What it does:

Controls the rate of cooling to reduce permanent stress in the glass.

Firing Schedule - Annealing

Why it is important:

Imagine a thick block of molten glass, sitting in a kiln, at a toasty 1500° F.  Suddenly, disaster strikes when a nearby lightening strike causes a power failure.

The temperature of the air around the glass falls quickly – more quickly, in fact, than the temperature of the glass.   Soon we find ourselves with a kiln full of 800° F air surrounding a block of 1200° F glass. This (relatively) cool blanket of air cools the surface of our glass block.
We now have a problem.  The center of our block of glass is much hotter than the surface. The cooler surface is trying to contract, but the middle of the glass isn’t ready to shrink.  Even though the surface is prevented from contracting, it still solidifies.

The heat in the interior glass will eventually escape.  When it does, the glass will contract and solidify.  As it contracts, though, it pulls away from the now solid surface.  This results in a permanent stress in the glass , which may result in the cracking of the work.

To avoid this scenario, we must cool the glass gradually to minimize the temperature gap between the inside and outside of the glass.  We call this Annealing the glass.

How it works:

It is impossible to keep all parts of the glass at exactly the same temperature.  First, there is no such thing as a kiln that heats and cools perfectly evenly.  Second, the glass that touches the air will always cool ahead of the glass that is wrapped in more glass.  Does that mean that glass always has some stress?   Absolutely – and that’s okay so long there isn’t enough stress to break our artwork.

The well-tested strategy for minimizing stress has two parts:

To successfully anneal a specific glass project, we need to know two things:

  1. How long do I need to hold the glass at the annealing point to ensure that the annealing process starts with all the glass at the same temperature?
  2. At what rate can I cool the glass so as not to create too much stress?

The science and math behind these answers is extraordinarily complex, so for answers we need to look to a trusted reference.  Here are some good online places to start:

Things to consider:

There are a number of things to look for that may be reason to extend the anneal hold and anneal cooling.  These include:

When in doubt, anneal for longer than you believe is required.  Unless you are lucky enough to have a good friend working in the labs at Corning, there isn’t a good way to test glass for proper annealing.

Since you cannot "over anneal" glass, an abundance of caution is your best insurance.