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|Hardness of Different Gold Alloys|
Q: Is it correct that 9 carat gold is harder wearing than 18 carat gold?
The simple answer is no!
Most people are surprised to hear this, because for some reason, they have always believed, or been told, that 9 carat gold is harder, or harder wearing than 18 carat gold. We sometimes have trouble convincing them, sometime they will not believe us until they see it in writing. That is why we have put it in writing, and included it in our leaflets and sales literature for some years now.
Why then does everybody tell you that 9 carat is harder?
What we have wondered for many years, is how and why people get to believe that 9 carat is harder. I believe there are two reasons why this myth has become established:-
This brings is to the point that hardness and durability are not one and the same thing. To give a simple example, a glass ball is harder than a rubber ball. Try throwing each onto a hard surface. The glass ball will break, but the rubber ball will bounce at remain intact, because the rubber ball is more durable than the glass one. The glass ball breaks because it is brittle. In the same way, metal alloy can also be brittle, and 9 hard carat gold alloys tend to be slightly brittle, whereas 18 carat gold alloys tend to be more resilient.
18 carat alloys are almost completely resistant to chemical attack in normal use, whereas 9 carat alloys are much less resistant. 9 carat alloys for example will go dull or even black merely from exposure to chemicals in the atmosphere, they will also discolour in contact with perspiration, some fabrics, bleach and other household chemicals.
Metallurgy & Alloys
To understand more fully, a little metallurgy is necessary. Each and every pure metal will have a particular hardness which will not vary much. If it is worked by rolling, stretching, bending, hammering, or other mechanical process, it will tend to become harder but more brittle. It can usually be annealed or softened by heating it. Two or more metals mixed together form an alloy. Alloys differ form pure metals, in that they often combine some of the mechanical properties possessed by their constituent metals, but often also in other less predictable ways. Many alloys for example can be hardened or softened by appropriate heat treatment. Heat treatment include heating to a variety of high or 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. 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.
Harness Table for Carat Gold Alloys
Although, as we have explained, there are many different alloy "recipes", to give you an indication of the hardness figures for 9 and 18 carat golds, the following table gives a range of typical "Vickers" hardness values for fairly common "recipes":-
|Alloy||Hardness as Cast||Maximum Annealed Hardness|
|9||70 to 105||160 to 170|
|14||125 to 165||150 to 180|
|18||85 to 125||170 to 230|
|22||70||60 to 90|
Key to Table
Silver = Typical sterling silver (925/1000)
SC = Soft casting alloy
HC = Hard casting alloy
What About 14 Carat?
Some observant folk may notice that 14 carat golds are quite hard as cast, and this is fairly accurate. In fact this can make 14 carat slightly difficult to work with in some manufacturing processes, especially for jewellers working mainly with 9 carat or 18 carat alloys. However, you will also notice that the 18 carat alloys tend to be hardenable to a higher figure than 14 carat, and providing that both standards had been correctly worked and conditioned, then the 18 carat alloys would almost always be both harder and harder wearing.
To finish with a very simple personal opinion. If there had to be only one single gold alloy purity standard, then I believe that 18 has such excellent all round properties that it would deserve to be the winner.
You may wish to visit some of our other pages:-
Allergies to Gold Jewellery
Gold Alloys by Weight & Volume
Moh's Hardness Scale
What is White Gold?
Density of Gold & Other Metals
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