I just purchased the double candle mold. Yesterday I fused a sheet streaky glass with a piece of clear. When this cooled, I tried to slump it onto the candle mold; the glass broke! I thought that maybe the glass was too heavy because I fused a piece of clear on top before slumping. I tried to slump using just one 3mm piece of glass over the candle mold, it broke!! What am I doing wrong???