Why does a glass tube show white bands after being heated with a torch? When I use a torch to split a glass tube, there's always a hazy white band which appears. Does anyone know what the cause of this is? 

It was a hydrogen-oxygen torch. The glass is fused quartz glass, which should be pure silica.
It is sublimed silica according to the following reference at page 8.

http://www.public.asu.edu/~aomdw/GLASS/book.pdf