Actually… The action of heating the sterling causes the copper
atoms in the surface metal to oxidize. Pickling removes these copper
oxides leaving a very thin surface of pure silver. A series of heatings
and picklings will, thus, deplete the outer surfaces of copper. This
very thin, pure silver layer melts at a higher temperature than the
internal material which is still sterling silver.
As you heat the material to reticulate it the inner sterling silver
melts and is held together by the outer layer of fine silver. I’ve
seen a master silversmith reticulate by heating the sheet up and then
thumping the table to cause waves to pass through the metal and then
removing the torch and freezing the material.
So it’s not copper on the inside and silver on the outside.
Copper melts at 1981 pure silver at 1761 and sterling at 1640. You
can see that if this were the case the copper inside would stay solid
and the silver on the outside would melt and slough off.