I'm also curious about how you sanitize the old bottles..Do you cook everything in boiling water like when Grandma used to can fruits/vegetables? or can you just wash and do a final rinse with ETOH? or Everclear (190 proof Ethyl Alcohol)?...Maybe even throw in a chunk of dry ice to add to the carbonation if it goes flat.
Everyone has their own process. I let my bottles soak in a solution of oxyclean and warm water to
clean the beer out. Then scrub them with a bottle brush to get everything out. You can then put them in a dishwasher and run the dishwasher on the dry cycle to heat the bottles up and kill most bacteria. I've done that once when I first started out, but don't anymore. My step after the oxyclean step (and after checking the bottles are clean) is to dump each bottle and fill them up with a solution of sanitizer. Home brew stores sell different kinds of sanitizers...many of them no-rinse sanitizers. You'll learn quickly that sanitizing is the most important process of brewing your beer. I'd rather spend the extra time sanitizing than have an entire 5 gallon batch destroyed because of mold.
be careful with dry ice. The idea behind carb'ing in bottles is that sugar is added and carbonation is slowly absorbed by the beer over several weeks. I think dry ice would be more of "shocking" the beer...not to mention you need to be
very careful putting dry ice in a closed container. Google "dry ice bombs"
Too much and you could have an explosion that could get you hurt.