Watermarks in paper are not the result of paper actually being marked by water. In fact, when a drop of water makes contact with a forming piece of paper, the resulting mark is considered an imperfection, and is known as a papermaker's tear (as in crying). So what's a true watermark?
If you are reading this post, you most surely know about the internet; and if you know about the internet, you possibly have heard about or seen watermarks.
Most people who have used the internet, at one point or another, will have seen an image (photo or text) with a word or logo in light grey superimposed over the image. The superimposed light grey word or logo allows the image to be viewable, but at the same time makes that image undesirable to be copied. Most people call those light grey words or logo images 'watermarks', but they are not true watermarks.
Those internet-based marks on images were given the name: watermarks, because, despite not being true watermarks, they function to provide security, which is one of the functions of a true watermark.
So we're back to the same question ~ what is a true watermark?
A watermark is an image, in the form of an individual design element, or of an overall pattern, created inside paper during the process of producing the paper. The watermark is created by causing certain areas of the pulp that makes up the paper to be thinner (or thicker) than the surrounding areas of the pulp. The watermark is normally viewed by backlighting the paper. All of the images in this post were photographed by allowing a light to shine through the paper. Hold any piece of recently printed U.S. dollar bills up to the light, and on the one side, seemingly inside the paper itself, you will see the image of the U.S. president, whose image is also included in the printed image. As light passes through watermarked papers, the areas in which the pulp was caused to be thinner will appear to be a bit lighter in shade as compared to the rest of the paper. Conversely, the areas in which the pulp was caused to be thicker will appear to be a bit darker in shade than the surrounding paper. This phenomenon is the result of variations in the opaqueness of the paper.
The watermark images displayed on this post were all created by the twisting and bending of a single strand of wire into an image, and then the attachment of the wire onto the wire mesh used in the process of making the paper. Since the strands of wire would cause the paper fibers to be thinner overtop them, the resulting lines of the watermark would be thinner, and appear white against the thicker paper 'background'.
The image created by the varying thickness in the paper itself is called a watermark merely because it is created in the paper while it still is (if ever so slightly) wet.
It is generally believed that the purpose of the watermark was originally to provide an identification of the paper manufacturer; it was essentially a signature mark. The purpose of the watermark today is primarily for security; a watermark verifies the authenticity of a document. If you get a paycheck from an employer, hold it up to the light, and you will probably see a watermark image in the paper.
Watermarks are formed by impressing a three-dimensional image in the wire mesh on which the paper fibers are laid in the process of papermaking. As the water in the paper pulp drains through the wire mesh, the paper fibers that remain will be thinner where the impressed image was thicker and thicker where the impressed image was thinner. As the paper dries, the thicker and thinner areas remain, becoming the watermark.