Gold purity, properties and the confusing issue of terminology.
I am often asked by customers questions about the precious metal they consider for their rings, questions showing the lack of proper understanding what is what in this field. This is my reason to write this article. Fine jewellery is an expensive purchase and I believe people should know the difference, options and what they are paying their money for.
In South Africa we have adopted the British purity system. The gold we are using is 9 and 18 karat purity in white, rose and yellow colour in order of popularity.
In order to achieve certain properties required for precision metal smiting the precious metals are alloyed (mixed in molten state) with other metals. These properties are colour, malleability, castability and hardness.
Gold is one of the two pure metals in nature which has a colour, it is bright deep yellow. The other one is copper. The rest of the pure metals, precious or not, are white or grey with different hues but not a definitive colour. The question that comes naturally is what is then the white gold? To answer this question I have to go to the alloys used in fine jewellery manufacturing. First I want to clarify the confusion about the word karat used for purity of gold as well as for weight of diamonds and precious stones. In short, there is no connection between the two. In diamond sand precious stone dealing carat is just a weight measurement equal to 0.2gr. In gold alloys karat is a measurement of purity. For distinction, the first is marked with ct. and the second with kt. Or k. Unfortunately, this is not always strictly followed.
Pure gold is marked with 24kt or 999.9. The number always indicates the parts of precious metal, in 1000 parts of the alloy. Fine gold with purity of 999.9 or higher has limited use in jewellery manufacturing. The highest purity widely used in jewellery in South Africa is hallmarked 18kt or 750. This as you probably guess means that 18k gold has 750 parts of fine gold per 1000 parts of alloy or 75% fine gold in the alloy. The remaining 25% in this alloy will determine the mechanical properties of this alloy as well as its colour. As long as there are 75% of fine gold in the alloy the item can be hallmarked 18kt. If I want to make 18k yellow gold I will put 75% fine gold 16.6% of silver and 8.4% copper, but that is the alloy I prefer. Other manufacturers can use different ratio and add small amount of other metals to meet their requirements, like to increase the amount of copper for rose gold or that of silver to get greenish hue. The constant is the amount of fine gold that must not be less than 75%. So, any white gold alloy hallmarked as 18k consist of 75% of fine gold and 25% whitening metals, usually silver, palladium and small amount of other metals depending of the purpose of the alloy (handwork, casting, chain etc). 9kt gold alloy has exactly half of the amount of fine gold compared to the 18kt alloy – 37.5%. It is hallmarked 9k, 9kt or 375. Because the amount of fine gold is so low, it is not possible to achieve the deep yellow colour associated with 18kt yellow gold. In white alloys however it is an advantage since the 62.5% of whitening metals we can add allow for whiter colour. Both the 18kt and 9kt white gold alloys have slight yellowish hue that is remedied by rhodium plating. Rhodium is a very white and hard metal from the platinum metal group which is deposited in a few microns thick film after the item is completely finished.
Which gold alloy is better? There is a balance between price and the value you buy for it. Obviously the 18kt alloy is far more expensive having double the amount of gold in the alloy. Thus, the intrinsic value of the metal is in the piece. Is it worth the huge difference in price? For yellow gold I would say definite yes. The colour of yellow gold looks like gold only in high purity and it is impossible to emulate. For rose I just have to note that although seldom some people’s perspiration my cause the low purity alloys to darken. I these rare cases 18kt is the solution. For white, at the current price of the platinum I will strongly recommend it over both 9 and 18kt white gold alloys. While 9kt is still cheaper, for a light item like a solitaire engagement ring the difference will be not substantial but the ring will last much longer with very little need for maintenance. 18kt white gold is still a choice for more conservative customers, who want to have a gold ring. I have to say that compared to 9kt, the 18kt gold alloy is more abrasion resistant and last longer but definitely not as long as a platinum ring. Here is a place for another note. Some intricate filigree designs are not possible to cast in platinum. While hand work of these designs in platinum is possible, the price will be much higher and if the budget is short 9kt gold CAD and cast is the option.
