Testing gold, specific gravity method
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There is also a very good book on the subject of gold, buying and testing
To see specific gravity testers click here. The SG attachments used with small diamond/gem balances cost about £100.00 (the balance is not included); the full-size diamond balance designed especially for measuring specific gravity cost upwards of £1000.00.
This is a 100% non-destructive test. Other methods (including electronic tests) require the item to be filed first (to remove any surface plating); also, chemicals (acids) can leave a stain. The stain can be polished off easily, but that's not the point: if you have a rare coin you cannot file it or stain it. Even XRF testers (from £12,000.00) will only test through very thin plating, so you still have to file the item.
Testing by specific gravity (SG) does not require filing the item and the only chemical involved is water. You can test loose gemstone (not stones mounted in jewellery) and gold.
Generally, the disadvantages SG testing are:
- it takes a few minutes to set up
- each test involves great care, patience, delicate placing of the item on the platform with tweezers, using a calculator to work out the answer, and you should do this two or three times to check that you have the same answer each time. This is not something staff can do whilst under pressure with a queue of customers.
- you can get quite good results for high-carat gold (22ct and above) but it is difficult to difficult to distinguish between 14ct and 18ct, or 18ct and 22ct, or copper and 9ct. Gold coins are usually 22ct. Jewellery (in the UK, USA and Europe) is usually between 9ct and 14ct.
- the item must be made of only one metal with not 'extras' such as gemstones.
Specifically, the disadvantages of the SG attachment (without the balance) are:
- you use with your existing (small) diamond/gem balance, the balance is not included in the price
- you can only test small items of gold (large items such as bangles and bracelets won't fit); you can only test gemstones (figurines etc won't fit)
- unlike the dedicated SG balances the attachment is not computerised, once you have taken the measurements you must get out your calculator to work out the answer).
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.
The item is weighed on the weighing platform on the weighing machine
Then the item is weighed on the little platform that is suspended in the water
Calculate: weight in air (i.e. just weigh it) divided by weight-in-air less weight-suspended-in-water (scroll down for an easy method of calculating this).
...and look up the chart.
(there's a chart for metals and a chart for gemstones)
SPECIFIC GRAVITY (relative density) - the theory
Density is the amount of 'stuff' in a given space. Take a wedding ring made of aluminium and an identical-looking item (same size) made of platinum. The platinum wedding ring will feel over eight times as heavy as the aluminium 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, even another liquid].
So, for instance, a result of SG 6.5 would mean your metal is 6.5X heavier than the same volume of water; for instance, a result of 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 an equivalent volume of water. In practice all you need to know is how to take the measurement:
Weigh the item. This is the weight in air.
Now weigh the item while it is suspended in water (this is what the specific gravity kit does). This is the weight in water.
Take the smaller number away from the bigger number. To aid your memory, draw a circle around the answer.
You will now, probably, need a calculator.
Calculate: item weight (weight in air) divided by the number you circled.
The answer is the specific gravity (SG).
Look up this number on the chart, there is one chart for metals and another chart for gemstones.
10.9 to 12.7
12.9 to 14.6
15.2 to 15.9
14.7 to 16.9
17.7 to 17.8
10.2 to 10.3
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.
It is, however, invaluable if you already have clues:
- You think you have a platinum wedding ring, you get a reading of 16.5, it might be white gold, it can't be platinum.
- You think you have a Victorian 15ct brooch, you get a reading of 10, it might be 9ct, it might be copper, but it can't be 15ct.
- You think you have a gold Sovereign, you know they are made of 22ct gold, you get a reading of 12, it can't be 22ct, it is a forgery.
It is especially useful for coins, because you know exactly what the genuine coin is made of, and you can also look up the exact weight and carat in The Gold & Silver Buyer's Handbook).
SPECIFIC GRAVITY KIT - A REVIEW
Assembling it: surprisingly easy, you can see by the picture, once you've got the hang of it, it will take less than five minutes. 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 be able to cope with a specific gravity kit. But it is quite easy to assemble.
Instructions for use:
- weigh the item. This is the weight in air
- weigh the item while it is suspended in water. This is the weight in water
- take the smaller number away from the bigger number. To aid your memory, draw a circle around the answer
- find a calculator
- calculate item weight (weight in air) divided by the number you circled
- all figures for 'specific gravity' are approximate. For metals, the actual densities vary according to the physical state of the metal e.g. cast, rolled, drawn, depending on porosity
- figures will vary with temperature
- metal alloys will vary considerably according to the exact mixture of metals they contain
- for gemstones there is variation too, the same stone might be found in various varieties and compositions.
Reasons for errors in the readings:
- sample too small, my readings on metals weren't very accurate on items weighing less than 0.5g
- edge of plastic holder touching wire of frame
- the platform (in the water) touching edge of beaker
- scale does not start at zero
- slight spillages = not exactly 50ml in beaker (small model) or 100ml in beaker (large model).
- there is only physically space for small items (see "Two models, two sizes" below)
- jewellery must contain one metal only, e.g. it won't work if a gold item contains steel rivets
- jewellery must be metal only, it won't work on jewellery containing gemstones
- as regards cost, the specific gravity kit does not include the weighing machine, we recommend one that weighs down to 0.001g.
- test each item at least twice, just to make sure you've weighed and calculated correctly
- if the item weighs less than 1g carry out three tests
- if the item weighs less than 0.5g carry out at least four tests and if not consistent, take an average
- check level of water after every test, the beaker must be filled to exactly 50mls (small model) or 100mls (large model).
Ideally (but more important for small gemstones than for gold):
- use distilled water
- the water should be at about room temperature (about 20 degrees C.)
- make sure there are no bubbles around the metal by very gently stroking the metal with a fine brush
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, 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.
Large: the platform measures 1.25 inches (about 30mm) in diameter, it will comfortably take a two-pound coin (27mm diameter X 2mm thick) or, with care and careful balancing, an object the size of a stack of five two-pound coins. The item can be larger, providing it doesn't fall off the platform, and providing it fits into the 100ml beaker of water.
ACCURACY - A CASE STUDY
I used the large specific gravity (SG) attachment and items weighing between 5g and 8g. I wanted to see how good this 'very cheap method' (about £100.00) was. If you were to use one of the dedicated SG balances (upwards of £1000.00) you would get more accurate and more consistent results.
The following tests were carried out on a sovereign (22ct). It weighed 7.987g. We know from the text books that it should weigh 7.988g plus-or-minus 0.1g 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.
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.
With the level of the water fractionally above the top of the "50ml" marker I got: 18.220 (then realised the water was a bit too high)
With level of the water just at the bottom of the "50ml" marker I got: 17.488.
What I learnt was the following:
- note the variations in the readings, when writing down the final answers, I recommend rounding them to two decimal places not three.
- my readings of between 17.5 and 18.2 were close enough to indicate 22ct, but you do need to take a few reading to be sure.
- the readings are slightly more accurate if the level of water is exactly 50ml and not a tiny bit more or a tiny bit less.
- the specific gravity method tells us whether gold is "about 20ct" or "about 22ct" but it's not quite accurate enough to measure to the nearest 1ct.
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.
I have therefore learnt:
- that even with pure gold there is a variation in the readings, but (in this case anyway) the readings are never lower than the official 19.3.
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 then 18.6
On the pure gold (officially, should be 19.3) I consistently got 20.36.
And so I learnt:
- using a 0.01 weighing machine (instead of 0.001) does give and indication of carat, it's fine if you only want to know if the item is "About 18ct" or "About 22ct" with little in between.
- using a 0.01 weighing machine (instead of 0.001) would not give any meaningful readings on lightweight items (e.g. 3 or 4 grams).
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 (the only electronic tester I have seen that is this good is the KEE tester), certainly 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) measuring specific gravity is not accurate enough.
It is also one of the standard methods for testing loose gemstones.