Evidence based speculation, page-44

I have refined gold and silver using nitric acid and hydrochloric acid, it turns gold into solution and everything else settles to be decanted off. Bi carb soda added neutralises the acid then sodium meta bisulphate drops the gold out of solution which looks like brown mud until melted.
Silver is nitric acid. There are products we all have at home that works but I have not tried them.
Btw it takes very little nitric acid to dissolve gold.
Silver I use electro winning which you let current do the work for you and watch pure silver crystals grow.

Anyway I'm sure the labs and bbx have better ways than what I'm doing at home.