比黄金还珍贵的颜料，到底是怎么消失的？(4)

“就群青色而言，‘暗淡而平淡’的外观是由于中间色素的降解——颜料的蓝色仍然存在，而降解的深蓝色颜料中的灰色外观是原始颜料的一部分。” Keune 说。
To explore ultramarine’s catalytic properties, Keune’s team prepared samples from lapis lazuli and obtained a synthetic commercial version. The chemists then carried out dehydration（脱水） of methanol（甲醇） to dimethylether（二甲醚） either in the presence of ultramarine or a quartz sand control.
为研究群青的催化性能，基恩的团队从天青石中提取样品。然后，化学家们将甲醇脱水，在群青或石英砂的控制下生成了二甲醚。
When ultramarine was present more dimethyl ether was produced than with the quartz sand control, with a conversion rate that was nearly 12 times greater, indicating catalytic activity. Keune suggests that the oil binder in ultramarine could be degraded and oxidised（氧化） due to the Brnsted and Lewis acid sites on the pigment’s cage structure.
当群青存在时，二甲醚的产率高于石英砂，转化率约为石英砂的12倍。这充分表明了群青的催化活性。基恩认为，群青中的粘合剂可以被降解和氧化，这是由于在色素笼状结构上存在溴离子和路易斯酸。
‘That a pigment with a zeolitic structure such as ultramarine should act as a catalyst for the hydrolysis（水解） and oxidation（氧化） of an oil paint binder makes perfect chemical sense,’ comments Jennifer Mass, who investigates pigment degradation at Bard Graduate Center, New York. ‘Keune’s group’s latest contribution（贡献） is another example of how the identification of paint alteration mechanisms guides us in the preservation of irreplaceable works of art.’