Lowest Voltage for Electro Etch

In reference to the electro etching technique utilising a DC power
source, what would be the lowest voltage/amperage needed to etch a
small piece (40mmx40mm approx) of st silver? I intend to purchase a
DC Lab Power Source and some only go up to 3amp. 5amp and up just
gets alot more expensive and if its not necessary for smaller
pieces, its worth saving some money. Can anyone shed light on this?
In relation to the electrolyte solution, I have read much about
ferric nitrate but Im also kinda curious about phosphoric acid as
perhaps being more user friendly. As mentioned by others, the upside
of electro etching is its relatively enviro friendly nature but when
it comes to etching silver, can the same still be said if one must
use an electrolyte such as ferric nitrate? Can anyone enlighten me
further on this subject?

Many thanks
Suzi