Bringing a mould to the same temperature as it takes to melt the
sterling will destroy the mold.
...unless, of course, somebody knows differently
You could do it with the various investment products which are
designed for platinum casting, as these are capable of withstanding
the temps needed for melting sterling. or one could carve an open
faced mold from graphite. A bitter problem will be the crappy
quality of casting one is likely to get, since this very slow heating
and cooling cycle will give one very serious/deep oxidation and fire
scale on the casting, and will also result in a truely coarse crystal
structure. metal with crystals as large and coarse as this likely
would yield would not be very strong, and might be difficult to get
a good polish on too.
Plus, there are simply easier ways to do simple at home casting...