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Since the dawn of time and its discovery, Gold has been treasured for its wonderfully rich color, its weight and relative rarity. In its original form, it is a beautiful warm yellow color that has led to its association with the sun, the Sun God, and with warmth and well-being.
CARAT...KARAT
The word has 2 meanings: Referred to diamonds and gems, 1 carat is a weight equal to 1/5 of a gram.
As a measure of gold purity, 1 karat is 1/24 part; Pure gold is 24kt, 18kt is 18/24 or 75% gold.
Karat golds are alloys of gold with other metals. There are different standards throughout the world. 18kt is commonly used in Europe. The other commonly found ones are:, 10, 14, 18, 22kt. Gold is very soft unless alloyed. The alloys are often used to vary the properties and color of gold.
PROPERTIES & CHARACTERISTICS:
Gold (Au) has a specific gravity of 19.32 (silver 10.5, platinum 21.45), Atomic weight 197.2.
COLOR
Most pure metals are gray or silvery in color except for gold and copper. When carat gold contains large amounts of copper, compared to other alloying metals (like silver & zinc), a red or rose gold is produced. Many different rose golds have been alloyed and it is difficult to match an old rose gold with modern alloys.
18 kt red gold is a subtle warm rosy color. Yellow gold usually contains gold, silver, zinc and other metals to enhance hardness. Alloyed with silver (Electrum), it is very soft and is greenish-gold in colour. 22kt gold is a very rich yellow color and is soft.
White gold is usually from gold, silver and nickel (see table) and sometimes palladium.
DISCOLORATION
Pure gold is a rich yellow color and does not tarnish. There is some tendency (not often though) for carat golds to discolor. Sulfides in the atmosphere, sweat from skin, perfumes, household chemicals can all contribute to this, although only the surface is affected and can be polished away.
White Gold is often more susceptible and can be yellowed by bleach. Antibiotics can alter the properties of sweat and promote discoloration.
COMPOSITIONS OF METAL FOR THE COMMON NAMES OF GOLD ALLOYS.
ALLOY NAME COMPOSITION
BLUE 18K 75% GOLD 25% IRON
YELLOW 14K 58% GOLD 4-28% SILVER 14-28% COPPER
YELLOW 18K 75% GOLD 10-20% SILVER 5-15% COPPER
YELLOW 22K 92% GOLD 4.2% SILVER 4.2% COPPER
GREEN 18K 75% GOLD 11-15% SILVER 13-0% CADMIUM
RED 18K 75% GOLD 25% COPPER
STERLING SILVER 92.5% SILVER 7.5% COPPER
WHITE 14Kt A 58.3% GOLD 17% COPPER 17% COPPER 7.6% ZINC
WHITE 14Kt B 59% GOLD 25.5% COPPER 12.3% NICKEL 3.2% ZINC
WHITE - 18K 75% GOLD 18.5% SILVER 1% COPPER 5.5% ZINC
(FROM: Engineering Alloys, Fifth Edition, Edition by Woldman & Gibbons)
MELTING POINTS
Pure metals have very precise melting points - silver 961C, gold 1063C, platinum 1769C. Alloys however, being mixtures melt over a range of temperature. 18kt yellow gold melts typically at 895-930C.
ANNEALING
Metals have a crystalline structure. When a metal is strained and distorted mechanically, the crystals become stressed and the metal hardens. To relieve this stress, and "relax" the structure, they must be heated to a temperature somewhat below the melting point. This is called annealing. Gold alloys must be cooled slowly after annealing.
HARDNESS
Pure gold is very soft (Mohs hardness 2.5-3). It is commonly wrongly believed that the purer the the alloy of gold, the softer it is. In fact, there are several different types of hardness.
Ductility refers to the flow characteristics of a metal under pressure, undergoing plastic deformation in stretching, bending etc.
MALLEABILITY
Describes how the metal deforms under impact from a hammer or punch.
After annealing, gold is generally quite ductile (soft). Fine golds are softer than carat golds.
Deforming metals by pressure or impact causes them to harden, at differing rates dependent on the alloy. As it hardens, it becomes less deformable, and more brittle.
Most alloys containing copper will age-harden. This may be apparent over months to years.
WEAR
The wear resistance is related to its surface hardness and age as well as to the actual metal. For example, the annealed hardness of 10 & 18 kt golds are similar. 18, 14, 10 kt golds are considerably harder than 22 or 24 kt golds. Annealing softens them, and working (bending, twisting) tends to work harden metals (this explains why forged steel is much stronger than cast steel for the same alloy content).
ALLERGIC REACTIONS
These are most commonly caused by allergies to nickel; if so, w/gold which contains nickel, should be avoided. In addition, tight fitting, poorly ventilated bracelets worn in humid climates are invitations to fungal infections.
SOME OTHER TRIVIA:
121,000 tons of gold have been mined so far (only 15% of this is missing). 43% of known gold lies in Central Banks. Over 80% of gold produced in a year goes to jewelery and (less) watches.
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