Thursday, 4 February 2016

Gold and Silver Metal Testing

25335_C20Jewellery is rarely made from pure gold or silver as both these metals are very soft. Normally gold chains for example are made from pure gold which has been alloyed with silver, copper and zinc.


To make jewellery that will wear well, gold and silver are mixed with other metals to improve the hardness and durability of the metals. Gold jewellery in Australia is most frequently either 9ct (375) or 18ct (750) and should be stamped as such. In the US, 10ct (417), 14ct (583) and 22ct (916) are popular.


A common misconception is that 18ct gold is much softer than 9ct because it contains twice as much pure gold. In hardness tests, 18ct gold will actually be very slightly harder and more durable than 9ct, the difference in hardness between the two metals is less than 1% but 18ct gold will be considerably more expensive.


Sterling silver is an alloy of 92.5% pure silver and 7.5% of another metal (nearly always copper). Silver jewellery from Mexico and many Asian countries will be less than 90% pure silver and may be considerably less. The percentages of one metal to another in an alloy is always by weight.


9ct yellow gold is 37.5% pure gold and 62.6% other metals, typically the majority is silver and copper with a small fraction of zinc. Varying the percentages of the silver and copper will cause a change in the actual colour of the gold alloy. More silver and less copper will give a brassier yellow hue and the opposite, more copper and less silver will give a pinkish hue to the alloy. Rose gold has a larger proportion of copper than silver.


Normally you can tell the carat of gold jewellery by the hallmark. This is a small stamping on the piece of jewellery of the carat weight, i.e. 9ct or the parts per thousand of gold i.e. 375. This mark can also be etched into the piece with a laser, instead of being stamped.


With the easy availability of carat stamps, some unscrupulous people are profiting from jewellery being incorrectly hallmarked. Cheap silver or brass chains can be gold plated and stamped with 375, and the buyer assumes they have purchased a 9ct solid gold chain necklace.


The only way to be absolutely sure of the actual carat weight of gold jewellery is by testing the metal chemically or with an electronic metal analyser.


Manufacturers are subject to random tests of their jewellery and undergo the most accurate testing. A random sample(s) is collected from their products and it is chemically analysed to around 1 part per thousand for gold content. This process is completely destructive and is only suitable for ensuring that manufacturers are adhering to the correct marking of carat weights of their products.


Gold testing kits are available quite cheaply and can be used to obtain a reasonably accurate measure of the gold content of a single piece of jewellery. They are however destructive in that a small section of the jewellery is removed by scratching the piece on a special stone used for testing. An area that is not normally visible should be used and the amount is very small.


The mark left on the stone is covered with a solution that will cause a chemical reaction and a colour change will be seen. Different chemicals are used depending on the actual carat of the gold being tested, so sometimes more than one scratch on the stone will be required. These chemicals are typically strong acids and require care during use. It may be best to have these tests performed by a jeweller who has the knowledge and experience to perform the tests safely and accurately for you.



In recent years electronic testers have come onto the market. Currently the price makes them not practical for anyone not using them regularly in a business. They are however very accurate and more importantly they are completely non-destructive. Handheld models with an LCD readout will indicate on the display the composition of the piece being tested and show the percentage of up to 14 different elements. More complex models can offer the detection up to 21 different elements.


X-ray Fluorescence (XRF) is the technique employed by these analysers. The device can be focused and an area as small as 1mm x 1mm and return a reading showing the percentage of each metal present in the sample without causing any destruction to the jewellery being tested.


If you are considering buying second hand jewellery you should arrange for a jeweller to test the item to make sure it is as expected. A seller who refuses or is reluctant to allow you do this should probably be avoided. False hallmarks have been widely reported on silver jewellery coming from China and frequently being sold on sites like and including eBay. If the price seems to good to be true, just move on, because it won’t be real.


Buying from a reputable store should guarantee you will not have issues with the jewellery not meeting the hallmarks applied. Buying within Australia also gives you protection under common law, consumer law and you have the backing of the consumer affairs and/or fair trading government departments.


 



Gold and Silver Metal Testing

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