科普專欄–薯不揀單 - 深入淺出–以化學角度審視吶喊名畫

吶喊為挪威畫家愛德華·孟克的作品，其呼喊的「怪異」樣貌形象非常鮮明。927Please respect copyright.ＰＥＮＡＮＡEUjCR3NUrc
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他本人在1893至1916年畫出多個版本，而其中一幅展示在孟克美術館（Munch Museum)的版本因多處黃色地方出現變白及脫落跡象關係（圖1的數字標記區域），自2006年起已甚少公開示人。927Please respect copyright.ＰＥＮＡＮＡs61NFqorA6
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作品現時正處於光線陰淡、攝氏18度、相對濕度約50%的環境備受保護。927Please respect copyright.ＰＥＮＡＮＡgpbwXVeHKi
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（圖2）為了解箇中顏料變色及脫落的機制，有科學家對吶喊作品作出深入研究，並在近日發表文章講述得出的結果（Ref1)。927Please respect copyright.ＰＥＮＡＮＡCu2bChqoza
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Cadmium sulfide，為該時期常用作為黃色顏料的化學物。927Please respect copyright.ＰＥＮＡＮＡcFG4JBYcv5
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同期的文森·梵谷 (Vincent van Gogh)、詹姆斯・恩索 (James Ensor)和亨利·馬諦斯(Henri Matisse)均有利用這種化學物所製成的顏料，而相關的作品亦有出現黃色顏料變白的現象。927Please respect copyright.ＰＥＮＡＮＡFEjrq1cAko
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該時期主要有兩種方式製作Cadmium sulfide，分別為Dry及Wet Method。927Please respect copyright.ＰＥＮＡＮＡOzibJKRD3U
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Dry Method為以300°C至600°C及零氧氣的方式，加上大量硫元素(Sulphur)調配Cadmium sulfide的方式，得出的成品相對上較為純正。927Please respect copyright.ＰＥＮＡＮＡgXFedTZ73Y
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Wet Method為將不同的化學物混合在一起（如圖中所示），再以沉澱的方式獲取固體Cadmium sulfide。927Please respect copyright.ＰＥＮＡＮＡJsB1w5Qky6
基於當時技術相對上不太先進，最終的產品會含有較多雜質及副產品。927Please respect copyright.ＰＥＮＡＮＡkuZGdmb9FN
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（圖3）科學家在文中嘗試挖取微量的吶喊畫碎粒，再利用各種技術檢測畫中的化學物成份。927Please respect copyright.ＰＥＮＡＮＡvcv57YZfEg
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圖3中間的圖片，其紅點代表黃色顏料Cadmium sulfide的存在。927Please respect copyright.ＰＥＮＡＮＡkw4RSFbo4N
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下方的圖片，其三種顏色的出現分別代表Wet Method的副產物。927Please respect copyright.ＰＥＮＡＮＡ2i7MLm2Bo9
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紅點:Cd(OH)Cl927Please respect copyright.ＰＥＮＡＮＡZ2UT23gSom
綠點:CdSO4927Please respect copyright.ＰＥＮＡＮＡEcetzLCGOy
藍點:CdCO3927Please respect copyright.ＰＥＮＡＮＡ4qFx5S1Hqz
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這三種副產物，在長久而來的知識中已被確認是顏料變色及脫落的兇手，與吶喊作品所出現的現象不謀而合。927Please respect copyright.ＰＥＮＡＮＡvsdGOrFtYT
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（圖4）為進一步了解顏料變色的現象，他們找來了吶喊畫同期帶有黃色顏料的作品(他們已7914數字命名樣本)。927Please respect copyright.ＰＥＮＡＮＡ49fs0sJ5ws
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拿取畫中碎片後，（圖4左方）他們同樣發現裡頭含有Wet Method的副產物（綠點）。927Please respect copyright.ＰＥＮＡＮＡwDOakKo63E
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為了解環境因素對於顏料變色的影響，他們對另一塊畫碎用上90 日相對濕度≥ 95%，攝氏為40°C，無光環境來模仿歲月的摧殘。927Please respect copyright.ＰＥＮＡＮＡ2gJXSwB61c
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他們發現得到Wet Method的副產物Cd(OH)Cl（綠點）數量大增且集中在一起，其目測得到黃色顏料變白的現象發生。927Please respect copyright.ＰＥＮＡＮＡDU4YiYRdiC
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然而，用上用上90 日相對濕度=45%，攝氏為30°C，300W光源直接照射的環境來模仿歲月摧殘的話，則沒有感測得到Wet Method副產物的產生和顏料變色現象發生。927Please respect copyright.ＰＥＮＡＮＡsJ2VHSecse
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（圖5）另外，他們亦對愛德華·孟克那時期捐贈予館舍的顏料油作出檢測。927Please respect copyright.ＰＥＮＡＮＡ5FgfZVL6ts
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在畫紙塗上顏料油後，已能測量得到有一定程度份量的Wet Method副產物Cd(OH)Cl（左圖綠點）。927Please respect copyright.ＰＥＮＡＮＡXdmzyCDC7H
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（右圖）用上90 日相對濕度≥ 95%，攝氏為40°C的方式摧殘後，綠點數量大增且集中在一起。927Please respect copyright.ＰＥＮＡＮＡQ8QYKQH4fn
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（圖6）換句話說，當時期以Wet Method製作的Cadmium sulfide顏料以及愛德華·孟克所使用的顏料油，乃是導致畫中黃色地方出現變白及有脫落跡象的元兇，而其變色的機制主要由相對濕度和溫度而非光線控制。927Please respect copyright.ＰＥＮＡＮＡHhPivgn85x
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基於這發現，在相對濕度及溫度嚴密的控制下，吶喊他日將來也許會有機會可以再度長期公開示人。927Please respect copyright.ＰＥＮＡＮＡs0RB2awfbk
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Ref1:https://advances.sciencemag.org/content/6/20/eaay3514927Please respect copyright.ＰＥＮＡＮＡn9XzJFCGB6