不良・失敗
reduction blackening
Japanese: 還元黒化
A phenomenon in which metal precipitates and turns black in a reducing atmosphere. This is caused by residual carbon and organic matter and is particularly noticeable in lead-based alloys.
Learn More
Reduction blackening is a phenomenon in which the metal ions contained in glass are reduced—causing the glass to turn black or dark—when the atmosphere inside an electric furnace becomes oxygen-deficient (a reducing atmosphere). Metal oxides such as iron oxide (Fe₂O₃ → FeO), copper (Cu²⁺ → Cu⁰), and manganese are particularly prone to reduction, causing them to lose their original color.
Since electric kilns typically do not perform firings that cut off oxygen, reductive blackening is unlikely to occur; however, when organic matter (such as excessive use of fusing glue, finger oils, or contamination from wood or paper) burns, a localized reducing atmosphere can form. The key to prevention is keeping the furnace clean and minimizing the introduction of organic materials.
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.