Chemically darkened farthings

British farthings used to be chemically darkened too make them black so that they could not be mistaken for half sovereigns which were similar in size and design, this seems to be common knowledge but what i cant seem to find out any where either in books or on-line is how they were darkened and what chemicals were used any reference to this subject i find just states that they were chemically darkened.    Any help would be much appreciated  Alastair

This won't be useful, but,

I've noticed coins from Baroda in India that have darkening, as well as a Fil from Iraq.

http://store.metalliferous.com/

This might be a place to start on chemical darkeners.

Also 2 clicks coins.com might tell you a little bit about coin toning.

I always thought it was a liquid, then a while ago I heard it was a gas. This is also what is on ...

http://www.aboutfarthings.co.uk/A5%20Artificial%20darkening.html

There are also two coins on their page, showing the difference ...

http://www.aboutfarthings.co.uk/Farthing%20-%201918.html

Thanks for the responses who would have thought an acidic gas was used, now that's a production line i wouldn't want to work on. Unfortunately though it dosent say what sort of acid was used.

Well fancy that...great website ZacUK. Thanks for the information in a way correcting me, or at least informing me: which is great I hasten to add!

http://www.rsc.org/Education/EiC/issues/2006Nov/AncientCoins.asp

   All sorts of interesting chemical / coin information on there; including the part ...
"Bronze (Cu-Sn alloy) farthings issued between 1897 and 1917 were darkened using Na2S2O3, resulting in a surface layer of copper sulphide. This was done to avoid confusion between a newly minted farthing and a gold half-sovereign, the latter being worth 480 times the former."

  So the answer is: Sodium Thiosulphate. The formula was copied and pasted but the lower numbers of 2 2 3 appear now on the same level as the Na S O letters. See the link given for how it is normally.
  Five elements of Copper [Cu] Tin {Sn} with Sodium [Na] Sulphur [S] Oxygen [O] leaves most of the bronze coin as it is, with just a layer of copper sulphide [Cu2S or CuS] - so the sodium and oxygen leave.
  The measurement of a half sovereign was 19.5mm and a farthing was 20.0mm. The half sovereign was heavier at 3.99gm and a farthing was rather lighter at 2.857gm (1895-1901) or 2.834gm (1902-1910).  

Cheers its a question that's been bugging me for a while now, its good to get such a comprehensive answer.