Over Annealing

Sometimes the statement is made that you can never over anneal.  This statement is true only under certain circumstances. 

Annealing

The statement is also dependent on the understanding of what anneal means.  Annealing is the process of stabilising the temperature, ensuring the piece is at the same temperature throughout, and then gradually cooling the piece to avoid heat shock.  This is to point out that annealing is both the soak and the slow cool.

Long Soaks

Long soaks at the annealing stabilisation temperature can be injurious to your piece if the temperature in your kiln is not even.  This can mean that one or more parts of your piece are at different temperatures. This sets up stress within it.

Placing

You can reduce the possibility of stress by placing the piece at the centre of the kiln or avoid placing the piece in the cool spots of the kiln. 

Cool Rates

Another method of avoiding locking in the stress to the piece is to reduce the cooling rate to less than normal.  This will reduce the temperature differential within the piece.

Mass Being Cooled

In all this you need to remember that the anneal cool rate is relative to the mass of material to be cooled.  Therefore, a thick piece needs a slower annealing cool than a thin one.

But it is not just the thickness of the glass to be cooled.  You need to think about the mass of the kiln shelf or mould that supports the glass.  An example is that glass on a ceramic shelf needs slower cooling than one on a fibre board shelf, because the mass of the shelf needs to be taken into account as well as the glass. Connected to this is whether the shelf is on the floor - slower cooling - or supported on posts, allowing air to circulate under the shelf.