Sounds as though this may be the stuff used by clockmakers to silver
a brass dial. If that is so, it will consist of variable quantities of
silver chloride, sodium chloride (salt, shouldn’t be much of this)
and sodium tartrate (“cream of tartar”). May also be some inert
filler as well.
The basic process is to thoroughly clean a piece of brass, mix some
of the powder to a stiff paste with water, and rub it onto the wet
brass, using fingers (traditional) or a cloth (more sensible). A
very thin film of silver is laid down, but it will be streaky and
grey. Rinse off the paste, sprinkle on sodium tartrate (buy it from
the grocer) and rub gently, to achieve a matt silver layer. All this
is done with the brass wet. Rinse, dry and lacquer. The silver is
much too thin to be of use on jewellery items, and repeating the
process does not thicken the layer, unfortunately. Any atempt to
polish the matt silver to a shiney finish is likely to result in total
removal if you do anything other than careful burnishing. It really
is only suitable for clock dials.
Kevin (NW England, UK)