Gold - Is 9ct Harder than 18ct

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The answer is simply no.

Metal hardness is measured by what is called the Vickers scale where harder metals receive a higher Vickers score than less hard metals.  9ct gold has a Vickers hardness ranging from 80 - 120, according to the composition and condition.  18ct gold has a Vickers hardness ranging from 135 - 165 (both in the annealed condition).  This shows 18ct gold as harder. 

So why is it often assumed that 9ct is harder than 18ct?

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 hardnesses of 9ct and 18ct 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.  18ct, which is 18 parts out of 24 pure, is equivalent to 75% gold, the remaining 25% is composed of the same type of alloying additions to 9ct).  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 9ct and also for 18ct.

What else affects the hardness of gold?

The process in which the item of jewellery is made wil also affect 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 (it can however be annealed to soften it).  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 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 followed by the stress relieving and others.

What are the differences between the two?

18 ct alloys are almost completely resistant to chemical attack in normal use whereas 9ct alloys are much less resistant.  9ct alloys, for example, will go dull or even black from exposure to 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 however be removed by polishing).

 

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