舊書報為什么會發(fā)黃？

If you look at old newspaper clippings, aging paper documents and books that are past their prime, you'll notice that they likely have a yellow tinge. But why do old paper products turn this golden hue?

看舊剪報、年頭長的紙質(zhì)文件和舊書時你會發(fā)現(xiàn)它們都微微泛黃。但是為什么舊的紙制品會出現(xiàn)這種情況呢?

It's not that books would rather be blond, but rather that paper is made from components that yellow over time — at least when they're exposed to oxygen, Susan Richardson, a chemistry professor at the University of South Carolina, told Live Science.

“這并不是因為書本身是泛黃的，而是紙中的一些成分會隨著時間推移而變黃——至少是當這些成分暴露在氧氣中時。”化學教授Susan Richardson解釋到。

Most paper is made from wood, which largely consists of cellulose and a natural wood component called lignin that gives land plant cell walls their rigidity and makes wood stiff and strong. Cellulose — a colorless substance — is remarkably good at reflecting light, which means we perceive it as being white. This is why paper — including the pages of everything from sheet music to dictionaries — is usually white.

大部分紙張是由木頭制成，這些木頭富含纖維素和一種叫做木質(zhì)素的成分，這些成分會使陸生植物的細胞壁具有一定硬度并使木質(zhì)強韌。無色的纖維素具有很好的反光效果，因此我們的眼睛感知到的它是白色的，這也是為什么從樂譜到字典的幾乎所有紙張通常都是白色。

But when lignin is exposed to light and the surrounding air, its molecular structure changes. Lignin is a polymer, meaning it's built from batches of the same molecular unit bonded together. In the case of lignin, those repeating units are alcohols consisting of oxygen and hydrogen with a smattering of carbon atoms thrown in, Richardson said.

但是當木質(zhì)素暴露在光線下和空氣中時，它的分子結構就發(fā)生了變化。木質(zhì)素是一種聚合物，這意味著它是由很多分子聚合而成的。Richardson教授說到，至于木質(zhì)素，它的重復單元是由氧和氫外加少許碳原子組成的醇類。

But lignin, and in part cellulose, is susceptible to oxidation — meaning it readily picks up extra oxygen molecules, and those molecules alter the polymer's structure. The added oxygen molecules break the bonds that hold those alcohol subunits together, creating molecular regions called chromophores. Chromophores (meaning "color bearers," or "color carriers" in Greek) reflect certain wavelengths of light that our eyes perceive as color. In the case of lignin oxidation, that color is yellow or brown.

但木質(zhì)素和部分纖維素都容易被氧化——這就意味著它很容易吸收額外的氧分子，而這些氧分子改變了聚合物的結構。額外的氧分子破壞了連接醇亞基的鍵，從而生成了被稱為發(fā)色團的分子區(qū)域。發(fā)色團(Chromophores)，在希臘語中是“顏色的載體”或“顏色的搬運工”的意思，它會反射出某種波長的光線，到我們眼睛中即被感知為色彩，由于木質(zhì)素的氧化，這個色彩就是黃色或棕色。

Typically, paper manufacturers try to remove as much lignin as possible by using a bleaching process, according to Richardson. The more lignin that's removed, the longer the paper will remain white. But newspaper — which is made cheaply — has more lignin in it than a typical textbook page, so it turns a yellow-brown color faster than other types of paper, she said.

根據(jù)理查德森的說法，紙張生產(chǎn)商會盡可能的通過漂白來去掉木質(zhì)素，去掉的木質(zhì)素越多，紙張保持潔白的時間也就越長。但是報紙所用的是廉價紙張，其中比其它的書本用紙保留了更多的木質(zhì)素，因此它也比其它種類紙張更易變黃。

Interestingly, the producers of brown paper grocery bags and cardboard shipping boxes take advantage of lignin because it makes their products sturdier. These paper products aren't bleached, leaving them much browner than a typical newspaper, but also stiff enough to give a bag carrying a milk carton and other groceries its strength.

相反，生產(chǎn)棕色紙袋和硬紙箱的廠商會利用木質(zhì)素，因為它能夠使產(chǎn)品更加堅固。這類紙制品沒有漂白流程，比報紙更容易變黃，但是也更加強韌。

According to Richardson, theoretically, you could preserve your high school yearbookin pristine condition, provided you kept out both oxygen and light indefinitely.

理查德森說，理論上，你可以通過隔離氧氣和光線來保持你書本的潔白。

"Oxygen is the enemy," she said. "Keep the book in a perfectly sealed box and replace the oxygen with nitrogen, argon or another inert [meaning it doesn't readily undergo chemical reactions] gas, and you're set."

Richardson博士說到，氧氣是敵人，將書保存在一個完美的密封盒內(nèi)，并用氮氣或其它惰性氣體擠出氧氣，你就勝利了。

But while oxygen-rich conditions are bad for paper, sunlight and high-moisture levels can also negatively impact paper preservation, Richardson noted. For instance, any book that's surrounded by oxygen will yellow, even if it's kept in a dark room. "Sunlight just speeds up the oxidation process," she said.

但理查德森指出，盡管富氧環(huán)境對紙張有害，但陽光和高濕度同樣也不利于紙張的保存，把紙張保存在無光但有氧氣的環(huán)境下，但紙張還是會變黃，陽光只是加速了氧化的過程。

Ensuring that our newspaper clippings stay crisp and legible is one thing, but preservationists, archivists and librarians wage a constant war against paper degradation and oxidation. Preserving important historical documents — anything from a nondigitized will to the Emancipation Proclamation — requires an awareness of what environmental factors can damage paper products.

保持剪報的清晰可讀是一回事兒，但是對文物保護者、檔案管理員和圖書管理員來說，則要一直與紙張的退化和氧化做斗爭。保留重要的歷史文件——從非數(shù)字形式的遺囑到獨立宣言——都需要注意這些會毀壞紙張的環(huán)境因素。