I was saving this particular coin from a heavy verdigris damage which took me more than half a year to deal with - some of the coins have been in olive oil bath more than one year (changing them out when need be).
This coin was completely covered in green - if you look them up on ebay, you can see good examples of them full of verdigris damage. This one turned out to be ok after conservation. Didn't pay a lot for it back then.
I found the string like damage to be interesting. You can actually look through it. Note, this is a cast coin. The Chinese call it the "sand variety". Maybe to describe the coarse details of the "coin"?
Here's a similar coin but struck.
Note the resemblence?
If I am not wrong, the second coin is actually the original coin that circulated, originated from Shanxi Province. The first coin however seems to be a comtempory counterfeit issued by Chinese warlord in Gansu Province. I haven't got to the point of researching how bad the Gansu economy became but I'm suspecting that because they needed a lot of money to finance, possibly military.
Both coins are actually quite easy to obtain and you don't have to fork out a lot of money to collect interesting pieces of history.