Regarding this coin, why would the Mint blacken the coin? [solved]

Hi folks! Regarding the above coin, from 1923 to 1942 they were minted blackened by the Pretoria mint. I wonder why this might be so. I have the opportunity to bid on a beautiful red 1942 example. How might the fact that it has escaped this blackening affect the price?

 Any information would be greatly appreciated.

 Thanks, AY

https://en.numista.com/forum/topic18157.html#p139081 

 The chemical used was Sodium Thiosulphate - 

to make them darker so they could not be confused with gold coins. 

Indeed, bronze can be quite yellow when new.

I have a 2012 edition of Hern's Handbook on South African Coins & Patterns, and this is what the King George VI farthing page says:

Prices are obviously in SA Rand and 12-13 years old. Hope this helps.

 Hello, All! Thanks for the information given here and in the links provided. It is now clear to me why and how this blackening was done. I'm going to go for the nice 1942 example. Wish me luck, I'll need it with all the South African ex-pats, now in NZ. I'm happy they're here. 😊 Many are Doctors, Nurses, Psychiatrists and other professionals that we desperately need in NZ.

 Much obliged, AY

Edit: to add last sentence.