Re: After paper gets wet, why does it dry wrinkly and bumpy instead of flat?

Jaime:
     That is an interesting observation.  One wonders what is occuring 
with the paper, when for example, you wet Cling-wrap, and dry it, it is no 
more wrinkly than it began.  The answer to your question has to do with a 
simple property of paper that we don't often think about... It is soluble 
in water.  
     When paper is made, wood from trees (Or sometimes linen, cotton, or 
hemp) is essentially made into a pulp, which is treated in various ways 
and then spread very thin, compacted and dried to make paper.  
     When you get a piece of paper wet, you are actually dissolving some 
of the paper, which is made predominantly of cellulose from trees.  Some 
of the cellulose dissolves, and many of the other things added to the 
paper do as well, Starch for example.  When the paper dries, its 
properties are very different now in terms of how much/where the material 
is, so it shrinks and wrinkles as a result.
      If you have ever made your own paper in an art class or elsewhere, 
you typically put newspapers in a blender and spread this pulp onto a 
screen where you dry your 'new' paper.  Because you do not have access to 
high pressure equipment and other facilities, this paper comes out rough 
and wrinkled too (And Thick).
     I would suspect that if you wetted a piece of paper and dried it 
under a very high-pressure press (Much more than a stack of books), it 
would look more or less normal.  
     I hope this answers your question, thank you for writing to the Mad 
Scientist

Here is a URL on how paper is made, I hope it is of use to you.
         http://www.idahoforests.org/paprmake.htm



-Matt-