Telling the difference between brass and aluminium-bronze

[FosseWay]

Does anyone know of a non-destructive way of telling the difference between aluminium-bronze and brass coins that are otherwise identical?

It has recently come to light that the token illustrated below (a tram token from Göteborgs Spårvägar used intermittently between 1924 and 1956) was produced in five separate orders during that period, and in two different metals. The first two emissions were in aluminium-bronze; the specification sent to the maker (Sporrong) in 1924 specifically referred to the new French coins of that metal. When Göteborgs Spårvägar ordered some more in the middle of WW2 they were told that aluminium-bronze was not available but they could be produced in brass, which was used thereafter until the last order in 1951.

I've looked at a number of these pieces and can't see any obvious difference. There shouldn't be a different in weight, as they were used in a portable ticket machine that required pieces of uniform weight, but if the metals are of different densities there could be a small difference in thickness - similar to the situation with UK 5p and 10p coins in Cu-Ni vs. steel - that is not evident unless you're looking for it. The researcher who unearthed the above information can't see any difference either, and he is much more experienced than me in these tokens, and being a former employee of the tram company in question has had the opportunity to handle lots of tokens.

[Figleaf]

I have no expertise in metallurgy, but as far as I am concerned, brass is an amalgam of bronze and aluminium. However, the respective amounts may vary, giving a darker colour when the amalgam contains more copper. Since the client referred to French coins, he probably referred to the 1920-27 issues of the Chamber of Commerce. Later issues, like the Morlons, are significantly lighter in colour.

My guess is that during the second world war, Sporrong could have plenty of brass from spent casing, but no bronze or aluminium. They must have convinced their client that the tokens would be just a bit lighter in colour, but with some age, you wouldn't even see that difference. The later amalgam would have been somewhat lighter, but if the weight had to be maintained, it could be done by increasing thickness very marginally.

Peter

[FosseWay]

I thought brass was an alloy of copper, tin and zinc, but not normally aluminium, hence the convention of differentiating between brass and aluminium-bronze in e.g. KM. There are some French colonial coins listed with different type numbers in KM where AFAIK the only difference is this variation in alloy, and I'm stumped there too.

[Figleaf]

You are right. Change all occurrences of aluminium to zinc, but keep in mind that the French coins contained aluminium as a hardener. Wiki says zinc content can vary from 5 to 40%. Sporrong would have been able to buy zinc, but not copper (useful for wires etc.) in wartime. Otherwise, the reasoning is the same.

Peter

[THCoins]

A very simple test could be to listen to the sound the coin makes when you let it fall spinning on a hard surface. Different alloys would be expected to give a different sound spectrum. But this would be only of use in a side by side comparison.

[FosseWay]

I will try the sound test.

Parallel to the above issue (with frozen date 1924) is another with frozen date 1949 and a hole. Both weigh the same (they were used in the same machines) but the 1949 is fractionally thicker to compensate for the hole. AFAIK all the 1949-dated issues are made of brass and not aluminium-bronze. If I discover that some 1924 pieces sound the same as 1949 ones and some don't, it is reasonable to assume the difference is due to the composition. If I don't hear any obvious difference, however, I don't know whether that's due to (a) that I happen to have only brass 1924s in my possession, (b) that there isn't a noticeable difference in sound between the two metals, or (c) that my hearing doesn't pick up the difference (much as tone-deaf people have problems with pitch, though I'm not tone-deaf).

[Figleaf]

I would expect that the hole changes the sound.

Maybe you can compare the tokens with the French coins the client specified.

Peter

[malj1]

I have this token which is much paler than the usual brass tokens and very similar to the British pound coin which I think must be made of aluminium bronze.

[constanius]

You should weigh them, as though you state they had to be of uniform weight, the ticket machines must have had some tolerance to account for worn tokens and if this tolerance was generous enough it could easily accept both types.

If that shows no weight difference then you could check volume by using water displacement, unfortunately you would need a graduated cylinder and if possible a pipette.   

[FosseWay]

My first challenge is going to be getting hold of a reasonable number. I thought I had rather a lot of spares but it seems I only have a couple. After the summer break I will talk to the guy who did the research I mentioned above and see if he has some piles of them lying around that we can mess with!

[Gusev]

Does anyone know of a non-destructive way of telling the difference between aluminium-bronze and brass coins that are otherwise identical?...........

40 years ago, I studied at the University of Aviation materials science. While the have existed simple spectrometers. Now, there are more modern methods of non-destructive testing.

For a determine the presence of Al. you can use the X-ray fluorescence spectrometer in these tokens.
In any major city, you'll find a few companies that use such devices. For example - a collection point for scrap metal.

Checking the sound - this will be an interesting experiment but it is necessary to invite a teacher of music.

[Figleaf]

Interesting machine. Can it be bought privately or is it much too expensive?

Peter

[Gusev]

Interesting machine. Can it be bought privately or is it much too expensive?

Dear Peter.
Of course you can freely buy.
For a one-time examination it will be expensive.
http://www.ebay.com.my/itm/Pocket-III-Spectrometer-/122042583495?hash=item1c6a4e09c7:g:1AQAAOSww9VXgkyS

[bgriff99]

Bronze technically is any alloy of copper, including brass.   Brass is specifically an alloy with zinc, but may include smaller amounts of any metal.   Conventionally, for Chinese coins, those with added zinc at any level are deemed brass, despite the coin may still be mostly lead and tin alloy.   

Both the bright metal color, and moderate oxidation color of customary brass and aluminum bronze are slightly different.   Cartridge brass is 30% zinc, 70% copper.   At that level it is most easily forged cold, and therefore also works best for striking coins.    Industrial aluminum bronze is 5% aluminum.    Obviously a couple percent more, or less than that would greatly change the color.   Striking properties probably too.   This coin is aluminum bronze.     

[Gusev]

Both the bright metal color, and moderate oxidation color of customary brass and aluminum bronze are slightly different.......... This coin is aluminum bronze.     

You're right - the color of brass or aluminum bronze is not very different.
If we evaluate the color of the alloy will be difficult to make the correct conclusion.