They were not found protected in pots, but loose finds. If copper coins are lost in acidic soil they will look like that, (or worst). This is very common with Sogdian coins due to soil conditions there, and I have read parts of the middle east are similar soil conditions.
I have a Sassanian coin with a similar granular surface. I too was told that it was caused by long term storage under ground. Nice avatar Mani. I no longer have this particular coin but it is a neat lion.
