設計・制御
Final Cool (Stage H)
Japanese: 最終冷却（H段階）
Stage 8 of the firing program—the final stage. This is the stage where the piece can be cooled relatively freely from below the distortion point to room temperature. However, caution must still be exercised to prevent cracking due to thermal shock.
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Final cooling (Stage H, Final Cool) is the eighth and final stage of the eight-stage firing program; it is the phase in which the piece is cooled from below the distortion point (442°C based on Bullseye COE 90) to room temperature (§8.1). In η-T-t trajectory terminology, this corresponds to the “late cooling” phase; the standard method is to allow the piece to cool to room temperature naturally (AFAP) without opening the kiln door. Since stress relaxation due to viscous flow does not occur below the strain point (rigid body state), no new residual stresses are generated after this stage.
Even though the H stage is described as allowing for “relatively free cooling,” the risk of thermal shock cracking still exists. In particular, for thick pieces, complex shapes, and assemblies of dissimilar glass types, gradual cooling (natural cooling) is safer even at the H stage. For thin sheets (6 mm or less) of Bullseye COE90, cooling to room temperature in 2 to 4 hours is standard; however, for thick pieces (19 mm or more), natural cooling overnight is recommended. The final step in quality control is to remove the piece after the H stage is complete and verify the retardation value using a polarizing filter measurement (§18).
Related Concepts
Understand the endpoint of the entire cooling phase in conjunction with annealing (Stage G) (firing-stage-g), the strain zone (strain-zone), the 8-stage firing program (firing-program-8-stages), and the polariscope test (polariscope-test).
This entry is written in Japanese by Rei Chikaoka and translated for readers outside Japan. If something reads oddly, the original is the source of truth.