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Contents of this article:
The components: frame with platform that hangs in the water, frame with beaker of water, bottom support enables you to weigh the item on the weighing platform ('weight in air') without dismantling the kit. THE WEIGHING MACHINE IS NOT INCLUDED.
Make the calculation listed below...
...and look up the chart.
(there's a chart for metals
The only 100% non-destructive test
Chemical tests (the acid test) can leave a stain behind, it's easily polished off with a cloth.
No acid test or electronic tester will test through plating, so the surface must be filed to removed any gold plating. This applies to all chemical testers. It also applies to electronic testers, you must file the surface: from the simplest electronic testers (under £150.00) which use an acid contact fluid, to the best of the low-cost electronic testers (about £400.00) which use a probe with a salt solution - none of these will test through plating. If the surface is gold plated, then the surface is gold and it will test as gold. In all cases, a small test-area must be filed with a fine steel (or diamond-impregnated) file.
Moving up-market, the 'scanner-type' XRF testers (about £12,000.00) and the 'cabinet-type' XRF testers (about £50,000.00) will test through very thin plating. If you set the machine to take long (several seconds) tests, and carry out each test a few times, and get inconsistent readings, then you probably have a plated item: file it, test it again.
In 99.99% of jewellery-testing, filing ('scratching') the item can be done 'discretely' in a place where it won't show (you only need file a tiny area), but there are times when you cannot file the item at all, e.g. rare coins.
Testing by specific gravity does not require filing the item.
Testing loose gemstones using the specific gravity method is a standard method in gemmology. This includes diamonds. The stones do have to be loose, you cannot test stones that are mounted in jewellery.
SPECIFIC GRAVITY (relative density) - the theory
Density is the amount of 'stuff' in a given space. Take a wedding ring made of alluminium and an identical-looking item (same size) made of platinum. The platinum wedding ring will feel over eight times as heavy as the alluminium wedding ring.
You will lift the aluminium item and say, "That's light!" You will lift the platinum item and say, "That's heavy!".
Of course, that's not what you mean. You mean, "That is much lighter / heavier than I expected, for the size." It is this 'being heavy or light, for the size' that is density, how much 'stuff' is squished into the space.
For the sake of the physics (which I shan't explain here) a cube of water measuring 1cm X 1cm X 1cm weighs 1g. Specific gravity is: how much a 1cm X 1cm X 1cm cube of unknown material* weighs compared with 1cm X 1cm X 1cm of water. [* This can be any 'material', e.g. metals, gemstones].
So, for instance, a result of SG 6.5 would mean your metal is 6.5X heavier than the same volume of water; or SG 1.5 would mean it was 50% heavier than the same volume of water (as you may notice, you don't actually have to cut the items into 1cm X 1cm X 1cm squares!)
But please don't worry if you don't understand any of that, you only need to know how to take the measurement.
SPECIFIC GRAVITY - the practice
In practice you really don't need to know about heaver-than or lighter-than or the weight of water. In practice all you need to know is how to take the measurement:
Weigh the item. This is the
weight in air.
You will now, probably, need
a calculator. Calculate: item weight (weight in air) divided by the number
you notice from the chart, pure metals have single values (though even
these can vary) whilst 'impure' metals (alloys containing mixtures of
metals) have a 'range' of values, and there are overlaps which can make
it difficult to distinguish between 14ct and 18ct, or 18ct and 22ct, or
copper and 9ct.
As you notice from the chart, pure metals have single values (though even these can vary) whilst 'impure' metals (alloys containing mixtures of metals) have a 'range' of values, and there are overlaps which can make it difficult to distinguish between 14ct and 18ct, or 18ct and 22ct, or copper and 9ct.
however, invaluable if you already have clues:
however, invaluable if you already have clues:
especially useful for coins, because you know exactly what a
genuine coin is made of, and you can also look up the exact weight
and carat (e.g. in The
Gold & Silver Buyer's Handbook).
It is especially useful for coins, because you know exactly what a genuine coin is made of, and you can also look up the exact weight and carat (e.g. in The Gold & Silver Buyer's Handbook).
SPECIFIC GRAVITY KIT- in more detail
Assembling it: surprisingly easy, you can see by the picture. The kit is delicate (especially the 'frame' that holds the equipment and the 'hanging wire' that holds the weighing platform in the water) so you do need to handle everything with care, if you're the type of person who is always knocking glasses of water over, you really won't have much a chance when it comes to using a specific gravity kit. But it is quite easy to assemble.
Instructions for use:
Reasons for errors in the readings:
Ideally (but more important
for small gemstones than for gold):
TWO MODELS, TWO SIZES
The two models are identical in construction, but different sizes.
Small: the platform measures
one inch (about 23mm) in diameter, it will comfortably take a one-pound
coin or, with care and careful balancing, an object the size of a stack
of three one-pound coins or six U.S. Nickels (they're the same diameter
but half the thickness of UK pound coins). The item can be larger, providing
it doesn't fall off the platform, and providing it fits into the 50ml
beaker of water.
ACCURACY - A CASE STUDY, USING THE LARGE SPECIFIC GRAVITY KIT AND ITEMS WEIGHING BETWEEN 5g AND 8g.
THE FOLLOWING TESTS WERE CARRIED OUT ON A SOEVEREIGN (22ct). It weighted 7.987g. We know from the text books that it should weigh 7.988g plus-or-minus 0.01g so if I were testing this to see if it were a genuine Sovereign, finding that the weight is correct is a good start. Officially (there is always variation) 22ct should be between 17.7 and 17.8, see the chart above.
I used a cheap (under £50.00) weighing machine that weighed down to 0.001g.
We should get a reading of 17.7 to 17.8, lower (about 15.5) would indicate 18ct, whilst higher (over 19) would be pure gold.
I took several readings.
With the level of the water
exactly level with the middle of the "50ml" marker I got: 17.593
then 17.550 then 17.630.
What I learnt was the following:
THE FOLLOWING TESTS WERE CARRIED OUT ON A PURE (.999 / 24ct) BAR OF GOLD, which we know, from the chart above, should read 19.3.
I took five readings and got:
19.4 then 20.01 then 19.55 then 19.85 then 19.4.
FINALLY, I WANTED TO KNOW WHAT WOULD HAPPEN IF I USED A VERY CHEAP (UNDER £20.00) WEIGHING MACHINE THAT ONLY WEIGHED DOWN TO 0.01g INSTEAD OF 0.001g.
On the Sovereign I got the
following readings (remember, officially it should be 17.7 to17.8): 17.8
Specific gravity is the recommended method to test rare and delicate antiques, collectors coins, and anything which must not be marked in any way. You do need patience, the equipment is delicate, the method is slow, you must take a few readings and expect to get a range of results for each item. What you end up with are results for high-carat items (22ct to 24ct) that are as accurate as using acids (and far more accurate than an electronic gold tester), certinaly good enough to measure to the nearest 2ct (providing you take a few readings), not really good enough to measure to the nearest 1ct. For lower carats (9ct to 18ct) this method competes well with the better electronic testers. It is also a standard method of testing loose gemstones (in addition to various optical and thermal tests).
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