温度・粘度
Lead Crystal Blackening
Japanese: 鉛クリスタル黒化
A phenomenon in which PbS and metallic Pb⁰ precipitate, causing black spots or discoloration, when lead-containing glass is fired with sulfate-containing refractory gypsum. This can be prevented by using lead-free gypsum or by limiting the maximum firing temperature.
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Lead Crystal Blackening is a phenomenon characterized by black spots, black streaks, and black discoloration that occurs when lead-based glass (such as Gaffer Casting Crystal) is fired in refractory plaster containing sulfates (treatise §5.1, §7.1, §10.3). The mechanism is as follows: SO₃ released from refractory gypsum at high temperatures (900°C or higher) reduces lead ions (Pb²⁺), causing them to precipitate as metallic lead (Pb⁰) or lead sulfide (PbS). An example of the reaction equation: Pb²⁺ + SO₃²⁻ → PbS + 3O₂ (Ingo et al. 2001, Rodrigues et al. 2017). The precipitated metallic Pb⁰ colloids and PbS absorb light, causing the material to appear black.
This phenomenon was also experimentally observed in the *Musashino Art University Bulletin*, Vol. 36 (2005, Saito, Chikaoka, and Shinohara), where a correlation with the sulfate content of the casting material (refractory plaster) was suggested (an experimental report in which Chikaoka participated as a co-author). There are two preventive measures: (1) Use lead-containing refractory gypsum (such as Crystalcast) or a lead-free gypsum mixture, and (2) limit the maximum firing temperature to 880–900°C or lower to suppress SO₃ emissions. Furthermore, since the process is accelerated in a reducing atmosphere (caused by the combustion of residual carbon and organic fibers), controlling organic matter within the furnace is also important. When used in combination with lead-based glass, the choice of refractory gypsum determines the final quality.
Related Concepts
硫化黒化（sulfide-blackening）・遷移金属redox（transition-metal-redox）・耐火石膏（refractory-plaster）・Gaffer Casting Crystal（gaffer-casting-crystal）・近岡式耐火石膏（chikaoka-plaster）・バルク結晶化（surface-crystallization）
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.