Question about gilding process at the Soho Mint

koincollect

I was looking at some Soho mint gilt coins which had a slight rub or impaired and they all show a black surface on the high points. The coin itself is copper or bronze and then the gilt is applied but then why does the rub on the high points look black? Is there anything applied to the coin prior to the gilding process? There was one interesting thread which I was able to search but did not give me this answer, though the possible process was discussed.

https://forums.collectors.com/discussion/876473/was-this-coin-gilt-gold-plated-before-or-after-striking

I was wondering about this for sometime but recently came across a gilt proof piece which seems to have some of the same black impressed on parts of the reverse of the coin and got me thinking if this was something done at the mint. I will try to get a picture up shortly.

Thanks!