Tonight I've been playing around with marbles in the microwave kiln. It's still cooling so I'll get back to you with the results later.
While doing a little research for said project I came across this image.
Turns out that these are marbles that were fused together during the Great Chicago Fire of 1871. How cool is that?! To see them in real life you have to go to the Chicago History Museum.
It reminded me of camping during the summers in the USA and how we could melt beer bottles in the fire over a period of a few hours. I forgot that fire could do that.