Which is Harder? 18 or 9 carat Gold?
Which is harder - 9 or 18 carat?
It is assumed that 9 carat gold is harder but this is not true. Metal
hardness is measured by what is called the Vickers scale where harder
metals receive a higher Vickers score than less hard metals. 9 carat
gold has a Vickers hardness ranging from 80 - 120, according to the
composition and condition. 18 carat gold has a Vickers hardness ranging
from 135 - 165 (both in the annealed condition i.e. to be heated and
then cooled). This shows 18 carat as harder.
So why is it often assumed that 9 carat is harder than 18 carat?
Most people are aware that gold in its purest form is too soft to be
used successfully in jewellery. It is also common knowledge that when
gold is alloyed with other metals it becomes harder. Many people assume
that the more "other" metals added the stronger and harder the resultant
alloy. As you can see from the compared Vickers hardness of 9 carat and
18 carat, this is not the case. There are many different combinations
of metals that can be used to make gold alloys and there are many
different compositions for both 9 carat and 18 carat (a carat simply
means 1/24th i.e. one part in 24. 9 carat must be at least 9 parts gold
out of 24 which is equivalent to 37.5% The other 62.5% is made up of
alloying additions such as copper, silver, palladium, and zinc. 18 carat
which is 18 parts out of 24 pure, is equivalent to 75% gold, the
remaining 25% is composed of the same type of allowing additions to 9
carat). You would be quite correct to assume that each different
composition has its own very different characteristics including
hardness. It is quite possible to make hard or soft compositions for 9
carat and also for 18 carat.
What else affects the hardness of gold?
The process in which an item of jewellery is made also affects its
hardness. If any metal is worked by rolling, stretching, bending,
hammering or other mechanical process it will tend to become harder and
more brittle. Many alloys can also be hardened or softened by
appropriate heat treatment. Heat treatment includes heating to a variety
of high temperatures followed by low temperatures for long or short
periods of time, followed by cooling at different rates. Each alloy will
have different hardness figures depending on its state and composition.
Common states to be considered include: as cast, after casting and
annealing, after age hardening (heat treatment), after cold working,
after cold working followed by stress relieving and others.
Corrosion Resistance
18 carat alloys are almost completely resistant to chemical attack in
normal use whereas 9 carat alloys for example are much less resistant. 9
carat alloys for example go dull or even black merely from exposure to
certain chemicals in the atmosphere. they will also discolour in contact
with perspiration, chlorine in swimming pools, medication taken by the
wearer, bleach and other household chemicals (tarnish can always be
removed by polishing).
Price
Because 18 carat contains twice as much gold as 9 carat it will be
considerably more expensive. The metal alloys used with gold are far
less expensive. Also there can be considerable variations in the quality
and price of alloys e.g. the use of palladium in white gold increase
the price but improve corrosion resistance.