What is Annealing?

[Stephen Richard]

OK. I just read the link to the description of devitrification. I mentioned annealing, which I have heard many times before but have no clue as to the meaning.

   ________________________________________~Audrey~


Audrey,
There is a long definition of two pages in Charles Bray’s Dictionary of Glass Materials and Techniques, 1995.  I will try to sumarise some of the important points.

Stress is induced into cooling glass through the outsides of the glass cooling more quickly than the interior.  This contraction causes residual stress.  Annealing is the process to relieve that stress.  The annealing soak temperature is determined by a number of factors, of which coefficient of expansion, viscosity, exposed surface, thickness are some.  “The relief from stress happens because of a process of viscous flow.  At the annealing point it can can take place within a few minutes whilst at the lower annealing temperature…. It can take a few hours.” (P.27)

The above statement is applicable to glass of a single colour from one manufacturer.  When combining colours in kiln forming, the colours absorb and give off heat at different rates and so you need to allow more time for the annealing – relieving of heat induced stress – to occur.  So, in the case of our work, annealing needs to be performed over a range. 

The annealing point soak has the purpose of allowing all the glass to be the same temperature from top to bottom, and side to side.  The annealing occurs during the slow cool past the lower strain point – usually about 50C below the annealing point.  The manufacturers give annealing and strain points for their glass.  These should be observed, rather than anything pre-programmed into your kiln’s controller.

Note the stress of incompatible glass cannot be relieved by annealing.

There are more notes on annealing at:
http://glasstips.blogspot.com/search?q=annealing

[Becki]

And a bit more reading on the subject!
http://www.warmglass.com/Basic_Process2.htm

[Ian]

Hiya Audrey
The way I understand annealing is as follows

 Lets take a piece of glass that has been heated to the stage where it is soft and now we want to cool it down.

The glass is the same temp throughout it's thickness so let's take it out of the kiln and see what happens

1 The outer skin of the glass will cool and harden BUT the inner core of the glass will still be hot and soft

2 this inner core will then start cooling and contracting in layers toward the centre of the glass but there is nowhere for it to contract to as it is trapped inside the cooled and hardened outer surface. The second layer now traps the third layer and so on. So now you have all these layers fighting each other and pulling every which way all encased in a hardened outer layer

3 When it is cooled you will either have a piece of glass that has shattered from the stresses that have built up or you will have a piece of glass that looks perfect and then one day just breaks for no apparent reason. [Ever been drying a drinking glass and it seperates into 2 pieces= bad Annealing]

This is where annealing comes in .
The glass has to be cooled down from a certain temp to a certain temp in a controlled manner to try and cool the entire piece at the same rate so that there is no stress left internally. And this part of the firing cycle is called the Annealing cycle.
Ian

[Audrey]

THAT I understand, Ian. Thanks to both of you for your explanations.

[Ian]

Not a problem Audrey.  :laugh:Ian