It seems as though every time I try to fire irridized glass, even though it states it is high fire, I fuse it at 1440 but still get cracking in the surface, some may call it crazing. Does anyone else have this problem?
This is natural and will increase with further firings of the piece. The irridised surface is a thin coating of metal that has different expansion and contraction characteristics than glass does. So yes, everyone experiences this.
I have no idea what a low fire irridised surface may be. Maybe it is the kind used by Spectrum art glass that fires off at lower temperatures, rather than the S96 irridised glasses.