Specific gravity test in action

[goldbones]

I've seen many people speak of it but never really seen anyone go into it or any detail of how its done and showed some math,  i did the following with essentially 3 types of metal, aluminium, 22ct and 24 ct, 22ct red( crown) gold is expected to have a density of about 17.5 while 24ct pure gold is expected to be around 19.3, while aluminium is 2.7:

In air (the normal weight checking on a scale):

Piece of aluminium= 1.5 grams

2015 Sovereign = 8.00 grams

1/2 ounce Krugerrand = 17.00 grams

Perth mint goat 1/2 ounce= 15.60 grams (see pic)

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suspended in water (this is the actual test):

aluminium=0.95 grams

krug= 16.02 grams

Sov= 7.54 grams (see pic)

Goat= 14.80 grams (see pic)

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The math:

weight in air - weight in water = displacement

weight in air / displacement= specific gravity

So Aluminium 1.5 - 0.95= 0.55 (displacement)

1.5 / 0.55= specific gravity 2.72

Krug 17.00 - 16.02= 0.98

17.00/ 0.98= specific gravity 17.34

The goat has a specific gravity of 19.5

The sovereign has a specific gravity of 17.39

That's more accurate than x-rays and less destructive than just about any other method. Considering i'm not set up to lab perfect conditions  this is pretty accurate.

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So the specific gravity results in order again:

Aluminium= 2.72

Krugerrand= 17.34

Sovereign = 17.39

Perth Goat = 19.5 

 

[HawkHybrid]

I'd normally zero(tar) the scales with the water to

just measure the weight of the equivalent volume

of water displaced by the object(coin).

less calculations means less chance of typos.

it's a good test.

 

HH

[goldbones]

3 minutes ago, HawkHybrid said:

I'd normally zero(tar) the scales with the water to

just measure the weight of the equivalent volume

of water displaced by the object(coin).

less calculations means less chance of typos.

it's a good test.

 

HH

If you mean measure the water on the scales and measure difference, i'm not doing that, the scales don't know the water is there and thats the way you need to keep it otherwise changes in volume (due to drips and so on) would alter the result.

[HawkHybrid]

I would:

1. weigh coin.

2. set up water etc and zero the scales with the

water etc. on it.

3. dip coin and read weight of water displaced.

(for a silver 1 oz it'll read something like <3 grams)

4. calculation would then be (~31 grams) divide

by (less than 3 grams). or a little over 10 for pure

silver.

by my method I would expect to be reading 0.4-0.5 grams

for the sovereign.

my set up is a little more crude than yours

 

HH

[goldbones]

1 hour ago, HawkHybrid said:

I would:

1. weigh coin.

2. set up water etc and zero the scales with the

water etc. on it.

3. dip coin and read weight of water displaced.

(for a silver 1 oz it'll read something like <3 grams)

4. calculation would then be (~31 grams) divide

by (less than 3 grams). or a little over 10 for pure

silver.

by my method I would expect to be reading 0.4-0.5 grams

for the sovereign.

my set up is a little more crude than yours

 

HH

That does work quiet well, but i find with things weighing less than say 15 grams the accuracy starts to drop. 

In true lab conditions you'd keep the water at 4 Celsius and only use distilled water and scales that measure accurately to within 0.00001g ,i'm not going that far.

You need it to check for lighter metals but for heavy metals like lead and tungsten the ping test (putting a coin on one finger and giving at tap to make it ring) is usually enough since lead and tungsten have a dull thud sound, it wouldnt ring, someone got a coin made of tungsten and the ping test pretty much confirmed it (along with the coin looking terrible because it can only be electroplated (electro brush) and cast (cant be struck).

 

[HawkHybrid]

for silver there's >15% difference with copper/bronze.

so the accuracy should not be a problem even with

smaller coins.

small coins can be tricky as well as fiddly

 

HH

 

[sovereignsteve]

That's quite an elaborate setup you have there GB. Is it home made or obtained from elsewhere?

I like the way it is only the coin that is acting on the scales. With methods that involve the container of water resting on the scales, it is difficult to get the required accuracy unless you can get a balance accurate to 4 dp that reads high enough to account for the water. The one I currently have at work only goes up to 120g. I think the previous one was up to 500g but it broke.

BTW How do yoususpend the coin?

Edited November 18, 2015 by sovereignsteve

[goldbones]

10 hours ago, sovereignsteve said:

That's quite an elaborate setup you have there GB. Is it home made or obtained from elsewhere?

I like the way it is only the coin that is acting on the scales. With methods that involve the container of water resting on the scales, it is difficult to get the required accuracy unless you can get a balance accurate to 4 dp that reads high enough to account for the water. The one I currently have at work only goes up to 120g. I think the previous one was up to 500g but it broke.

BTW How do yoususpend the coin?

I purchased it from quicktest UK, you can see in photos how its suspended below. I supose you could make a setup like this without spending like £100, but i'm too lazy for that, it's overpriced for what it is but they are the only ones who do it as far as i know.

For bullion dealers/coin dealers i'd suggest this : Densometer  product from china, which is a laboratory accurate automatic specific density balance, you stick the item on the top press a button then throw it in the tank below and press a button and it spits out SG value and can be programmed with various copper/silver ratios and other stuff.

Edited November 19, 2015 by goldbones

[goldbones]

Additionally this website calculator lets you calculate density in a given alloy composition, so you have a reference number rather than a guess:

http://www.handymath.com/cgi-bin/density.cgi?submit=Entry

 

Edited November 21, 2015 by goldbones

[goldbones]

Oh i know how you do the calculation now:

For a gold eagle very interesting:

(1/19.3x916.67) + (1/8.96x53.33) + (1/10.49x30)= 56.30

1000/56.30=  SG 17.76

A gold eagle has a specific gravity of 17.76, that cannot be coincidental lol 1776 the date of independence.

[Cointreau]

What weighing scales are you using there goldbones?

[sovereignsteve]

1 hour ago, goldbones said:

Oh i know how you do the calculation now:

For a gold eagle very interesting:

(1/19.3x916.67) + (1/8.96x53.33) + (1/10.49x30)= 56.30

1000/56.30=  SG 17.76

A gold eagle has a specific gravity of 17.76, that cannot be coincidental lol 1776 the date of independence.

So it's a straightforward proportion calculation. I'm surprised metal alloys behave like that, I always thought there was more interaction between the elements on being alloyed which would affect the volume (obviously the weight would be fixed) and thus the density.

[goldbones]

6 hours ago, sovereignsteve said:

So it's a straightforward proportion calculation. I'm surprised metal alloys behave like that, I always thought there was more interaction between the elements on being alloyed which would affect the volume (obviously the weight would be fixed) and thus the density.

The elements are stable and do not react with each other and do not react with water i suppose.

The volume only changes when there is a reaction, the only reaction you can create is temperature change, the volume will change with delta (like sub zero or over 50 Celsius).

Edited November 23, 2015 by goldbones

[trickyt57]

Finding Specific Gravity for different metals

I find it difficult to find the specific gravity of certain metals.  There are plenty of tables on the internet, but the results often don't agree and are slightly different.

As far as I can tell the below specific gravity of metals in g/cm2:

Gold: 19.32 (Some sites say 19.3)

Silver: 10.49 (Some sites say 10.50)

Copper: 8.96 (Some sites say between 8.80 and 8.95)  

Specific Gravity of an alloy

There appears to be more than one opinion as to what the correct formula is.  As far as I can tell the correct formula is:

1. Find the reciprocal of the specific gravity of each metal in the alloy. This is done by dividing by the specific gravity. For example, the
specific gravity of silver is 10.49 and the reciprocal is 1 divided by 10.49 or 0.094966.
2. Multiply each reciprocal by the number of parts per thousand of that metal to be used.
3. Add the results of the multiplication together.
4. Divide 1000 by this total - the answer is the specific gravity of the alloy.

So in Excel the formula would be: =1000/(1/SG1*P1+1/SG2*P2)

Where:

SG1 = specific gravity of metal 1

SH2 = specific gravity of metal 2

p1 =purity of Metal 1 in parts per 1000 of the coin

p2= purity of metal 2 in parts per 1000 of the coin

So, for a Krugerrand which is 91.67% (22ct) gold and 8.33% Copper the formula would be:  =1000/(1/19.32*916.7+1/8.96*83.3) =17.62266 (However I found the specific gravity quoted in one place as 17.77.  Close enough.

Testing for specific gravity

Most digital scales won't do the job well enough.  Some only measure weight to 1 decimal place (you need at least 2. Three decimal places is better.)  Some can't weigh more than 125g or 150g.  That's fine for weighing a coin (1oz = 31.1035g), but with specific gravity test you need a glass of water.  That will likely weigh at least 150g.  Some scales don't give very accurate readings, particularly for heavier weights.   Accurate scales are more expensive.  Be prepared to pay up if you want accuracy.

Before doing the test you need to calibrate the scale.  This doesn't mean just pressing the Tare button and seeing 0.000 on the scale.  Calibrating means that you have a metal calibration weight (e.g. 100g). 

The calibration process will vary according to the manufacturer, but generally the calibration process goes something like this:

1) Turn on the scale. If necessary press the Tare button so that the scale shows 0.000g.  2) Press and hold the "M" button (could also be the button used to choose between grams and ounces).  After a second or two it will show a weight (e.g. 200.000g).  Press the Tare button as many time as necessary until the weight showing corresponds with the weight shown on metal calibration weight. 3) When the display shows the same number of grams as embossed on your metal calibration weight, press the "M" button again.  You should see the word "CAL" flash on the display. 4) Place the metal calibration weight on the scale.   It should say "Pass".  5) You can now remove the metal calibration weight and commence weighing (you might need to turn off and restart, or just wait a minute depending on the manufacturer of the scale.)

Testing equipment

Apart from the scales you don't need any special equipment.   For suspending the coin in the water I use dental floss with a slip-knot tied across the diameter of the coin.  Dental floss is almost weightless, so it won't affect the results much.  String would be too heavy and thick and would absorb some of the water.

Other ways of testing your gold or silver

Make sure the weight and size of your coin corresponds to the official specification.  For size the best way is to compare it side-by-side with a coin which is known to be genuine.  In particular look at how fat it is.

Use a "ping" tester that you can buy for around $3 per month on your phone.  You enter the coin name and year, (or specifications if it's not in their database).

Use a surface purity tester - often called a "diamond tester".  It only checks the surface, not the interior.  But it will help you quickly distinguish between gold or silver-looking and genuine gold or silver, at the surface level.   Gold or silver plated coins would pass this test.

Magnet test.  Buy a large circular magnet ("fishing magnet").  Tilt it at 45 degrees.  A silver coin should slide slowly, maybe in a semi-circular pattern, whereas a cupro-nickel coin will slide fast.  Gold is non-magnetic, so it should slide fast.  Anything which sticks to the magnet is not a genuine precious metal. 

Visual test: Buy a coin magnifying loup and examine your coin carefully comparing every detail with pictures of the real thing.  Often modern genuine coins will have features that are hard to replicate, such as very fine hair-like lines, or a hologram.  If the lettering seems less distinct, or if the fine lines seem less fine than on the image, you could be handling a fake