My guess is that they are
My guess is that they are cut from a 5mm sheet and then have the edges ground.
Probably too much hassle to cast all those wee squares. If they were fused, they’d want to be 6mm and I’d expect to see the top bulge due to surface tension. Also, the edges look a little cloudy, as if they’d been ground smooth.
Just my initial thoughts, hopefully someone more knowledgable will have a better answer though….
J 
