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    Darkened Farthings

    Farthings issued between 1897 and 1917 were chemically darkened so that newly made examples with a good lustre could not be passed of as a Half Sovereign's. It may seem a little unlikely that any one could make this mistake but when you take into consideration the fact that the Farthing and Half Sovereign essentially had the same designs and the diameter of a Half Sovereign is 19.5 millimetres and the Farthing is 20 millimetres it seems more plausible, however the weight of a Half Sovereign is quite a bit more than a Farthing. The Half Sovereign weighs 3.99 grams and the Farthing weighs 2.857 grams. The darkening took place after the Farthings were struck they were turned black by exposing them to a gaseous acid which when it reacted with the surface layer of bronze turned the coin the desired black colour. The acid used was Sodium Thiosulphate (Na2S2O3) it was used as it did not react through the whole coin, the black substance left on the surface caused by the reaction between the bronze and the Sodium Thiosulphate is copper sulphide (CuS). It does seem as well like a lot off effort to avoid mistakes which are unlikely but still possible but it is worth remembering that a Farthing is worth 480 times less than a Half Sovereign.