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I have knoticed on some of the glass we temper there is kind of a rainbow (burn color) effect near the corners of the glass, around the holes in the glass and sometime near the cut-outs in the glass.
Any Ideas???
It does not happen to all of the glass so my first thought is "over heating"...
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If the glass has a coating then yes, these areas are probably suffering from overheating and burning the coating.
However if the glass is not coated, it is more likely that you are seeing iridescence caused by the varying levels of stress around the corners and holes when the glass is viewed with reflected light.
The stress levels in the glass will change around holes and cutouts because, during heating, the edges act like surfaces and absorb radiation making the edges slightly hotter than the surrounding glass. During quenching the hotter temperatures will result in a greater temperature difference between centre and surfaces and therefore produce a higher level of compressive stress in the surface immediately around the hole or cutout
Note: Iridescence is an integral side effect of the tempering process and is apparent in all tempered glass to some degree. It is seen when polarised (reflected) light hits the surface of stressed glass causing a colour change in the light to appear where the stress level changes.