Hi all, just joined up.
I also worked at a aluminum foundry (auto wheels) for 18 years. We had quite a few times were we had explosions.
You all are right about moisture getting under the liquid metal. Most of the time it was people putting wet gloves or trash in the scrap hoppers. They were then dumped in to be remelted. From what we could tell the object was colder then the metal and would get in-cased then the steam would start to build (think hand grenade)
As for things dropping on top. not a good idea, but not as bad. most would just burn off. Snow would come in through the roof and fall into the open furnace (Michigan) Someone came up with the idea of adding potatoes to help degas / flux the metal. Still not a good idea. When you add bees wax to the top of the lead to flux it, that's what your doing,,, adding a liquid to the top of the metal... just like a sweat drop.
Another source was cold / damp sow molds (1,300 lbs) that were pulled inside to be filled. They would get about 1/2 full before they would pop. All the sows that were added to the furnaces would be set on the ledge to get hot, then pushed in with the next one when needed. Steele sample molds (3" round, 1/2" thick) would reach the roof if it was wet. If a low psi. mold would leak on a pour and spill to the ground, the concrete would have enough moisture to flip over a 4"x 3' x 3' thick cooling sheet aluminum.
I don't want to come across as a know it all, still got a lot to learn and been checking out some good stuff here. Just be safe. BT