Question

the great bath was made waterlight with a layer of

Answer 1

The floor of the tank was watertight due to finely fitted bricks and mud laid on edge with a kind of plaster and the side walls were constructed in a similar manner. To make the tank even more watertight, a thick layer of bitumen (waterproof tar) was laid along the sides of the pool and presumably also on the floor.

Answer 2

basalt.......hope it helped you