Hi Dietmar..I use a NOrtel Minor burner, I just have problems with Green glass-Always have. I either go too hot or too cool
The bell core on this one was just steel wool wound in a ball, dipped 2 x's in bead release and then flame dried...this was the first attepmt. I had better l uck with this than witht he complicated cores I made from the suggestions here. The other cores I made that were are dried, sanded, dipped (about 4 times) didn't work for me at all.
This one was shaped with tools. I laid down the "petals" with a strip of glass from the top of the mandrel down the core and melted it in and finished shaoing with tools.
I had made 3 other bells at the same time, and all of them broke when I tried to remove the core after anealing.
When I worked this one I was very careful to keep it evenly heated so I didn't have to attempt to repair anything!
good luck with your cores. I don't think I will attempt them again..at least for a wile anyway!