Literally Yellow Paper!

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Offline artistic

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Literally Yellow Paper!
« on: 17/08/2006 14:56:42 »
Yellow is a sign of "old" age for paper, but why does it become yellow??[:)]

Don't make me angry, I have advanced "Green Technology".


Offline eric l

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Re: Literally Yellow Paper!
« Reply #1 on: 17/08/2006 15:36:13 »
1) You will have noticed that some papers yellow more quickly than others.  The more the paper is made out of "groundwood" the faster will be the yellowing. Are you familiar with paper technology ?  Otherwise, start by reading the wikipedia article on the subject. (  It is mainly the lignin from the wood that is responsible for the yellowing, and you'll have more lignin in groundwood than in "chemical pulp".
2) Direct sunlight or uv-light increase the rate the yellowing, but you'll have it even with indirect light.  Even in complete darkness, some yellowing will occur (in papers with high groundwood content).
3) The acidity of paper also increases the rate of yellowing.  Now, paper is made at lower accidity levels than say in the middle of the 20th century, when it was generally much higher than before the 20th century.
4) Our current standards for whiteness (for paper) are much higher than they used to be, even around the 1950's.  Old paper may be yellower simply because it always has been less white than paper produced now !  Optical brightening agents for example only appear in the course of the 20th century.
5) In coated pāper, you can not always be sure wether it is the patper that is yellowing, or the coating.  Somethimes it is both.  
« Last Edit: 17/08/2006 15:37:43 by eric l »
"Wonder is no wonder" (Simon Stevin 1548-1620)