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Author Topic: Trying to identify metals by density  (Read 383 times)

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Online bagerap

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Trying to identify metals by density
« on: July 23, 2016, 06:50:29 PM »
It's too hot here to work today, so I thought that I'd exercise my brain. (Triumph of hope over experience)
Following on from the excellent Fosseway thread on Brass v Aluminium-Bronze, I'm trying to identify a particularly dense medal.


Looking at the engraving to the reverse, the underlying metal would appear to be brass except that it feels very heavy for brass. The density of brass expressed as kg/m3 ranges from 8400 - 8730.
This piece weighs 50 gr and measures 38 x 4 mm.
So unless my figures have gone sour the density is: 50gr/4538 mm3.

That's giving me 11018 kg/m3 and the nearest thing I can find to that is lead.

Any critiques or suggestions welcomed.

Offline Figleaf

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Re: Trying to identify metals by density
« Reply #1 on: July 24, 2016, 11:06:37 AM »
Interesting experiment. I think the weak point in the calculation is measuring the thickness of the coin. You can set up a small spreadsheet and play with thickness to get a feel for its importance. An alternative for calculating content is to get a tapered measuring glass, with the finest measures you can find and as narrow as you can get (but larger than 40 mm at the highest point of the scale). Simply cover the medal in whisky, measure its volume and reward yourself ;)

Lead is not very suitable for medals. It is quite soft and poisonous. Assuming that the medal was made of metals produced in India, major Indian mining products are iron, aluminium and magnesium, but there is also copper and zinc production, so brass with a white metal coating is a possibility.

Peter
An unidentified coin is a piece of metal. An identified coin is a piece of history.

Offline FosseWay

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Re: Trying to identify metals by density
« Reply #2 on: July 24, 2016, 08:41:46 PM »
There is a problem with measuring the thickness of coins/similar objects struck by machine using a collar, that the thickness of the rim is no guide to the thickness of the overall coin. A particularly extreme example of this variation can be seen in the UK 5p issued 1990-1992, where the edges can vary considerably in thickness but the mass of the coins is always within the official tolerance.

If you extrapolated the various thicknesses of the 5p rims to the whole coin and then related your figures to the known mass of the coin, you could easily end up with variations in apparent density of similar proportions to what you report when comparing your medal to the known density of brass.

Online bagerap

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Re: Trying to identify metals by density
« Reply #3 on: July 24, 2016, 10:17:38 PM »
I will be using a graduated measuring cylinder tomorrow. Should give a more precise result.

Offline natko

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Re: Trying to identify metals by density
« Reply #4 on: January 05, 2017, 01:38:06 PM »
Coin thickness could only be estimated at the fields, since they make a vast majority of the coin area. If they're uniform through all the coin, which is questionable, it would still give some errors in calculation (precision, plus raised part, which lowers density in our simplified model). There are easier ways to determine the volume, and more precise as well. Don't be afraid of emerging it in water.

This looks like silver to me.

Offline bruce61813

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Re: Trying to identify metals by density
« Reply #5 on: January 06, 2017, 03:06:25 AM »
How accurately can you measure mass [weight] and volume ? 1 ml = 1 cubic centimeter, 1 cubic centimeter at room temperature = 1.0028 grams. If you have some to measure 2 volume of water, you can accurately determine the volume of the coin. You need something that allows a consistent max capacity , say the plastic top to a bottle, just larger in diameter of your item and deep enough to be filled above the item. The easiest way then is to place the cap on a scale, tare the scale, fill it to the top with water, and record the weight{1}. Empty the top, tare the scale , place item in the top and fill the top. Note the weight{2}. The weight {1} - weight {2} will be equal to the volume of the medal/coin . Just divide its weight by the volume and you will have the density of your item. But this works only for pure metals, as the density of alloys can be really complxt.

Bruce

Offline FosseWay

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Re: Trying to identify metals by density
« Reply #6 on: January 06, 2017, 09:34:29 AM »
This looks like silver to me.

Yes, that thought had crossed my mind as well. It can be hard to tell from a photo though, especially when there are deposits on the coin.