I just read your posting on trying to etch silver plate over nickel.
Your solution is probably too strong to be the most effective. You
need to get your solution down to 10%-30% strength. Use distilled
water, not tap water; acid into water.
Now then, to rain on your parade--no one who hasn't had *chemical
hazard* training and access to a fully equipped lab setting, with an
eye wash station and dump shower, should be using the highly caustic
acids. There is just too much potential for a serious accident,
particularly with your lung tissue. Please consider using a safer
alternative, such as ferric nitrate. It has a slower etch, yes, but
the nice advantage is that it will stop when it gets to the nickel.
Useful for multiple metal laminations, say when you want a contrast
situation, or to remove some material in mokume, so there is more