It was “sick”, you clean it up with either nitric acid or boiling and re-condensing it out (both of which you need to know what the hell you’re doing before you try it).
“Sick” mercury has got dirt on it, and a film of oxide. It does poorly at amalgamating gold with that layer, and it has to be cleaned to be useful.
Back “in the day” there were copper troughs on the crush end of some stamp mills for gold ores with sizeable (as opposed to microscopic) free metal. You would take and rub the clean trough down with mercury until it was silver with it, then crush the ore and have water washing through the crushed down the trough. After X time of operation you’d stop, get putty knives, and scrape off the mercury/gold amalgam from the troughs and wad it up into putty like balls for placing in the retort to boil off the mercury and get the rough gold out. The chimney of the retort was often shaped in such a way and high enough that the mercury would condense out and run down the inside in a small track.
You’d lose some that way, of the ~90K tons humped over to the Comstock/N.NV mines they lost about 10% in production.
Yes...that is what we saw, it had a nasty film and stopped looking nice, clean and shiny!
I have a degree in chemistry, never used it, but I did get some nitric acid on my skin one time, and it sucked. I got a large swath of skin that turned hard and dark brown, and stayed that way for a long time.
Oddly, I don’t remember it blistering, but I remember it hurt like Hell.