温度・粘度
Viscosity Fixed Points
Japanese: 粘度定点
A general term for characteristic temperatures defined by standards that correspond to specific log η values, such as the softening point, slow-cooling point, and distortion point. These serve as reference points for firing design.
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"Viscosity Fixed Points" is a general term for characteristic points defined by standards as the temperatures at which the viscosity η of glass takes on specific values. The terms "softening point," "slow-cooling point," and "distortion point," which kiln workers use on a daily basis, are all defined as these viscosity fixed points (treatise §1.1). The main fixed points and their corresponding standards are listed below: Working point (log η = 4, T⁴ = flow processing range), Littleton softening point (log η = 6.6, ASTM C338, onset of self-weight deformation), ASTM softening point (log η = 7.6, lower limit of tack fusion), slow-cooling point (log η = 13, JIS R 3103-2, completion of stress relaxation), and strain point (log η = 14.5, starting point of effective rigidity).
Viscosity reference points are listed in the glass brand catalogs and are used as reference points for designing firing programs. For the Bullseye COE90 series, the slow-cooling point is approximately 482°C, and the distortion point is approximately 442°C. Since these values vary by brand, when mixing glasses of different compositions, it is necessary to verify not only the COE (coefficient of thermal expansion) but also the compatibility of the viscosity curves. While the measurement of viscosity fixed points follows standards such as ASTM C336 (slow-cooling point), C338 (softening point), and C598 (distortion point), artists in the field often use the values listed in each brand’s TechNotes or data sheets.
Related Concepts
粘度-温度曲線（viscosity-temperature-curve）・ワーキングポイント（working-point）・VFT式（vft-equation）・4ゾーン（four-zones）・歪域（strain-zone）・annealing域（annealing-zone）
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.