hope it helps! =)
and powdery picric acid (i.e. dry) is an explosive, so you should be extremely carefully...
the one I use is solution based. and may I know what do you mean by 'etch' the skin? I simply stain the hair with solution. the same concept when u dye your hair. i used it on ICR mice. the solution did not cause the hair to fall off or give any scar. but i don't guarantee the same thing if it is on nude mice.
sorry If i cause misunderstanding
And with etching I meant that it's a quite strong acid and wondered if the marking is an acid burn, but actually it is not (luckily).