Over the past 32 years of fusing glass and after thousands of test firings, I have learned a lot about heating, annealing and cooling glass. When clay is fused with glass the glass dictates the schedules needed for annealing. Stepping through the critical temperature points slowly, is required. The one thing I have learned in all of my tests firings is there are 4 critical zones of firing.
Room temperature to 600°F. To be on the safe side you must be careful not to push the glass too fast up to 600°F. This is also a critical zone coming down. If the glass cracks, this is the temperature range where it will happen. Rate of climb and or cooling is determined by thickness of glass.
To make sure you are well soaked at 600°F I fire to 650°F
Above 650°F, once soaked, you can go as fast as you want and you will not crack the glass.
I can honestly say I still do not know what the Annealing point of the clay is, but I do believe the clay is somewhat flexible because it is not in a totally vitrified state. If the clay were totally vitrified it would be a glass and behave like a glass. The clay is somewhat porous.
In eastern Oregon I visited an obsidian bed where volcanic glass has flowed out into magnificent formations, it is really quite something, to not only see natural glass on such a scale, but to walk through the rough and sharp terrain. The interesting thing is that alongside the glassious rock formations was pumice. The pumice is the same glass as the obsidian but full of microscopic bubbles. The obsidian fractures just like you would think glass will fracture however the pumice breaks in irregular breaks, its structure makes it behave differently. Fired clay is measured by its porosity and it shrinks as it becomes more glass like. I do know it is Important to have the glass annealed properly.
Shoot for a temperature just above the annealing point. Every kiln behaves differently you may not be at the temperature you think you are. Even if you are just fusing glass without clay, Use this method. Glass and your time is too precious to take risks. The annealing point for Bullseye glass is 900°F. I will start annealing at 940°F or 920°F and come down through the annealing point. Objects with dimension will have temperature gradients within the piece of glass. Shoot high 20° or 40°F and hold for required time for thickest dimension, and then slowly move down the next 80°. And then cool. Under 650°F never cool faster than you would go up from room temperature to 650°F.
For ¼ inch thick glass and below, the residual heat from the kiln will anneal the glass sufficiently. When blowing glass I will start annealing by soaking the glass at 1025°F to establish a consistent temperature gradient before coming down.
What stepping looks like for ¼ inch thick fuse:
What stepping looks like for ½ inch thick fuse:
What stepping looks like for 1-inch thick fuse:
Delta T Annealing was introduced at the Tucson GAS conference in 1997 By Dan Watson who was annealing 10 ton telescope lenses. The process was further refined for studio applications by Ray Ahlgren, co-founder of Bullseye Glass Co. He explained the process to me in the late 1990’s.
By the use of three thermocouples you can track the temperatures of the inside of the glass, top of the glass and, bottom of the glass. By controlling the heat in your kiln, you can add heat to the glass where needed to maintain a + or – 10°F temperature gradient throughout the glass.