18 ct gold vs 9 ct gold

Is 9 ct gold harder than 18ct gold? This is a question we get asked again and again at Metal Urges Fine Jewellery. The simple and correct answer is NO! Many of our new clients are surprised to hear this answer. The commonly held opinion of many (including lots of people in the jewellery trade) is that 9 carat gold is harder, and therefore wears better than 18 carat gold. At Metal Urges we believe in the saying "Knowledge Is Power" and take satisfaction in empowering our clients with correct information and knowledge! So here goes....

The reason people believe 9ct gold to be harder than 18ct gold is simple...they have been told this by someone they trust. Some of these people just think they know the correct answers and other actually do but seek to mislead for there own financial reasons.

Sales staff and even jewellers in many stores do not always have the correct information, and because 9ct gold is more commonly offered for sale than 18ct, they level their sales pitch to sell the 9ct gold they have in stock. When you have a shop full of 9ct gold it must be very tempting to push its benefits...however incorrect these may be.

Gold facts!

Pure gold is a beautiful rich yellow metal worshipped and highly prized by many societies throughout human history. It is well known and accurate to say that pure gold is a soft metal not suitable for jewellery manufacture. Some cultures do prefer pure gold, or near pure gold (22ct) jewellery and pay for it either by low durability, or financially by making pieces extremely thick and heavy in an attempt to add strength.

Pure gold is combined or 'alloyed' with other metals to add strength, durability and hardness. The amount and type of other materials (alloys) added determines the carat (gold content) of the finished gold alloy. There are many different recipes for gold alloys, for our purposes we will just describe the more common western jewellery alloys.

24ct (twenty four carat) gold is pure gold, so all 24 parts are pure gold.

18ct gold is 18 parts pure gold or 75% pure, hence the stamp 750 found inside pieces made from this alloy. This is the main metal used at Metal Urges, it is strong, beautiful, durable and does not tarnish, corrode or react with its owners skin. Engagement rings and wedding jewellery have to be all these things!

9ct gold is 9 parts pure gold or 37.5% pure, hence the stamp 375 found inside pieces made of this alloy. Due to this metals lower hardness and tendency to tarnish, corrode and react with its wearers skin, Metal Urges choose not to use it. No one wants their diamond engagement ring to turn black on their finger!

The obvious question here is... why would an alloy like the 375 above even be labeled as gold. Would it not be more accurate to call it by the name of its other much larger % components? It is after all only just a little over third pure gold!

Ok so now we come to the confusing bit!

Pure gold is soft... CORRECT!

Therefore the more other metal (alloys) we add, the harder it becomes... WRONG!

This is a common and understandable mistake to make, both consumers and many jewellers within the trade have commonly made the same mistake over a long period of time. This mistake has become lazily accepted as the truth.
The truth is that the alloying of gold is an incredibly complicated process when it comes to generating fine alloys for jewellery production. If you ask at Metal Urges we are very happy to run through the process and show you our alloying area and recepies. Unlike most Australian Jewellery businesses, we regularly mix our own metals, allowing us much finer control of our end product. Each different alloy has its own characteristics including colour, workability and hardness. It is quite possible to make harder or softer alloys of 9ct and 18ct golds. This is useful, as we often find that different components within a piece require different hardnesses and workability. The ability to control our materials will see our pieces last, in order to become heirloom jewellery for future generations.

The other major factor affecting gold alloy hardness is how the material is worked after alloying. Some gold alloys can be hardened through the manufacturing process. This is called 'work hardening' and is normally the result of compression and or the alignment of the metal molecules within the material. As a general rule: materials that have been work hardened last longer and wear harder than those that have not. A work hardened alloy can be somewhat softened again through the process of annealing. This involves heating and cooling the particular alloy involved through a certain cycle. Different alloys require different treatment to achieve this. Just to make things really complicated: Maximum hardness doesn't nessasarily mean maximum durability! An in depth knowledge and understanding of each alloy is required. At Metal Urges we pride ourselves on our current depth of knowledge and our continuing quest for further understanding.

To give a simple example, a sheet of glass is harder than a sheet of Perspex (an advanced plastic). Try kicking each. The glass will break, but Perspex will remain intact, because the perspex is more durable than the glass. The glass breaks because it is brittle. In the same way, gold alloys can also be brittle, 9 carat gold alloys tend to be slightly brittle, and 18 carat gold alloys tend to be more resilient. This is a huge factor in the amount of wear and tear a piece of jewellery can withstand before becoming too worn to wear or breaking all together.

Due to the inert nature of pure gold, 18 carat alloys are almost completely resistant to corrosion in typical everyday use. In stark contrast, 9 carat alloys are much less resistant due to their low pure gold content, and will quickly turn dull or blacken when in contact with perspiration, some clothing, regular household chemicals (such as bleach), and even the atmosphere. Not all that appealing for an exclusive custom made engagement ring!

In conclusion, we at Metal Urges Fine Jewellery strongly believe that if there we were to be limited to only one gold alloy....we would love it to be 18ct gold! Its excellent all round properties make it indispensible for the production of fine handmade, designer, art jewellery!

And then there is the differences between platinum, paladium and 18ct white gold... but that's another day on the blog!