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:

  • Anneal Hold: Hold the kiln temperature steady long enough to ensure that the temperature of the glass is even throughout.  This temperature, called the annealing point, varies among different types of glass.  To determine the annealing point of your glass, check with the manufacturer.  Bullseye specifies 900° F for their fusing compatible glass.  Spectrum’s System 96 glass requires an anneal hold of 950° F.
  • Anneal Cooling: Decrease the temperature gradually to minimize the temperature difference between the center and surface of the glass.  This slow cooling continues until the strain point (also specified by the manufacture).   This is the temperature at which any remaining stress is permanent.

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:

  • As projects get thicker, annealing gets longer at an increasing rate.  For example, one half inch of Bullseye glass has a total annealing time of a little more than 3.5 hours .  Double the thickness to one inch and the total annealing time jumps to almost 10 hours.  Double it again to two inches and you can expect to wait over 30 hours.  A four inch piece will anneal for over 100 hours (that’s more than four days!).
  • Most firing schedules assume that the glass is a simple, evenly thick slab.  If this isn’t the case, you should anneal for the thickest part of the work.
  • Consider the placement of the elements in the kiln.  Once loaded in your kiln, is one end of the glass a lot closer to the elements than the other end?
  • Make sure to read any notes that come with a firing schedule.  Bullseye’s firings schedules for thick slabs, for example, state that their recommendations assume that the glass is setup to cool equally from both top and bottom (not usually the case) and, if not, to anneal for double the thickness.
  • Never open the kiln during annealing.  Even a small rush of room temperature air can introduce stress into the glass.

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.

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