THE BIRMINGHAM ASSAY OFFICE, U.K.
or see the latest AGT3 GemOro AuRACLE electronic gold tester
There is also a very good book on the subject
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 - they are THE foremost experts in testing. 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 that the Birmingham Assay Office use.
METHOD: I give away a steel needle file with each set, you can
file thoroughly over a very small area, which causes the minimum of damage.
Also to be recommended is a diamond-impregnated file, it's harder, cuts
deeper, and is more durable.
PLATED ITEMS: Gold plated items can be 'flash' plated (very thin) or 'hard' plated (very durable); gold can be rolled into very thin sheets 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 and test the metal underneath; large silver items covered in a very thick layer of silver are made by 'electroforming' - the wax shape is coated with silver nitrate then heavily plated with silver.
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 platinum group metal alloys to discover
their exact chemical composition. Fire Assay Testing:
This is still 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 a several sets of sample 'needles' (each set has about ten samples of different carats), the user guesses the carat then selects a sample that matches the colour of the unknown item. The known sample is rubbed across a stone (touchstone) which leaves a gold streak. The user then streaks the unknown sample across the stone, 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 carat of the unknown item is the same as the known sample. 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 (if you use a TROYTEST or QUICKTEST tester) have a choice of 6 gold samples instead of about 100, two acid mixtures instead of 9, and you won't be supervised while you learn for six months. The TROYTEST recommended by the Assay Office for retail use (and also QUICKTEST testers) are designed to be used without touchstone method...though you are welcome to buy the needles and touchstone if you wish: click on the pictures below.
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 about £50,000.00 each. As with ICP (see above) it gives a detailed analysis of the chemical composition but, unlike ICP, it is totally non-contact / non-destructive. However, it will not test through plating (the best clue to plated items is inconsistent or unexpected readings).
(handheld). Scrap dealers and antiques traders tend to use the
'cheap' XRF machines (handheld 'scanner' type), which you can now get
for as little as £12,000.00. These are not used by the Birmingham
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), and the main items they test are costume jewellery, toys and fabrics, further information.
QUICKTEST, Watford, WD18 8PH, Tel. 01923 220206, email info(at)quicktest.co.uk