Flip any plastic container over and it'll have a number in a little triangle on the bottom that identifies the type of plastic the container is made of. The best for our purposes HDPE or #2. Those 5-gallon white pails that salt mixes come in are HDPE.
There are many "food grade" plastics that are unsuitable for this hobby including polystyrene (#6), for example.
I think the clear Rubbermaid containers are polypropylene (#5) which is another good choice.
http://www.americanplasticscouncil.o...ID=313&DID=931