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There is also a very good book on the subject
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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
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 ring (same size) made of platinum. The platinum 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 / much 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) we 'expect' that a cube of water measuring 1cm X 1cm X 1cm will weigh 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. [When I say, "unknown material" I mean 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 practise
In practice you really don't need to know about heavier-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
Now look up the standard chart for gemstones.
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 and using it is not difficult providing you work slowly and carefully. Carrying out a test, and taking a second or third test, just to be certain, and checking your calculations - all that can take four or five minutes, it is not 'instant'.
Instructions for use:
Reasons for errors in the readings:
Ideally (but more important
for small gemstones bigger mineral samples):
TWO MODELS, TWO SIZES
The two models are identical in construction, but different sizes. The small size is for testing gemstones and small gold items (up to the size of a couple of one pound coins); the larger model is or larger items (up to the size of a stack of five two-pound coins).
platform measures one inch (about 23mm) in diameter, it will take any
gemstone of a size you are likely to find in jewellery.
QUICKTEST, Watford, WD18 8PH, Tel. 01923 220206, email info(at)quicktest.co.uk