Thank you very much for all
Thank you very much for all your replies, I think I have now solved the problem. My kiln controller gives time per segment and temperature to be attained at the end of the segment (it’s an Italian kiln!). The more conventional glass kiln controllers give a rate of temp increase, a target temp and a hold time. Coupled with the fact that my European kiln working in Celsius rather than Fahrenheit has led me to drift somewhat from the ideal schedules as it is difficult to immediately compare them. The nice lady at Spectrum Glass sent me some new schedules which I have translated for my kiln and for the first time in ages I have fired a piece of System 96 white glass without devit!
Again, many thanks for all your thoughts and comments, they have all been read with interest.