I buy mine in an 8-pound plastic bottle at my LHS. I'm sure it's not the most economical way to purchase it, but it works for me. I go through this size pretty slowly, perhaps taking a couple of years to finish it. It's a bottle with a tight-fitting screw top, so it's well sealed. However, when my last bottle got to the bottom I found that the last half-inch or so would not make good castings, and had probably absorbed water just from sitting around for so long.
So, unless you are going through a lot of Hyrdrocal or you can protect it some other way, getting a 50 pound bag might not be the deal that it seems.
