Old Postcard Silver Patina

I'm trying to identify the patina that is on the surface of this old
photo-postcard. It looks to me like (and probably is) silver when held to
the light at an angle. What would cause this? Bad fixing? Could it have been
deliberate? The card dates between 1910 up to the first world war. Purchased
recently in a market in Zagreb for $1. There is a mark on the centre right
which shows the original black tones. Possibly a scuff. Is it
possible/advisable to clean the silver off this card? Is it possible to
stabilize the image using modern stabilizers? Thx. Any history on
photo-postcards would also be appreciated.

~m

http://www.mondotrasho.ca/pages/alt-photo/cards/Postcard.jpg
Received on Wed Jan 4 21:53:23 2006