Glasses at Risk of Compatibility Shift

Many people take their fusing glasses beyond the tested parameters of the manufacturers in pot and screen melts and combing and casting operations. It has been speculated that there are compatibility shifts of hot colours and of opalescents.

Reading, and some experience, lead me to the belief that is the colouring minerals that are the key to which glass will shift in compatibility. Colours made with sulphur and selenium are more likely to opalise and also change their compatibility at extended times at high temperatures. Extended time is in the region of an hour or more. High temperatures are those over 850ºC

The colours at most risk of compatibility shift seem to be:

Reds

Oranges

Browns

Ambers

and a few bright and olive greens, but not dark greens.

http://www.warmtips.com/20070207.htm

Of course testing, using polarising light filters, is required to determine which will remain compatible after long, high temperature firings.  A method of testing is given here.

High temperature compatibility shifts are discussed here.