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Yes there is some stress between the blue and grey in the second photo. To determine whether this is annealing or compatibility stress, you need to run a test.
The following is a method that should give you answers.
http://glasstips.blogspot.co.uk/2009/08 ... tests.html

On a piece that has no stress, I would expect to see only dark. If there are clear spots originating from an edge or around a piece fused into the glass, that would indicate stress of some degree. Coloured indicates extreme stress.

It is also unlikely to be very successful ever, as older window glass was produced without any concern for compatibility between batches. It will be best appreciated used for projects as cold glass not kiln formed ones.

Any light bulb. Put the glass between layers of polarised light filters. The steress will show up as light areas.
Method here:
http://glasstips.blogspot.co.uk/2009/07 ... lters.html

If the pieces fit - glue it. That is the least intervention. Only then try flatening it, as that has a greater intervention, and probably less chance of success.

Not a good idea in my view

I think you have to remove that layer by grinding or sandblasting.
You also need to think about why the kiln wash stuck. Common reasons are: firing too hot or too long, firing a second time to tack fusing temperatures or above on the same kiln wash.

Brad beat me to it. They are simple to construct and glue together.

Mike, I know you are an experienced worker. However, I have found a number of people who have been cutting glass for a long time tend to press too hard on opalescent glass, listening for the sound of the cut rather than feeling the pressure. Most opalescent glass does not make a sound when scored wi...

A simple rule of thumb that does not always work Qom course, is to anneal for about wide the thickest part of the piece. You can look up th s heddles for thick pieces on the bullseye site (they apply to any glass)
http://www.bullseyeglass.com/methods-id ... slabs.html

If you look at the edges of the glass you will see that one side is darker than the other

Havi,
The heat originates at the bottom at the burners. It the flow up to the top of the kiln through baffles. So the radiant heat is effectively from the top.

There is no reason why glass would not fire in a gas fired kiln. I have found that I can fire at least twice as fast in a gas fired kiln than an electric one. The heat in a gas ceramic kiln is coming from the top and so acts more like a top fired glass kiln. If you fire fast, you will not have a lot...

Or you could do as jewellers do and use a pickling solution.

Yeah. I thought you were in the UK ( one of the drawbacks of not stating your location in your profile)
Any small tile cutter will do. Your local DIY store will have one and it will do.

I use a simple one from B&Q. The larger one is about £50. The smaller one is £40 or so, but the extra is worth it for the slightly larger depth of cut

Anthea,
You can contact me at info@stained-glass-supply.co.uk for polarised light filters if you like, but as Brad says, they are for use after fusing.

Surface texture on flip and fire: I find that a good scrub with a stiff bristled brush is sufficient to clear off any thinfire, shelf wash, or ceramic fibres. Then a sufficiently high firing is required to alter the surface - for me this is the region of 775ºC for a couple of minutes, done at the ra...

Laurie,
I don't know what ceramic fibre board you use, but mine holds together. It is not hard, but certainly firm. Even my 3mm ceramic fibre paper does not turn to dust.

OK. Try a circle of fibre board a little larger than the kiln post. This may keep the glass away from the side of the post. You probably will have to dig out the fibre board though.