Initial Heating (Stage A)

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Initial heating (Stage A) is the first stage of the eight-stage firing program (Stages A–H) systematized by Rei Chikaoka (2012, p. 95) (§8.1). The main purpose of this stage, during which the temperature is gradually raised from room temperature to approximately 500°C, is to prevent sudden thermal shock cracking of the glass. In this heat-conduction-limited heating phase, the standard setting for Bullseye COE90 is 100°C/h; however, the thicker and larger the piece, the slower the heating rate must be. Since the temperature difference (ΔT) between the surface and the interior manifests as tensile stress (σ = αEΔT/(1−ν)), controlling the heating rate is the first critical step in the process.

Stage A corresponds to the “pre-heat-up stage” of the η-T-t trajectory language (§2.2) and passes through the temperature range (below the strain point) where the glass behaves as a solid (rigid body). Since stress relaxation due to viscous flow occurs only minimally in this region, thermal stress arising from temperature differences tends to accumulate easily. Bullseye recommends reducing the cooling rate to approximately 50°C/h for 19 mm-thick slabs and 20°C/h for 50 mm-thick slabs (in accordance with the Bullseye Annealing Thick Slabs Chart). For works with complex layouts or those containing dissimilar materials, even slower rates may be necessary due to differences in thermal expansion coefficients.

Related Concepts

Temperature Homogenization (Stage B) (firing-stage-b) · 8-Stage Firing Program (firing-program-8-stages) · η-T-t framework (η-T-t framework) and thermal diffusivity κ (thermal diffusivity) to understand the design principles of the heating phase.