A length is gradually warmed in the center, finally heated at that point until soft, drawn out, cut apart and annealed.
"Laboratory Manual of Glass-Blowing"
Francis C. Frary
This must be carefully annealed.
"Laboratory Manual of Glass-Blowing"
Francis C. Frary
The parts should be annealed by holding above the flame for a short time, to cool slowly.
"An Introduction to Chemical Science"
R.P. Williams