Gold is yellow in color when pure. It is the most malleable and ductile of all metals. It can be hammered into sheets less than 1/300,000 of an inch thick, and one ounce of gold can be drawn into a 35 miles long wire.
Gold is chemically inactive, that is to say not affected by oxygen, sulphur, or acids.
It can be dissolved by aqua regia, which is a mixture of 1 part nitric and 3 parts hydrochloric acid.
Gold is greatly used for jewelry, coinage, decorative, scientific, and dental purposes.
Pure gold is very soft.
It is alloyed with other metals to give it different colour appearance & make possible for it to resist wear & tear.
Yellow gold is the most accepted for jewelry and is comparatively easy to work.
White gold is typically used with diamonds, since its white color harmonizes with the stone. Comparatively white gold is difficult to work because of its hardness and brittle nature. Red gold is often used with yellow for contrast effects.
Green gold is used for antique jewelry.
White...gold, nickel, copper, and zinc
yellow...gold, silver, copper, and zinc
green...more silver-less copper than yellow gold
red...more copper-less silver than yellow gold
The sum of alloy metal used with the gold depends upon the karat desired.
The term karat denotes a measure of purity; 24 karat is pure gold.
14 karat gold is by weight 14/24 fine gold and the balance (10/24) is alloy metal, for instance.
Purity and melting points of gold
24K fine gold...1.000.... 1945 F
18K white ... .750.... 1730 F
18K yellow ... .750.... 1700 F
14K white ... .5833 ... 1825 F
14K yellow ... .5833 ... 1615 F
10K yellow ... .4167 ... 1665 F
Many jewelers prepare their own gold alloy by melting the proper weight of gold and base metal in a crucible; a small amount of boric acid powder is added to check oxidation of the base metal. The molten gold is mixed with a steel or carbon rod to obtain a uniform batch.
For handmade objects or stamping, the gold can be purchased already alloyed in the desired karat, color, shape and guage.
Analysis for Gold
-a black stone slab
-testing needles of different karats and colors
-a bottle of nitric acid
-and a bottle of aqua regia (1 part nitric and 3 parts hydrochloric acid).
File a deep cut into the piece to be tested. Apply a drop of nitric acid. A bright green result indicates gold plate on copper or brass, a pinkish cream color indicates gold plate on silver. Ten karat gold will show a slight reaction-over ten-karat, there is little or no reaction.
To verify the karat of a gold object, rub it several times on the testing stone to make a distinct mark about 1/4 inch wide. Be-side it make a mark equally heavy with a test needle nearest in karat and color to what you suppose the gold to be. Apply acid to both marks at the same time. Use nitric acid for 12 karat or less and aqua regia for higher karats. If the needle marking reacts (disappears) sooner than the gold, wipe the stone dry and retest with a higher karat needle-if slower try a lower karat needle. When you have matched the reactions you have determined the karat.
Green gold reacts more slowly than yellow gold because it contains more silver. White gold reacts slowly because of the nickel or palladium in the alloy.
Instead of testing needles you can also use pieces of gold of known karat.