Lemon or lime juice
How to do it:
(Click on the images for an enlarged picture of each step)
Tear the edges of your paper for a ragged effect. Pour a little lemon juice into a plastic container, and brush or sponge around outer edge of paper. Juice might also be splattered or sponged across the full sheet—try a couple of practice sheets to see what happens.
Place juicy paper on a piece of tile. Begin heating with heat gun. Keep gun moving slowly. Paper will begin to brown in places that have been painted with juice.
Continue heating juice until paper has browned to your satisfaction.
An example of a finished book layout using juice burned papers.