THE METAL OF CHOICE – 9ct or 18ct
We are often asked by customers if 9 carat gold lasts longer than 18 carat gold?
It’s surprising for most people to learn that the answer is no – because for some reason, they have always believed, or been told, that 9 carat gold is harder, or harder wearing than 18 carat gold.
Why then does everybody tell you that 9 carat is harder?There are some general misconceptions formed because 9 carat jewellery is more commonly stocked in jewellery stores than 18 carat jewellery, making 9 carat gold more accessible to consumers than 18 carat gold.
Also, pure gold is quite soft, too soft in fact to be used successfully in jewellery. Although in some cultures, consumers prefer pure gold jewellery, but it is rather soft, so needs to be alloyed for greater strength. It is fairly well known that if gold is alloyed with other metals, it becomes harder. Where many people make a mistake is to assume that the more metals added, the stronger and harder the finished alloy becomes. This is an easy and understandable mistake to make, and consumers and many working within the jewellery trade have commonly made the same mistake over a long period of time.
When gold is worked by rolling, stretching, bending, hammering, or other mechanical processes, it will tend to become harder. So it can usually be annealed or softened by heating it. Heat treatments include heating the metal 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. A method used to determine these figures, known as the Vickers hardness test method, consists of indenting the test material with a diamond indenter then establishing the relationship between the load applied and the area of indentation.
The approximate maximum Vickers hardness value of 9 carat is 170, compared to 18 carat which has a hardness value of 230. Basically, the higher the number the harder the alloy is.
However, hardness and durability are not 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 and 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 alloys can also be brittle, and 9 carat gold alloys tend to be slightly brittle, whereas 18 carat gold alloys tend to be more resilient.
This can be further influenced by mechanical working and appropriate heat treatments; however, in general, most common 18 carat gold alloys are both harder and more hard wearing than their 9 carat equivalent.
