In last week's digest, Fredericka Cohen made several good suggestions on
measuring and proofing.
Using discount store plastic storage boxes for proofing formed loaves of
bread would likely be safe, as long as the dough does not come in contact
with the plastic.
However, it is not a good idea to use a hardware store paint bucket for
rising dough in the interest of economy.
There are many different grades of plastic, and when a plastic is intended
for non-food use, it may very well react adversely with ingredients in
dough (such as the acids in sourdough, for example). Plastic products that
are intended for food use are generally marked as such, and commercial
products in particular are approved by the NSF. The National Sanitation
Foundation makes sure that items intended for food use have no harmful
characteristics.
Being safe in the interest of one's health is surely worth a few dollars
for the proper container.
Happy Safe Baking!
Carolyn