I'm the crazy person trying to make lampshades out of wine bottles. The success of my second attempt had me thinking(?) that I had it figured out (successful 2nd attempt @ http://www.glassimpressions.net/bottles.htm
). My third attempt ended up like the first failure: several pieces, apparent thermal shock on the ramp up. There WERE some differences this firing: different bottles(thicker glass), and the position of the form/wok(upside down) was higher in the kiln, to get more heat around the bottom of the shade during the final stages of the slump. The bottom opening of the wok is about 2" off of the bottom of the kiln, resting on 3 fire bricks.
The elements closest to the bottle "disc" edges (disc 28.5" diam., kiln 30" x 60") were baffled with 110J. The disc was centered on the top(bottom) of the wok, the only contact being the "hub" of the disc, where the bottle "openings" touch each other forming a ring in the center of the disc, with about a 2.5" opening in the very center.
Possible reasons for failure: Raising the wok in the kiln brings the highest point ABOVE the position of the temp sensor, which means the initial temperature (50/hr ramp up)is a little higher than with my original positioning. The glass in the disc is very irregular in thickness across the disc, though radially symetrical. Since the ONLY contact, initially, with the wok, is in the CENTER(one of the thicker glass areas), is it possible the wok is heating up and transmitting too much heat to that area? Would a couple of thicknesses of 110J between the glass and the wok help in this circumstance? Or, should I just lower the wok back to it's original position, and drop the ramp up to 25/hr? Any and all information/theories would be appreciated for consideration.