温度・粘度
Annealing Zone
Japanese: 徐冷域（アニール域）
A temperature range of approximately 482°C (based on COE 90). This is the temperature range in which internal stresses can be removed most efficiently. By allowing sufficient time, permanent deformation can be reduced to virtually zero.
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The annealing zone is the temperature range surrounding the “annealing point” at a fixed viscosity of the glass; for the Bullseye COE90 series, this corresponds to approximately 482°C (§1.1). The annealing point is defined as log η = 13.0 (Pa·s); at this temperature, the glass is capable of viscous flow, allowing internal stresses to be fully relieved (structural relaxation model by Narayanaswamy 1971). By holding the glass in the annealing zone for a sufficient amount of time, permanent strain is reduced to nearly zero (§4.2).
The "F" stage (annealing soak) of the firing program corresponds to holding the piece within this temperature range; for Bullseye COE90, the standard specification is to hold at 482°C for at least one hour (or longer for thicker pieces). According to the Adams-Williamson rule, the permissible cooling rate is inversely proportional to the square of the thickness (q ∝ 1/t²); therefore, the thicker the piece, the more important it is to manage the holding time in the slow-cooling zone and the cooling rate. Polarizing plate measurement (measurement of the retardation value) is used to verify after the fact whether this region was passed through properly.
Related Concepts
Understand the design and quality control of the slow-cooling process in conjunction with the strain zone, annealing soak (F stage) (firing-stage-f), the Adams-Williamson rule, the polariscope test, and retardation.
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.