Precious Metal Properties
Precious Metal Properties
Gold is one of the most precious metals in the world. It is present in the earth's crust and trace amounts are present in plants and animals. It is, however, difficult and expensive to extract. In modern mining operations approximately 3 tons of ore are needed to extract one ounce of gold. The many desirable qualities found in gold, along with its scarcity, have made it the most popular metal for use in jewelry today.
Properties of Gold
Gold in its pure state:
- Has a melting point of 1945 degrees Fahrenheit (1063 degrees Celsius). When alloyed (chemically combined) with other base metals the melting temperature of the resulting alloy is changed. 18K yellow gold has a melting point of 1675 degrees Fahrenheit and 14K yellow gold has a melting point of about 1550 degrees Fahrenheit.
- Has a specific gravity of 19.33. It is relatively heavy compared to most metals, such as silver (SG 10.7) or iron (SG 7.8). A notable exception is platinum (SG 21.4).
- Is more malleable than any other metal and can be hammered into foil so thin that it is almost transparent.
- Has a unique ductility property allowing it to be drawn into wire so fine it can barely be seen.
- Is deep yellow in color. Its great reflective properties help keep its brightness and color from fading with time.
- Will not rust, tarnish or corrode. Gold jewelry recovered from ancient Egyptian tombs is in the same state as when placed over 4000 years ago.
- Is softer than most other metals. On the Mohs scale of hardness (which is a measure of a gemstone or mineral's resistance to scratching), gold has a hardness value of 2 to 2.5. Diamonds have a value of 10. Pure gold may easily be scratched. Fortunately, gold becomes harder when alloyed with other base metals.
- Is relatively scarce and therefore expensive. It is estimated that only 125,000 tons of gold have been mined the world over since the beginning of time.
- Is able to bond with other base metals. This property gives rise to the many different colors available in modern gold alloys.
Fineness (Karat value)
Since ancient times the purity of gold has been defined by the term karat, which is 1/24 part of pure gold by weight. Pure gold is equivalent to 24K. Gold purity may also be described by its fineness, which is the amount of pure gold in parts per 1000. For example, a gold ring containing 583 fine god has 583 parts (58.3%) gold and 417 parts (41.7%) of other base metals.
Federal Trade Commission rules require that all jewelry items sold in the United States as gold shall be described by "a correct designation of the karat fineness of the alloy." No jewelry item less than 10K may be sold in the United States as gold jewelry.
Fineness of Gold Karats
|United States Markings||Parts Gold||Gold%||European Markings|
|24K||22/24||100%||1000 or 999|
|22K||22/24||91.7%||916 or 917|
|14K||14/24||58.3%||583 or 585|
Weighing Precious Metals
The weight of a piece of gold jewelry is a factor that helps to determine its value. It is important because it is an indication of the amount of fine gold in an item of jewelry. Grams (g) and pennyweights (dwt) are the units of weight most commonly used in weighing gold. Gold and silver are almost always weighted in the troy system of weights where one pound troy equals twelve troy ounces and twenty pennyweights equals one troy ounce. The Avoirdupois weight system, where one pound equals 16 ounces, is used in the United States for most everything except precious metals. The following table summarizes useful weight conversions.
Weight Conversion Table
1 gram (g) = 0.643 dwt = 0.0032 oz t = 0.035 oz av
1 pennyweight (dwt) = 1.555 g = 0.05 oz t = 0.055 oz av
1 troy ounce (oz t) - 31.103 g = 20 dwt = 1.097 oz av
1 ounce avoirdupois (oz av) = 28.3495 g = 18.229 dwt = 0.911 oz t
Gold, Silver, Platinum Group
All precious metal weights are given in troy ounce (oz t); there are 12 troy ounces per pund (lb. t) in contrast to 16 "regular" (avoirdupois) ounces (oz) per pound (lb.).
Purity or "fineness" denoted by karatage (K); 24 karat gold is pure gold.
12 karat gold is 50% gold and 50% alloyed metal.
Gold will alloy with nearly all base metals (Cu, Pb, Ni, etc.); different colors of gold are produced by alloying with different base metals. Common alloys are Ni and Cu.
Gold purity can be tested with inexpensive devices that measure electrical conduction, or by the older method of guaging the extent of reaction with dilute acids. Pure gold is inert with respect to dilute HCI and HNO3; most alloys will react slowly in these acids. Gold plating or filled can be detected by testing a scratch in the item.
F. T. C. Guidelines governing sales of gold and gold jewelry in the United States are the following:
1. If karat-age is to be stamped on, it must be at least equal to that designated or higher, and also must be accompanied by a trademark.
2. Terms "pure gold" and "solid gold" can only be used for 24 karat gold.
3. Term "gold" can only be used if gold content is 10K or better.
4. Term "gold filled" applies only to items composed of a layer of gold pressed (not alloyed) onto a base metal where the weight of the gold compromises at least 1/20th of the total weight of the item. Fineness must be shown by stamp, e.g. "1/20 12K G.F." means the gold layer is 12 karat gold and compromises 1/20th of the total weight of the item.
5. Term "rolled gold plate": is the same, but gold compromises less than 1/20 of total weight; e.g. stamped "1/30 12 k G.P." means 1/30 of the metal weight is from 12 karat gold plate.
6. "Gold electroplate" or "gold flash" or "gold washed" mean a gold coating that is 10K or better gold at least 0.000007 inches thick. A variant, "heavy gold electroplate" (H.G.E.P) is at least 0.0001 inches thick.
Pure silver has a specific gravity of 10.5
"Fine Silver" is 99.9% Ag
"Britannia Silver" is 95.84% Ag
"Mexican Silver" is 95% Ag + 5% Cu
"Sterling Silver" is at least 92.5% Ag; harder than pur Ag.
U.S. "Coin Silver" is 90% Ag
Vermeil (Vur-MAY) is sterling silver with gold plating
Crudefineness testing is done with dilute nitric acid. Silver + dilute HNO3 yields a creamy white to brown liquid. Alloys containing Cu, Ni and brass effervesce and yield a green, yellowish green of bluish green color. More precise technique involves gauging reaction and resulting color with Schwerter solution (HNO3 + KCr2 + H2O).
Platinum Group Metals (Pt, Ir, Pd, Ru, Rh, Os)
Group of precious metals (platinum, iridium, palladium, ruthenium, rhodium and osmium) that share a similar chemical and metallurgical properties.
Most "platinum" used in jewelry is composed of 90% platinum and 10% iridium, which makes it much harder than pure platinum. 5% ruthenium serves same purpose.
Much like gold, platinum purity and alloys are commonly stamped, in abbreviated fashion, on items. Unlike gold, in the U.S., purity is given relative to 1000 parts (per mil), e.g. "950Plat" is 95% pure platinum.
Palladium and small amount of ruthenium is used in setting heads for platinum jewelry; lighter, less expensive.
Rhodium is used as plating metal. It gives white gold and platinum a hard, bright finish and slows tarnish on stainless steel.
Platinum has a specific gravity of 21.45.
The Strengths of Platinum
Beneath its subtle beauty and understated elegance lie the properties which make platinum truly unique.
Platinum is Strong
It is the heaviest of the precious metals, weighing almost twice as much as karat gold. Its strength ideally secures diamonds and other precious gems.
Even after many years, platinum will not wear away or wear down. For example, after many years of wear, a gold wedding bands shank will wear down and become thinner. This is not the case with platinum.
As with all precious metals (gold, silver, etc.), platinum can be scratched. However, with platinum there is actually no material lost from the scratch as there is with gold. If your platinum jewelry becomes scratched, simply take it to your jeweler for a quick polish.
Platinum is Pure
In America, platinum jewelry contains 90% or 95% pure platinum. By comparison, 18 karat gold is 75% pure and 14 karat is 58% pure gold. Platinum will never tarnish or lose its rich white luster.
Platinum is Rare
Ten tons of ore must be mined to produce a single ounce of platinum. It takes five months to process platinum order into pure platinum. Only after this time can skilled hands work their creativity and craftsmanship, transforming platinum into pieces of wearable art.
Source: Indygem 10/17/2007