Or something unexpected could happening.
I have been rusting paper, mostly newsprint packing paper with nice results.
This is a detail of a piece of rusted paper with embroidered code.

The result of a piece of newsprint laid on top of wet rusty nails and weighted with a brick.

A wallpiece in progress that is in the process of rusting.

Brown packing paper, safety pins, embroidery cotton, and a strip of drop cloth.
Solution of peroxide and sodium was used to rust the safety pins.
Newsprint with a solution of peroxide and sodium.

The unexpected result–I was planning to make a couple of pieces similar to the Indelible Mark detail.
I cut and folded paper from a drawing pad that I picked up at Five Below.

I was expecting lovely rust and this is what happened–

White paper wrapped around newsprint–

Immediately thought the sizing in the paper was the reason for the peculiar rust color.
When I applied peroxide and sodium to a scrap of the white paper, this is what happened–

Lesson Learned–Always, always test new materials.