温度・粘度
Transition Metal Redox
Japanese: 遷移金属redox
Mechanisms of color development and color change due to changes in the redox state of transition metal ions. When the valence of ions such as Fe²⁺/Fe³⁺ and Cu²⁺/Cu⁺ changes, their absorption spectra change.
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Transition Metal Redox is a mechanism in which trace amounts of transition metal ions (Fe, Cu, Mn, Cr, Ni, Co, etc.) present in trace amounts in the glass change their valence depending on the redox conditions of the atmosphere during firing, causing coloration or discoloration as a result of changes in their absorption spectra (treatise §5.1). Examples: Fe²⁺ (blue-green) ⇔ Fe³⁺ (yellow-brown), Cu²⁺ (blue) ⇔ Cu⁺ (colorless) ⇔ Cu⁰ colloid (golden-red), Mn²⁺ (colorless to pale pink) ⇔ Mn³⁺ (purple), Cr³⁺ (green) ⇔ Cr⁶⁺ (yellow). Even the same type of glass will change color depending on the firing atmosphere (oxidizing/reducing) and temperature.
There are several practical implications for kiln operations. While electric furnaces generally operate in an oxidizing atmosphere, the combustion of organic materials (fiber-based materials, paper, adhesives) can create localized reducing atmospheres. Furthermore, in lead-based glass, SO₃ from sulfur-containing refractory gypsum acts as a reducing agent, causing Pb²⁺ to reduce to Pb⁰ (metallic lead) and the precipitation of sulfides (sulfidation and blackening). Furthermore, in Mn-containing glass, prolonged exposure to high temperatures causes Mn²⁺ to oxidize to Mn³⁺ (solarization), resulting in a purple discoloration. To avoid the risk of discoloration, it is important to prevent the accumulation of unburned organic matter, use lead-free refractory gypsum, and avoid excessive exposure to high temperatures.
Related Concepts
Sulfide Blackening · lead-crystal blackening, metal-colloid coloring, striker glass, coloring vs. discoloration, metal oxides
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.