I tried it before and it didn't work for me. Yours look great.
I tried a few times today and failed and finally found the magic temperature.
It's 500 degrees for 20 minutes - I did them on a cookie sheet lined with parchment paper and just slid them into an ice water bath right off the cookie sheet and it worked like a charm.
I tried 350, 375 and 400 with no luck. Trying to heat them multiple times just resulted in breaks. 500 did the trick!
Thanks for a beautiful idea!! I love these so much. I'll bake the rest of the two pound bag tomorrow - LOL!!
PS - mine are floral marbles from Hobby Lobby - the flat backed kind.