How does an ancient Greek mosaic get from excavation site to museum? [video]

I vacationed in Rome several years ago, and I took the subways to Ostia to see the ruins.

The park is extraordinarily blasé about these tile mosaics, of which there are many. They asked that, if you walk on them, you do not walk on any portions that are cracking, frayed, or otherwise delicate. They are piled up in sideways stacks in several places around the park.

Ostia is quite an experience. They have the best and most complete ruins in the vicinity of Rome, and Ostia is second only to Pompeii in the size of the collection and site.

Anyone know what the white material they covered the mosaic with is? Then later washed off with what looked like hot steam? I'm guessing some sort of plaster or glue to protect it during transportation.

Can anyone explain why the stone structures of the acropolis aren’t coated with the stone sealer I buy at Home Depot?

Thanks.

What does the mosaic get covered with that then gets peeled off using water?