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Properties
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.
Uses
Gold is highly 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 & enable it to resist wear & tear.
Colors
Yellow gold is the most popular for jewelry and is
comparatively easy to work.
White gold is usually 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
Karat
The amount of alloy (base) 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
Alloying
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.
Test for Gold
Equipment required:
-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).
Procedure:
File a deep notch into the piece to be tested. Apply a drop
of nitric acid. A bright green reaction 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 determine 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.
In place of testing needles you can also use pieces of gold
of known karat.
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