This MAY work - but if it doesn't at least it's not "chemically"
Get some denture cleaner tablets - generic brand's okay - dissolve at least 3 in a large bowl of WARM not hot water - soak a couple of pieces overnight - see if it loosens the stuff -
The good news for you is that if it can be scraped off, it's not into the glass
a bit more chemical in nature - CLR - but - read the label carefully - you don't want to "etch" or scratch the glass -
Did you consider that since the stuff is on the outside, and on the bottoms, that they might have been set onto a painted shelf and it's just white paint? You could try a little nail polish remover on a cotton swab -
Nice big set - let us know how it comes out - KB