DerbyBorn explained :
Chris Hogg wrote in
:

https://tinyurl.com/yc4nkcrv

Thanks - beyond me!

I have seen it done in a glass factory, making fancy drinking glasses
amongst mass produced items like milk bottles, beer bottles and cheap
molded drinking glasses. For the fancy hand made ones, they had them on
an automatic process, a powered revolving table and a small gas flame
would come in for just a few seconds on the rotating rim. The cooling
glasses would then move along on a conveyor.

In my teens I would cut, bend join and carry out the above process on
glass tube for chemistry experiments. It was quite a satisfying
process, I remember.

If it is chipped, you probably have nothing to loose in trying, or take
it to a neon sign maker, to see if they could do it.