Adventures in etching I

From one of the members of The Steampunk forum I got the instructions for etching metal.It is a very simple process:put half a cup of salt in water(I used about 500 ml),get a battery(or batteries)or a solarcell,use some clips,put a nail or screw on the - cable and put the material you want etched on the + cable.
Of course you have to cover the part you don't want etched with tape(or something else that can be removed easily).
The results I got so far are mixed:a piece of iron worked pretty good,copper not so good(but that might have been the particular kind I used) and aluminium gave some interesting pitting.
Next test I'll up the voltage,maybe even to 12 volts........