Twelve methods of testing gold, silver and platinum
HOW TO CHECK GOLD AND OTHER PRECIOUS METALS
CONTENTS
VISUAL TESTS
MAGNETIC TESTS
HARDNESS / MALLEABILITY
THE ACID TEST (including the Touchstone method)
DE-PLATING
XRF (X-ray Fluorescence)
SPECIFIC GRAVITY, SG (RELATIVE DENSITY)
RESISTIVITY (for coins and bars)
RESISTIVITY (metal detectors)
ULTRASOUND
MELT-AND-ASSAY (FIRE ASSAY)
ICP
RELATED ARTICLES
VISUAL TESTS
Before you start, LOOK at the item. And if you can’t see clearly (and nobody can see tiny hallmarks with the naked eye!) buy a good quality jewellers loupe, it will be the best ‘tester’ you ever own. All modern gold, silver and platinum items made in (or imported into) the U.K. must be hallmarked.
Hallmark charts illustrating hallmarks must (by law, in the UK) be displayed wherever precious metals are bought or sold. Familiarize yourself with these marks.
Alongside the hallmark is also a date letter you can look up in a hallmark book, there's one for British hallmarks and one for international hallmarks. This is useful for dating antique items.
There are dozens more 'general knowledge' hints and tips in The Gold & Silver Buyer's Handbook,
MAGNETIC TESTS
Gold and silver are not magnetic. If it's magnetic it can't be gold or silver. So using a magnet is a good way of removing non-gold/silver items before going to the trouble of testing. However, NOT being magnetic doesn't prove anything, copper (which is often plated with gold) is not magnetic, some steel is not magnetic. And some some platinum is magnetic. We have a large range of magnets.
Some metals (e.g. copper and silver) are diamagnetic. Slide a tiny magnet down a flat item (or a coin) and it will set up a "field" that will slow it down. We sell special sets of magnets for this purpose. There is also an article specifically about testing gold, silver and platinum with magnets.
HARDNESS / MALLEABILITY
Precious metals are soft, large thin gold items (especially if made of high-carat gold) bend easily in and out of shape. Also, before testing the item you will have to file the surface with a fine steel file (otherwise you will merely test the surface plating). This is a good test in itself, you will soon see how easy it is to file gold or silver whereas the steel file will 'bounce off' steel. A very fine steel or diamond-impregnated file (needle-file) is supplied with every testing set and electronic tester. It doesn't matter which shape of file you use but we suggest you buy a few because they wear out.
THE ACID TEST (including the Touchstone method)
Summary
When we speak, colloquially, of "The acid test is" we mean, the absolute proof. This saying comes from the acid test for gold. Even if you have an electronic tester for testing yellow gold, you will benefit from a set of acids, because they also test for silver, platinum, Palladium and steel. Some traders say, "I don't bother testing white metals". If you're a trader you should be making money from buying all precious metals!
For GOLD, acids are designed to test to the nearest carat commonly used in jewellery (in the UK, 9ct, 14ct 18ct, 22ct) but, with practice, results to within 5% can be achieved. SILVER: in addition to non-silver you can distinguish Sterling (.925), 'low-grade' (.800) and (just about) very low grade (.500) silver. OTHER: there is also an acid for platinum, Palladium and steel.
See also, our Guide to chemical testers (gold-testing acids)
Operation:
File the surface (a fine needle- file is included with each set). Do this firmly but only over a small area, and in a place it won't show (eg the inside of a ring shank, the back of a brooch, the clasp of a chain). This is to ensure that you are testing the item underneath any plating. Put a spot of acid on the filed area of the item. The acid will change colour, and that tells you whether the item is gold (and also the carat). There are also acids to test for silver, platinum, Palladium and steel. Our instructions are very clear, they even describe what the reactions look like to the colourblind.
An alternative to using a needle-file is to use a steel scraper and scrape (scratch*) the surface, see . This advanced method takes more skill than using a file, read about how this works.
* This is the origin of the expression, coming up to scratch.
Another method uses a touchstone instead of putting the acid directly onto the item. There is a simple (relatively!) method using a touchstone and (test-needles) supplied by Quicktest, or an advanced method as used at the Birmingham Assay office.
How it works
The acid dissolves the non-gold in the alloy but not the gold. So 9ct (an alloy containing of 9/24ths gold) reacts quickly; high carat (from 18ct) requires a stronger mixture of acids; white metals use acids with special additives.
Advantages
Cheap, accurate, also tests white metals. It can't 'break down', when a bottle of acid is use up, just buy another bottle, they are not expensive and a bottle usually lasts a couple of years.
Disadvantages
Good eyesight and a steady hand required, you must work slowly and carefully.
Acids burn. If using glass bottles of acid please protect your face against splashes by wearing goggles. Please note, 99% of our customers use the plastic bottles which are much safer, no goggles required. Always wear acid-proof gloves.
Availability
We manufacture these, small quantities (a handful of testing sets or a few dozen refill bottles) always available, larger quantities (dozens of sets or hundreds of bottles) take us a few days to produce.
The two most popular brands of tester are QUICKTEST and TROYTEST. They comprise small bottles of acid in a wooden box. All sets test for gold (all carats) and silver; advanced sets also test for platinum, palladium and steel. There is a short article listing what each set (and each bottle) tests, (there is also detailed information with each product description).
DE-PLATING
Summary
This is the most common type of 'low-cost' (£350.00 to £650.00) tester. Good for testing 9ct to 18ct yellow gold, with careful calibration the KEE tester can also test 22ct to 24ct accurately. Not so accurate on white gold, can't test silver.
Operation
Place the item on the test-plate, gently touch it with the pen-probe, a meter (or digital display) gives the reading.
How it works
A microscopic amount of the metal is 'dissolved' into an acid or salt solution - but it's only the non-gold that will dissolve, so the machine calculates the percentage of metal that has not dissolved = the percentage (in 'carat') of gold.
Advantages
Quick and very easy, the KEE tester does not require acid (some of our rival's models do!).
Disadvantages
Not reliable on platinum, not very accurate on white metals (including white gold), can't test silver. Will not 'read' through plating, you must file the item so that you are testing whatever might be underneath any plating.
Safety
There are no safety issues with our KEE tester, the pen-probe is filled with harmless salt solution. Some models (which we don't sell) use a strong acid.
XRF (X-ray Fluorescence)
How they work: they fire x-rays a fraction of a millimetre into the item and see if it 'glows' (fluoresces). A display lists the percentage of all the metals in the sample, not just the precious Metals. Ease-of-use: reasonably easy to use. Within a few seconds it gives an approximate result, a detailed analysis take a minute or two. Limitations: Some models give you a 'probability' that the item might be plated but, to be certain, you need to file the surface so that you are testing whatever might be underneath. Safety: They work on x-rays, we see market traders holding each sample in their fingers whilst testing, they will get cancer. However, these testers are quite safe if used properly. Cost of tester: prices have come down immensely in the last few years, now as low as £10,000.00 for the handheld models, about £30.000 for the 'cabinet' models. Cost of consumables: there are no 'consumables' but there are a few parts that cost £1000.00 each to replace when they wear out. They also need regular servicing which costs a few hundred pounds. Availability: we do not sell these, good makes are Niton and Fischer.
SPECIFIC GRAVITY, SG (RELATIVE DENSITY)
Summary
Measures the relative weight, e.g. a gold ring feels very 'heavy' compared with an aluminium ring of the same size.
Operation
The QUICKTEST Specific Gravity tester is for small items (up to about the size of a stack of three pound coins). Weigh the item normally (in air) then again whilst suspended in water. Then find yourself a calculator and do a calculation. The large table balance with a Specific Gravity chamber (they call it a Densimeter) will test large items, and it's computerised, so you simply get it to 'memorise' each weight and it does the calculation for you.
The Sovereign tester works on the same principle but is simpler, no weighing involved. Weigh the coin (weighing scales included), measure the diameter (does it fit exactly into the circle on the gauge?), measure the thickness (does it fit neatly through the slot in the gauge?), if it's the correct weight, correct diameter and correct thickness - then it's 22ct. This doesn't guarantee it's a genuine coin, it could (though unlikely) be a forgery made of 22ct, though that wouldn't make a huge difference in value.
See an article all about specific gravity testing.
Advantages
Totally non-destructive, the only chemical involved is water, you can even test rare coins.
Versatile, you can test gemstones and small mineral samples too.
Disadvantages
The process is fiddly, you need to work very slowly and carefully, not a method you would use in a busy shop or on a market stall.
When weighing in water you need to take a few readings then take an average.
Availability
We stock these.
RESISTIVITY (for coins and bars)
Summary
These are not jewellery testers that tell you the purity (carat) of the precious metal, they are for checking that coins and bars are exactly what you think they are. For instance, you think you have a gold Sovereign, or you think you have a 100g bar of 999 silver, or you think you have a 400oz (12.8kg) bar of gold - how likely is this?
Operation
Place the coin (or bar) on the test-plate. For small items you use the probe (“wand”) provided; for large items you use a 'bridge' attachment. You then select the carat (i.e. amount of gold in the coin/bar) and alloy (i.e. mixture of other metals in the coin/bar). Usually, you can do this by selecting the name of the coin from the database built into the tester. The display indicates if the reading for your particular coin / size of bar is ‘correct’, ‘probably not correct’ or ‘definitely not correct’.
How it works
NON-TECHNICALLY: the sensor in the tester has a coil of wire which creates a magnetic field in the coin/bar, this gives a reading on the display. TECHNICALLY: a transmitter coil induces a high-frequency magnetic field in the sample, this is detected by a receiver-coil and the resistivity is calculated.
Conditions
- the coin/bar must be at approximately room temperature
- there are limitations as to size and thickness when using a basic model
- the basic models can penetrate the surface to about 2mm (which will show if the item is plated), the top models can penetrate a 400oz bar all the way through (which will show if the item is “filled”).
Advantages
- reasonably easy to use.
- portable, battery-operated, robust.
- totally non-destructive, you can test rare collectors’ coins.
- you can even test coins and small bars that are sealed in plastic.
- better than XRF in that it can penetrates right into the middle of the item instead of just fraction of a millimetre.
Disadvantages
- you need to know if the item you are testing is gold or silver and, fairly precisely, the composition of the alloy (because other metals may give the same reading as gold and silver). In practise this means you need to be skilled enough to recognise when the item is obviously not gold or silver (e.g. a gold-plated brass coin).
- it won’t tell you if the coin is genuine, i.e. a 22ct forgery of a rare coin will register “22ct” (in fact, it’s not sensitive enough to distinguish 22ct from a 21ct forgery).
- to be more certain of what the metal is, you also need to check the specific gravity (relative density), the top models will do this.
Cost: £1,000.00 to £2,500.00
Availability
We do not sell these, they are available from Trusti Metrix.
RESISTIVITY (metal detectors)
Summary
To find metal under the ground, as used by 'detectorists'.
Operation
Scan the ground, listen to the bleeps, watch the display.
How it works
Same as the coins and bars testers, see above, but in the form of a 'metal detector' you sweep along the ground.
Conditions
Take care to choose a model specifically designed to detect gold. The cheaper models will 'reach' to about six inches below the ground, the best models up to 24 inches.
Small items need to be very close to the surface, large nuggets or gold coins can be found a few inches under the ground. But this depends on the quality of the metal detector and the composition of the ground.
A metal detector doesn't guarantee you have found gold (for instance, iron gives similar readings) you will need to test the item with a gold tester.
Availability
We do not sell these.
ULTRASOUND
How they work: they pass sound-waves through the object to see if it is the same all the way through or if it is 'filled' with another substance, e.g. a gold bar filled with non gold. Ease-of-use: (not sure, we've never used one). These are standard testers in the world of engineering, e.g. to see if steel is rusting or concrete is corroding, but when used for precious metals it requires specialist software to translate the reading into something meaningful. Limitations: it will readily show if an item is not homogenous but it won't tell you if the item is made of gold. Safety: safe, the same 'sound waves' as used in ante-natal testing. Cost: £750.00 to £1000.00. Availability: we do not sell these.
MELT-AND-ASSAY (FIRE ASSAY)
How it works: take a sample and weigh it. Then melt it and refine it, to leave pure gold. Weigh it again and calculate the percentage (in carat) of the gold. Ease-of-use this is not a 'tester', this is the method used in the laboratories of smelters. Limitations: it's destructive. Safety: smelting / refining is very specialist and skilled. Cost: a smelter will charge you about £80.00 per test, though if you have a reasonable amount and sell them the precious metal(s), they won't charge.
ICP
Inductively-coupled plasma mass spectrometry.
How it works: a tiny amount of the gold is vaporised and the resulting light analysed. It uses a high-vacuum system, high energy electron beam and an X-ray detector. Ease-of-use this is not a 'tester', this is the method used in research laboratories and some Assay Offices. Limitations: it's destructive. Cost: from £50,000.00.
To see all precious metal testers, click here.
See also, The Gold & Silver Buyer's Handbook,
RELATED ARTICLES
More methods of testing gold (and other precious metals)
Acid tests, what he various testing kits do
Details / review of KEE electronic gold tester
Testing gold, specific gravity method