Don’t try this at home! It may be counter-intuitive, but sometimes to fix a water damaged item you need to soak it in water.
Due to the extensive tide marks from previous water damage, our book conservators decided to wash these plates from Exposition Universelle De 1889 in de-ionized water to remove impurities and tone down the tide marks.
You can see the marks in the lower left and right corner of the plate. By re-immersing the plates, the acid build up in the paper can be reduced and the tide marks washed mostly away. The plates have since been safely dried and rehoused.

I’m always thrilled when people walk in the lab and see me working on something in the washing sink…”You can wash paper?!” is their usual response of amazement.

Sometimes conservators will wash an ENTIRE book too, either separately or in pieces. Here’s a post on the fabulous Flash of the Hand blog about washing a book intact. 

But yup, you totally can wash paper, but it requires a lot of knowledge and training to do it safely and right, so don’t try this at home (or with your library books!).