20-30% acid, agitation and possibly slightly higher temperature. The
process produces heat so once it gets going.... be careful with
heating. It is a good acid but don't inhale the nitrous gases, they
are not benign. If you get acid on skin -rinse with plenty of water-
you'll get yellow spot, a kind of burn but it won't kill you.
And you can easily recycle the silver from the used liquid once it