May 03, 2010

What is Annealing? Why is it important?

If you've ever shopped for handmade glass... you'll often see "glass has been properly annealed".
But what does this mean?  Why should you care whether or not it was annealed?

"Annealing is a process of slowly cooling glass to relieve internal stresses after it was formed. The process may be carried out in a temperature-controlled kiln.  Glass which has not been annealed is liable to crack or shatter when subjected to a relatively small temperature change or mechanical shock. Annealing glass is critical to its durability. If glass is not annealed, it will retain many of the thermal stresses and significantly decrease the overall strength of the glass.

The glass is heated until the temperature reaches a stress-relief point, that is, the annealing temperature at which the glass is still too hard to deform, but is soft enough for the stresses to relax. The piece is then allowed to heat-soak until its temperature is even throughout. The time necessary for this step varies depending on the type of glass and its maximum thickness."

In my studio I anneal all of my glass... when the glass is cooling down from the highest tempurature (1465F for a full fuse) I hold it at 960F for an hour before letting the temperature come down back to room temperature.  Annealing time is longer if the glass is thicker.

This ensures that the glass will last much longer and be very strong and stable.

So next time you're interested in shopping for a hand made piece of warm or hot glass... make sure it has been properly annealed!


  1. This is also true of metals, and I could explain in detail just how this works on a molecular level - but I don't anyone really cares :) Nice explanation for laypeople though.

  2. Very cool. I didn't know that. Temperature is very important when making candles as well. But annealing sounds stressful! lol