There is also a very good book on the subject.
ALCHEMY AND THE TESTING OF GOLD
In the middle ages alchemists made it their life’s aim to find 'The Philosopher's Stone', a substance that would turn ‘base’ metals into the ‘noble’ metal of gold. They were unsuccessful. Even today, with the latest technology, it just isn't possible. Current-day jewellers have a similar ambition. To find a method of testing precious metals without the use of acids, using an electronic portable low-cost (under £300.00) non-destructive machine. Alas, even today, with the latest technology, this just isn't possible. It is therefore the traditional ‘acid test’ that is the universal test for precious metals, either on its own or with an electronic tester (more about those later).
Before you start, LOOK at the item. And if you can’t see clearly, buy a good quality magnifier, it will be the best ‘tester’ you ever own. All modern gold, silver and platinum items made in the U.K. or imported into the U.K. should be hallmarked.
Wall charts explaining hallmarks are shown (by law) wherever precious metals are bought or sold, familiarize yourself with these marks, it’s as important as being familiar with coins. Forged hallmarks (as with forged coins) exist but are rare, possibly because the authorities go to lengths to track down forgers, with a maximum penalty of seven years in jail.
No hallmark? Then start testing! The first two tests do not require acids.
Firstly, a magnet. Gold and silver are not magnetic. So if it's magnetic it can't be gold or silver. But NOT being magnetic doesn’t prove anything, copper (which is often plated with gold) is not magnetic, some steel is not magnetic. Also, a magnet won't help test white metals, some steel is not magnetic, some platinum is magnetic.
Secondly, 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' an item made steel - no need to test any further.
THE ACID TEST
You've tried all of the above, you may have your suspicions, but now you need absolute proof: the acid test.
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.
This is what you do.
Choose a place on the item that is not normally seen and file the surface (a fine needle file is included with each set). Do this firmly but only over a very small area. This is to get past any plating, because if the item is gold plated, the surface IS gold and will test as such. Now put a tiny 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) or silver.
For gold, acids are designed to test to the nearest carat commonly used in jewellery, 9ct, 14ct 18ct, 22ct – but, with practice, results to within 5% can be achieved, very accurate! On silver, the acid gives a very definite reaction on Sterling (.925) silver, a slight reaction on 'low-grade' (.800) silver, and hardly any reaction at all on very low grade (.500) silver.
ELECTRONIC GOLD TESTERS
Low-cost testers that use an acid contact fluid or gel. Some have a row of lights to indicate commonly-used carats or a digital readout. For £95.00 you get to see readings for not gold, 10K, 14K and 18K (we have discontinued this one), for £149.00 you get nine lights, for 9, 10, 12, 14, 16, 18, 20, 22 and 24K - we sell this one (all are American and display 'K' for Karat rather than 'ct' for carat). The problem with these is that you never know if the machine is rounding a reading up or down. Other machines have a digital readout that displays the exact carat. The problem with these is that although it looks as if you are measuring to the nearest 1ct, it is nearly always incorrect, in practice it is no more accurate as the versions with the lights. Applying the acid is either from tiny bottles with droppers, or as an acid gel with a 'pen dispenser'. Typcially, you will file the item, fill the 'well' (a hollow in the machine) with 'activator fluid' (acid), clip part of the item to the crocodile clip, then dip another part of the item very carefully into the fluid without it touching the sides of the 'well'. After taking the reading you must meticulously clean away all the acid or it will dissovle the machine. You must always file the item first as these will not test through plating (they will test through very thin plating, but since you don't know if the item is plated, you have to file it first).
Low-cost testers that don't use an acid contact fluid or gel. We sell the AGT1, click here to see detailed review. Summary: it's fast, it does not use any acid liquid or gel whatsoever, it has a row of 32 LED lights enabling very accurate readings on 9ct to 14ct, adequate readings on 14ct to 18ct, not so accurate on 18ct to 24ct. But far better than the testers (above) that have a handful of lights. £399.00. You can see a video of one of these action.
ICP (Inductively-coupled plasma mass spectrometry): requires high-vacuum system, high energy electron beam and an X-ray detector. I don't really know anything about this method except that the equipment costs about £35,000.00. We do not sell these.
Table-top XRF (X-Ray Fluorescence Analysis technology). The larger machines weigh 20Kg, connect to a computer (not supplied), and each test takes a minute or two. They say they are easy to use (I've never used one). They cost about £8,000.00. Advantages: it's more accurate than acids and it's non-destructive. Disadvantages: they are large and heavy, relatively slow to use, will not test through heavy plating, and require careful setting up and regular maintenance. We do not sell these.
Handheld XRF machines cost £12,000.00 to £15,000.00. I have spent a few hours playing with one of these, they are impressive: within a few seconds a breakdown of the chemical composition appears on the screen. They are easy to use and require no setting up other than selecting the correct profile on the touch-screen. Disadvantages: they can only test through very thin plating, in other words you still have to file the item; although they do not involve acid, they use x-rays (highly dangerous if pointed at someone). We do not sell these.
There is also the Melt-and-Assay (Fire Assay) method of testing (not electronic). Melt the item (or part of it - if only a fraction of a gram). The service costs (if we get it done for you) £95.00 and takes two to three weeks.
QUICKTEST, Watford, WD18 8PH, Tel. 01923 220206, email info(at)quicktest.co.uk