Asia is often as high as 22K.
The various colors of karat gold depend upon the other metals in the alloy. These include silver, copper, nickel, zinc, platinum, and palladium. The specific formulas used by refineries are trade secrets, but in general, green, pin, and the different tones of yellow gold are made by alloying gold with varying proportions of silver and copper. A high percentage of copper gives pink (or “rose” or “red”) gold its color, while the alloying metal in 18K green gold is practically all silver.
White gold contains about 10 to 20 percent nickel, plus zinc, copper, platinum, and palladium (but, contrary to the popular assumption, no silver). These alloys make white gold harder and somewhat tougher to work with than yellow gold.
Gold itself is impervious to tarnishing , and it takes very strong and dangerous chemicals to dissolve it. After centuries in saltwater, gold artifacts retrieved from shipwrecks look as yellow and shiny as the day they were made.
Nevertheless, some forms of karat gold, particularly pink gold and the 10K alloys, do darken noticeably over a period of time. The culprits are the alloying metals involved–the high copper content in pink gold, for instance. Occasionally, body acids react with the alloys, even in 14K gold, leaving a dark residue on the skin. The chemical in cosmetics can cause a similar effect. The chlorine in bleaches and swimming pools will attack and pit gold jewelry in time; caution your customers about this.