Sorry for taking so long to reply to your question. I have just moved
to Botswana and it has taken some time to get set up with e-mail here.
Phosphoric acid definitely does etch fine silver. I have used 10%
solutions to do this a number of times. I guess that there is not
enough of the stuff in coke after all. Hope this helps.
Nils Schwarz, on the edge of Sua Pan in Botswana, where the
temperature at 20:30 is about 30’C - Time to get an aircon!!