The annealing schedules should only be taken as a rough guide to follow for annealing a piece that is able to cool reasonably evenly on all sides. They are quite conservative, but one should always be guided by the temperature difference, ∆T, between two or more thermocouples in the firing. One thermocouple should be placed as close as possible to where you think the glass will cool slowest and the other in the open air of the kiln close to the centre of the mold. If the ∆T between the thermocouples is growing too large then the rate of cooling should be slowed down.
Ideally the kiln should have elements on the floor as well as the sides. For very large scale work, especially if it is very thick, two or three banks of elements controlled by separate controllers should be employed. One or more of the controllers should be a slave to the master controller. This will keep the kiln’s temperature more even throughout.
The schedules start at the upper temperature end of the annealing range. In the past we have closely followed the original Corning equations where the 1st cooling range gets progressively wider as the glass gets thicker. However taken to its to its logical conclusion once the glass thickness reaches 250 mm (10”) the first stage reaches zero degrees.
We cast 6 pieces at Gaffer in 2009 that weighed 250 kgs each and were up to 225mm thick (9”). Utilizing two slave circuits and 4 thermocouples we were confident that there was no need for the 1st stage of cooling to go all the way to 40oC which the original Corning formula called for. At 320oC we were confident that all of the glass mass was comfortably below the strain point so that we could move onto the 2nd stage cooling. The schedules now have 3 temperature ranges for Stage One cooling where the bottom temperature is 360, 340 and 320oC respectively depending on glass thickness. We think this is reasonably conservative but this speeded up schedule does rely heavily on using two or more thermocouples.
Gaffer casting crystal’s low Young’s Modulus means that the glass will allow a greater ∆T (nearly 20%) than an ordinary non-lead glass because it is more elastic. According to Dan Watson (see the accompanying article on annealing in this technical section which we strongly recommend you read) for 100 psi residual annealing stress levels Gaffer crystal can have ∆T of 1.3oC versus say Bullseye’s glass of 1.11oC. We feel that 250 psi residual stress is an adequate level, in which case the ∆T could be 3.3oC or nearly 6oF.