色・発色
Sulfide Blackening
Japanese: 硫化黒化
A phenomenon in which PbS, FeS, and other compounds precipitate and cause the glass to turn black when lead-based glass is fired using refractory gypsum containing sulfates. This can be prevented by selecting the appropriate refractory gypsum.
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Sulfide blackening is a process in which lead (Pb) -based glass is fired in refractory gypsum containing sulfates (which applies to many standard products), SO₃ is released from the gypsum and reacts with Pb²⁺, causing PbS (lead sulfide) and metallic Pb⁰ to precipitate, resulting in black spots and black streaks within the glass (Ingo et al. 2001, Rodrigues 2017). The chemical reaction is given as Pb²⁺ + SO₃²⁻ → PbS + 3O₂. An experimental report co-authored by Chikaoka, published in the Musashino Art University Bulletin, Vol. 36 (2005), includes on-site verification of this mechanism.
Methods to prevent sulfidation and blackening include: (1) using lead-free refractory plaster (such as Crystalcast or other lead-compatible products); (2) limiting the maximum firing temperature to 880°C or below (SO₃ release accelerates at temperatures of 900°C or higher); and (3) avoiding a reducing atmosphere (caused by excessive use of residual charcoal or organic fibers). Particular caution is required when casting with lead-containing glass such as Gaffer Casting Crystal, as the choice of refractory plaster directly affects the quality of the finished piece. This problem does not occur when using lead-free products such as Bullseye or OYF.
Related Concepts
Understand how to handle lead-based glass and the mechanisms of color formation and discoloration, in conjunction with solarization, striker (striker glass), refractory plaster, and lost-wax casting.
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.