Is this a fourree? Or just bad silver?

I wonder since the seller has not labeled it as fourree:


Source: https://www.vcoins.com/en/stores/ho..._constantinople_977989_ad/634087/Default.aspx

This is a fouree since silver cannot transform into that condition. - Right?

 

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What is the typical fineness of these coins? Because billon certainly can corrode like that.

 

I call mine fourree but it is a fair question. I believe the originals were reasonably good silver.