More methods of testing gold (and other precious metals)
Various methods of testing gold including advanced method of touch acid testing at the Birmingham Assay Office, U.K.
I visited the Birmingham Assay office for their special training course for touch acid testing.
They started by explaining that to test the fineness (carat) of precious metals for the purpose of hallmarking, the Birmingham Assay Office uses XRF Testing and the referee cupellation method (both explained below). Both of these methods are very accurate. They explained that before the introduction of XRF testing, they used Touch Acid testing to give an approximate indication of fineness of a precious metal and the homogeneity of a batch.
They then introduced the 'industry standard' tester: the Troytest 4 (which is manufactured by us, Quicktest).
After half an hour of theory (going through the instructions), we had a long practical session, testing dozens of samples. This was interesting. When we manufacture the testing sets here at Quicktest, we check the acids against a dozen known samples. On the course, I had the opportunity to test the acids against 45 known samples.
The people running the course spend their working lives testing gold (and other precious metals); the Birmingham Assay Office hallmarks between five and six million items per year, most of which are tested. And so I was curious to know if they used any techniques I had not come across. They did. And so here are some 'advanced' instructions, and also a list of other testing methods used by the Birmingham Assay Office for testing precious metals.
ASSAY OFFICE METHOD: they remove the surface with a 'scraper' - a three-edged steel tool (very sharp!). Because they are scraping a relatively large area (unlike using a fine needle file) there is no resulting 'notch', it's much more 'gentle', in skilled hands you can hardly see that the item has been 'scratched'. The downside is that it takes practice to use, it's not as easy a needle file. However, I'm impressed and I now stock scrapers, click here to see them.
PLATED ITEMS: Gold plated items can be 'flash' plated (very thin) or 'hard' plated (very durable); gold can be rolled (rolled gold) by rolling two very thin sheets of gold around a central sheet of copper; white gold can be plated with rhodium to improve the colour (or to speed up the polishing process at the manufactures) - in all cases you must file through the surface to test the 'true' metal underneath. While this is all in my book, The Gold & Silver Buyer's Handbook, I came away with a curious cluster of extra facts.
Filing through very heavy gold plating with a steel file is hard work compared with using a diamond-impregnated file; XRF testers (below) cannot test through most plating (unless the plating is very thin) but they can be programmed to 'ignore' rhodium plating; large silver items covered in a very thick layer of silver are made by 'electroforming' - a wax shape is coated with silver nitrate then heavily plated with silver. In all of these cases you must file through the surface so that you are testing the metal underneath.
ICP (inductively coupled plasma mass spectrometry): a tiny sample is removed, weighed and dissolved into the appropriate solution. The solution is then sprayed through an ultra hot flame where until it is destroyed in a burst of plasma and the resulting spectrum is analysed. This is the most accurate method of analysing precious metals to discover their exact chemical composition.
As you see from the pictures, these are not 'testing machines', these are laboratories and cost tens of thousands of pounds.
FIRE ASSAY: this is the referee method of assaying gold alloys. A small gold sample is weighed, silver added, and then wrapped in a lead envelope. The sample is melted at 1200C in a cupel. This drives off the base metals and leaves a ball of fine silver and gold. The ball is flattened and put into acid to dissolve the silver away, leaving a "cornet" of pure (24ct) gold. Then it's weighed again and the carat (percentage of pure gold) is calculated. Each test takes two and a half hours.
TOUCHSTONE TESTING. ASSAY OFFICE METHOD: a sample of gold is selected from several sets of sample 'needles' (each set has about ten samples of different carats), the tester (the person doing the testing) must guess the carat of the unknown item (e.g. 9ct) then go to their bundle of 9ct 'needles' and select one that matches the colour of the unknown item. The known sample is rubbed across a stone (touchstone) which leaves a gold streak, and just below this 'known' streak the unknown sample is streaked - so there are now two streaks of gold, parallel to each other. The tester (the person doing the testing) then selects an acid from a set of 9 different strengths (mixed in the laboratory at the Assay Office), puts the acid across both streaks and observes which steak dissolves first. The process is repeated by selecting a colour-match from each set of samples and trying different strengths of acid until both streaks dissolve at the same rate - the 'unknown' streak will be of exactly the same carat as the known streak. The girl in the photo is the apprentice, she has been doing the job for six months and is making good progress.
SIMPLE METHOD: we stock one set of six gold samples ('needles') and a touchstone. The method is the same as above but you will have a choice of 6 gold samples instead of about 100, and two acid mixtures instead of 9, and you won't be supervised for six months while you learn. The testers we manufacture (including our TROYTEST, recommended by the Assay Office for retail use) are designed to be used without touchstone method...though you are welcome to buy the test-needles and touchstone if you wish.
Click here to buy the test-needles (samples of gold of difference carats)
Click here to buy the test-needles (samples of gold of difference carats)
XRF (x-ray fluorescence), sometimes know as "the x-ray machines".
XRF (cabinet). This is an example of a 'cabinet style' XRF tester but not one used by the Birmingham Assay Office. The Birmingham Assay Office's XRF instruments cost £50,000.00 each [that was a few years ago, today you can get small versions for as little as £15,000.00]. As with ICP (see above) it gives a detailed analysis of the chemical composition but, unlike ICP, it is totally non-contact. However, it will not usually test through plating (the best clue to plated items is inconsistent or unexpected readings) so you still have to file the item first.
XRF (handheld). Scrap dealers and antiques traders tend to use these 'cheap' XRF machines (handheld 'scanner' type), which you can now get find for as little as £10,000.00 (these are not used by the Birmingham Assay Office).
Experienced users acknowledge that, even with these, care must be taken to ensure you are not testing surface plating (or rolled gold) or you will get incorrect readings.
OTHER TESTING. It is not only the testing of precious metals that takes place at the Birmingham Assay Office, they test for nickel (high levels on nickel can trigger allergies, especially jewellery used for piercings); they test for lead (not found in precious metals but it used to be common in paints and in children's toys); they test for cadmium (in production, molten cadmium is very toxic); they mainly test costume jewellery, toys and fabrics, further information.