They didn’t leak; had they leaked, they’d have never made it even a few miles. Besides the exterior pitch, they were built to remain tight while being flexible in the rise and fall of the water.
sorry, I read it as loose grain used as base for amphorae, not as grain in jars.
Yeah, the waterproofed plugs retained integrity even after being submerged for so long.
Something else about these clay containers. If some that were NOT glazed were aboard ship then ropes could be afixed and one could be placed into the sea and water would pass through to the interior and maybe produce drinking water by filtering out most of the salt.
Might be somewhat brackish but I’d give that a try as a supplemental supply to the shipboard stores. Kinda the reverse of a garden pot.