On WetCanvas there is a thread from back in 2005 about flame annealing, a topic which was dispensed with in several of my blog entries. From out of nowhere, this little gem showed up a couple of days ago:
I make miniature marbles and beads, everything I do miniature is flame annealed. All it really mean is heating the glass to a uniform temperature, once it is heated to a uniform temperature it is annealed, uniform cooling is a seperate issue really regardless of if it’s annealed in a kiln or in the flame. That’s my two cents but I’ve been flame annealing for ten years.
Hmmmmm…really? Are you sure about that? Perhaps you should go look up the definition of annealing. What you are really doing is heating your marble/bead to a uniform temperature, you are not annealing.
Uniform temperature and uniform cooling are the two parts of the annealing equation. You start by ensuring that the marble/bead/pendant/whatever is at a uniform temperature (the annealing point) and hold it there long enough to ensure that all the molecules of glass have realigned themselves, thereby removing stress. Then, you slowly and uniformly cool the piece down to the stress point (the temperature where the glass molecules stop moving), and then cool down to room temperature.
You see, I could uniformly cool the piece down very fast and set up enormous stress — this is called case hardening and is what is done to eyeglass lenses all the time, as well as tempered glass for automobiles and windows. The outside of the glass cools faster than the inside, freezes, and sets up internal stress. The piece won’t break unless scratched or impacted, but when it is scratched or impacted, the internal stress is released and the the glass breaks, sometimes spectacularly.