旧书报为什么会发黄？

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?

看旧剪报、年头长的纸质文件和旧书时你会发现它们都微微泛黄。但是为什么旧的纸制品会出现这种情况呢?

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.

“这并不是因为书本身是泛黄的，而是纸中的一些成分会随着时间推移而变黄——至少是当这些成分暴露在氧气中时。”化学教授Susan Richardson解释到。

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.

大部分纸张是由木头制成，这些木头富含纤维素和一种叫做木质素的成分，这些成分会使陆生植物的细胞壁具有一定硬度并使木质强韧。无色的纤维素具有很好的反光效果，因此我们的眼睛感知到的它是白色的，这也是为什么从乐谱到字典的几乎所有纸张通常都是白色。

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.

但是当木质素暴露在光线下和空气中时，它的分子结构就发生了变化。木质素是一种聚合物，这意味着它是由很多分子聚合而成的。Richardson教授说到，至于木质素，它的重复单元是由氧和氢外加少许碳原子组成的醇类。

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.

但木质素和部分纤维素都容易被氧化——这就意味着它很容易吸收额外的氧分子，而这些氧分子改变了聚合物的结构。额外的氧分子破坏了连接醇亚基的键，从而生成了被称为发色团的分子区域。发色团(Chromophores)，在希腊语中是“颜色的载体”或“颜色的搬运工”的意思，它会反射出某种波长的光线，到我们眼睛中即被感知为色彩，由于木质素的氧化，这个色彩就是黄色或棕色。

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.

根据理查德森的说法，纸张生产商会尽可能的通过漂白来去掉木质素，去掉的木质素越多，纸张保持洁白的时间也就越长。但是报纸所用的是廉价纸张，其中比其它的书本用纸保留了更多的木质素，因此它也比其它种类纸张更易变黄。

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，生产棕色纸袋和硬纸箱的厂商会利用木质素，因为它能够使产品更加坚固。这类纸制品没有漂白流程，比报纸更容易变黄，但是也更加强韧。

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

理查德森说，理论上，你可以通过隔离氧气和光线来保持你书本的洁白。

"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博士说到，氧气是敌人，将书保存在一个完美的密封盒内，并用氮气或其它惰性气体挤出氧气，你就胜利了。

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，尽管富氧环境对纸张有害，但阳光和高湿度同样也不利于纸张的保存，把纸张保存在无光但有氧气的环境下，但纸张还是会变黄，阳光只是加速了氧化的过程。

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.

保持剪报的清晰可读是一回事儿，但是对文物保护者、档案管理员和图书管理员来说，则要一直与纸张的退化和氧化做斗争。保留重要的历史文件——从非数字形式的遗嘱到独立宣言——都需要注意这些会毁坏纸张的环境因素。