The problem is that pewter (tin), and for that matter similar
metals like lead, if placed on a piece of silver, copper, gold
alloys, or the like, and then heated to the normal soldering or
annealing temps for those other metals, will melt long before
that point, and in essence, burn into the other metals. So if
you work pewter in the same area as you also work silver, you
run the risk of little bits of pewter, such as filings, etc,
being on your silver when you solder or anneal it. Even little
tiny filings will leave deep pits in the silver. Larger bits can
burn holes right through a piece of thin sheet metal. This is
generally not something you wanted to happen. Thus the need to
keep pewter and similar low melting metals (including soft
solders like stay brite, tix, tin/lead, etc) seperate from your
work with higher melting metals.
You don’t absolutely need to have a seperate bench to work with
pewter, if you’re careful to clean up completely when switching
between metals. But it’s a lot safer to use a seperate area if
you can do so. At the very least, use seperate soldering
pads/surfaces, solder pics, and the like… Be sure, also that
your files, used with pewter, don’t retain bits of pewter in the
teeth which could then come out later and contaminate your
precious metals. It’s easy to do by accident, and the scars that
result can be a royal pain to clean up again.
A somewhat similar situation, by the way, also pertains to
working with platinum. In this case, platinum can be similarly
contaminated by filings and other bits of gold or silver, which
will burn into the platinum if heated to the normal soldering and
annealing temps of platinum.
Hope this helps.