This thread is annoyingly familiar to me as it’s a problem I also
suffer from. I get this problem when making my own sheet -though not
every time - but I have encountered it with purchased stock too. I
assumed that I had overheated the metal, causing the blistering. Now
I’m wondering if I’m getting gas bubbles when I’m melting my silver.
Possible lightbulb moment! I’ve been coating the silver to be melted
in Pripps flux, whereas I used to drag it through the sludge at the
bottom of my super-saturated boric acid in alcohol jar. I think I’ve
only been getting this problem since changing flux. May be
coincidence, I’m not sure.