It is possible to break the glass in heating it up by going too fast during the initial temperature rise. How fast you can increase the temperature is dependent on how even the heat is within your kiln. So any suggestions have to be tested within your own kiln and setup rather than relying exclusively on others' experience. Some of the considerations relating to the kiln are given in this blog about initial rates of advance.
So with those precautions, I put forward a suggestion based on my experience and information gleaned from the Bullseye site, education section and from Graham Stone's work. These lead me to suggest that the initial rate of advance can be twice the actual or planned first cooling segment. This rate of advance applies up to the softening point of the glass.
So this theory implies that a piece of glass 6mm thick - that might be annealed at 80ºC per hour during the first cooling segment - can be taken up at rate of 160ºC/hour to the softening point. And by extension:
- A 12mm thick piece could be taken up at 110ºC
- A 19mm piece could have an initial rate of advance of 50ºC/hr
- A 25mm thick piece of glass could be taken up at 30ºC/hour.
These all depend on a number of factors:
- how the glass is supported,
- the nature of the shelf,
- the composition of the mould, and
- the kiln characteristics as well as
- the colour combinations and
- whether the piece is tack fused or full fused.
Slower rates of advance are indicated if
- the glass is supported only at a few points,
- or if the kiln is side fired or has cool spots.
- If the piece is tack fused, you need to slow the rates of advance.
- Consider the rate of advance for the next thicker glass as your starting point as a minimum.
Remember that these numbers can only be used as a guide in conducting you own experiments.