Anneal Cool (Stage G)

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Anneal Cool (Stage G) is the seventh stage of the eight-stage firing program; it is the phase in which the piece is slowly cooled at a specified rate from the slow-cooling point (482°C) to the distortion point (442°C, based on Bullseye COE 90) (§8.1). In η-T-t trajectory terminology, this corresponds to the “pre-cooling” phase. If the cooling rate during this stage is too fast, the surface will solidify first, causing tensile stress to accumulate while the interior is still contracting; this stress is then locked in as residual stress. The standard setting for Bullseye COE90 is a cooling rate of 80°C/h down to 371°C; however, for thick pieces, a rate of 40°C/h or lower is required.

The G stage is the phase during which the workpiece passes through the final temperature range—between the slow-cooling point and the strain point—where stress relief is effective; cooling control during this interval determines the final stress state of the workpiece. After stress relief is performed during the F stage (annealing soak), the workpiece is cooled even more slowly during the G stage to prevent the accumulation of new residual stresses during cooling. Rei Chikaoka (2021, p. 158) minimizes stress accumulation during the G stage—even in thick pieces—by incorporating an additional hold at 371°C for 30 minutes at the end of the G stage. Although this additional hold is not included in the standard Bullseye program, it serves as an effective safety measure for thick pieces.

Related Concepts

Understand the overall design of the cooling phase, taking into account annealing soak (Stage F) (firing-stage-f), final cooling (Stage H) (firing-stage-h), the strain zone, the polariscope test, and the Adams-Williamson law.