Stare at This Handmade Paper Watermark (Animated GIF)
Elusive, mysterious, surprising. Watermarks can be an art form, a papermaker's mark, security method, or design effect unique to paper.
Regardless of purpose, it's an eye-widening moment when you hold a deceptively blank sheet of paper to light and a hidden image suddenly appears.
How does this work?
The simple explanation is: the thickness and thinness of the sheet itself is manipulated during the papermaking process, when the sheet is first formed and wet. Where the paper is thinner, more light shines through, and where the paper is thicker, the opacity of the sheet blocks light. (Okay okay, let's get nerdier about how one might create a watermark. Traditionally in western papermaking, a flat design is formed with wire, and attached onto the surface of a hand papermaking mould. During sheet formation, less pulp settles where the wire is, making the sheet thinner where the wire design is located. The pulp settles as usual on the rest of the mould surface.)
More on watermarks:
Take a look at this brief overview of western watermark history.
Several hand papermaking artists who use watermarks.
A look into Japanese handmade paper watermarks and history