cutting glass bottles

I discovered something which may be of use to some of you. I got one of those butane blow torches they use for cooking. I suspended a bottle from a string with the aid of a cork with a hook in the centre. I set it on a steady spin and then aimed my fine flame so that the tip of the blue flame was touching the glass. I gave it about 30 seconds then touched the bottle as it was spinning at the point I had heated it with a fine paintbrush that was slightly wet, there was a "ping" and the bottle split into 2 with no fractures. I want to try this on a demi-john on acid carboy..........