How does one break a glass bottle and have smooth edges

Kmccall

Okay, here is a question that I have not been able to get answered. My Grandmother, when she was alive made crafts out of broken glass. The goal was to have broken glass with no sharp edges. These are the steps. First she would heat the bottle in the oven. Then she would remove the bottle using a broom stick. She placed the bottle in a bucket of ice water. She told me this would prevent the glass from having sharp edges. So I tried doing this and I was not successful. She is not alive for me to ask her what steps I may have missed. I have talked with other friends and they recall this art form. The glass is all broken and there are no sharp edges and the glue the small chunks together in the shape of a Christmas tree on plywood. I know this is long and drawn out, but if there is anyone in the physics forum that can help me it would be so helpful. I am an artist and I would love to use broken glass with no sharp edges as a medium for my art.
Thank you for you time in helping me with this problem.
kathleenmccallartist.com

edward

Grandmothers are so special.

I think that the temperature that the glass is heated to would be crucial. It may take some trial and error. And be careful. I couldn't find anything on Google.

I am not sure that current glass bottles are of the same type as your grandmother used.

I have used silicon carbide sand paper to smooth pieces of sharp glass, so that I could glue them to a rock. Be really careful with this also. I wore heavy welding gloves.

I also have read that pieces of broken glass can be smoothed off in a rock tumbler (polisher).

Heating pieces of glass in a kiln can also make the edges slump.