Why are some farthings artificially darkened?

The decision to artificially darken farthings was taken in 1897, due to the fact that on occasion uncirculated farthings had been passed off as half sovereigns. The method adopted was to use acid fumes to artificially darken the coins giving them a “black” appearance, however on some coins this has resulted in shades down to light brown. This method of artificially darkening farthings was undertaken on farthings dated between 1897 and 1917.

Freeman M.J. states that “very occasionally a farthing which had not been thus treated escaped from the mint”

These varieties have not been included in this catalogue due to the fact that I consider them to be error coins rather than varieties, with only the years where we know that both issues were released intentionally classified as varieties.