Before writing my commentary today I thought I would have a brief look at what others have said in answer to this question.
Here’s one of the most amusing answers I found (it came from Answers.com)
“Because the light shines into the paper and turns it yellow”
Well, yes, up to a point I suppose that’s right – but it doesn’t really take me much closer to the “why?” in my question. Why does sunlight turn paper yellow?
The fact is that paper is made from wood which itself is made up of white cellulose plus a dark substance called lignin. It is the exposure of lignin to air and sunlight that turns paper yellow.
Lignin (sometimes written lignen – the word comes from “lignum” – the Latin for wood) makes up about 30% of the mass of wood. Its effect is to strengthen the wood so that it can more readily stand erect (somewhat akin to the function of concrete in buildings). If you want to be technical you might like to know that it is just about the most abundant organic polymer on the planet.
How yellow paper will get over time depends not only on how much sunlight it is exposed to, but also how much lignin is in the paper. Newsprint, for example, goes yellow quickly because it is one of the cheaper forms of paper (generally intended to have a short lifespan). So when produced the lignin is not separated out – which makes newsprint cheaper.
It doesn’t take much of an intellectual jump to realise at this point that cardboard is stiff because it has a high percentage of lignin in it.
At the other extreme, because pure cellulose is white, if the paper manufacturer puts the wood through a chemical solvent process to separate out the lignin, you will end up with white paper that will not yellow – or at least not easily and not for quite a long time.
So there are three factors: the quality of the paper, the sunlight and the air.
Storage in a dark dry space will certainly help reduce the level of yellowing quite considerably.
Which is where we come in – we offer document storage in the Midlands.
There’s more information on our website or give us a call on 01922 632227.