比黄金还珍贵的颜料，到底是怎么消失的？

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Luxury blue paint pigment（颜料）
catalyses（催化） its own ‘disease’
奢侈的蓝色颜料催化自身的疾病

Prized for its deep blue colour and used in oil paintings since medieval times, ultramarine（群青色） can lose its bright colour and become ‘sick’. Now, researchers have found that this phenomenon（现象）, called ‘ultramarine disease’, is likely caused by catalytic（起催化作用的） activity of the pigment itself. The finding could aid（援助） conservation（保护） of renaissance（文艺复兴） artworks.
群青因深蓝色而备受青睐，自中世纪以来就用于油画中。但是，群青会随时时间流逝而失去其明亮的色彩，就像长病了一样。研究人员发现，这种被称为“群青综合症”的现象，很可能是由颜料本身的催化活性引起的。这一发现可以帮助保护文艺复兴时期的艺术品。
Ultramarine was traditionally produced by grinding（研磨） lapis lazuli（杂青金石） rock from mines in Afghanistan into a fine powder and blending it with oils and waxes（蜡）. It was the most expensive blue used by Renaissance painters and more precious（珍贵的） than gold. However, a synthetic（人造的） version was invented in 1826 making it far cheaper.
群青色，是将阿富汗出产的天青石研磨成细粉，然后与油和蜡混合而成的蓝色颜料。这种蓝色颜料是文艺复兴时期画家使用的最昂贵的蓝色，比黄金还要珍贵。直到人工合成的群青色于1826年得以发明，群青色的价格才跌下神坛。
Ultramarine sickness – which results in the paint becoming a dulled（使变得无光泽的） grey-blue colour – occurs in both the natural and synthetic versions of the pigment. Previously, researchers assigned the malady（疾病） either to pigment loss or to tiny cracks forming between pigment particles and organic（有机的） binders（粘合剂） in the paint causing light to scatter（使分散） and dull the perceived colour – despite the blue pigment still being there.

群青综合症，会导致颜料变成暗淡的灰蓝色，这种情况在天然和人工合成的群青色颜料中都会发生。此前，研究人员将此归咎于颜料丢失或颜料颗粒与有机粘合剂之间的细小裂缝，认为这些裂缝导致光线分散并使人们感知到的颜色变暗——尽管蓝色颜料仍然存在。
‘If you look at the structure of ultramarine it makes complete sense it has catalytic activity, as it is a zeolite（沸石） and analogous（相似的） to commercial catalysts,’ explains Katrien Keune at the University of Amsterdam, Netherlands. The zeolites could be responsible for the pigment’s ‘disease’ by degrading the binder to produce the light scattering effect.

荷兰阿姆斯特丹大学的凯特琳·基恩解释说：“如果你观察一下群青的结构，就会发现它具有催化活性。因为群青是一种沸石，与催化剂类似，它的分子可通过降解粘合剂来产生光散射效应。”
Keune first had a hunch that ultramarine could be catalytically active in 2006 when she artificially aged various oil paints that contained different pigments but which had the same linseed oil binder. The paint containing ultramarine was the only one that crumbled（使破碎，成碎屑） and results showed that the binder had completely degraded（降解）.

2006年，基恩第一次有了这样的想法。当时，她老化了各种含有不同色素但含有相同亚麻籽油粘合剂的油画颜料。其中，含有群青的颜料是唯一一个破碎的样品。结果显示，群青色的粘合剂已经完全降解。
‘In the case of ultramarine, the “dull and flat”appearance is due to the degradation in the medium –the blue of the pigment is still there and the greyish appearance in the degraded ultramarine paint is part of the original paint,’ says Keune.
“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.’
“一种具有沸石结构的颜料，例如群青，可以作为水解和氧化油漆粘合剂的催化剂，这是完美的化学意义。”纽约巴德研究所的颜料降解专家詹妮弗·玛斯评论道，“基恩小组的最新贡献是另一个例子，说明了涂料改变机制是如何指导我们保存不可替代的艺术作品的。”