Fair estimate of the gold content

Years ago when the prices sailed up in gold I bought scrap from
individuals. Being in a very rural location, everything was what ever
individuals managed to scrape up, from old gold dental crowns to
broken rings. I made a testing plate out of black slate and marked it
with the diferent gold karats by streaking the plate. That is,
drawing a karat gold down the plate to make a mark. Then I took the
unknown gold scrap and drew a line near each of my karat marks,
using nitrite acid on a toothpick, I then touched the marks. The rate
of the alloy dissolving and the amount of visiable residue left on
the ‘plate’ usually gave a fair estimate of the gold content of the
piece in question. I have also used a slab of black jade as a streak
plate… At least my customers were satisfied in seeing this happen
since they were able to see the same process enacted with my own
marked and known karat gold streaks.