Aloha Barbara!
I've bought many OLD collections over the years & there isn't any easy
answers. Basicly larger non-porcelan surfaced shells I put into the
dishwasher. This works great w/Tritons & Helmets. Soaking in light clorox
solution, say 20%, works well on most shells.
Sometimes brushing w/soap & hot water helps if there's a oily film. A
combination of cleaning methods works wonders. Last year I acquired a HUGE
collection from the 1950's that must number in the thousands and after
cleaning most of the World Record Sizes & other rarities I still have 90% to
ponder over. I wish you luck.                Mahalo! Chris Takahashi