Watermarks are created by shapes woven into the wire trays that paper pulp is placed upon to make sheets of paper. When the water is removed from the pulp, the shapes are left on the paper. Since 1840, with Great Britain's first stamp--the Penny Black-- watermarked paper has been used in the printing of postage stamps. Because watermarked paper was not always used, certain stamps are more valuable then their counterparts. This makes detecting watermarks on stamps an exciting pastime for stamp collectors.
Wash your hands to remove any oils that can damage your stamps. Use stamp tongs to place your stamps facedown on the black, acid-free paper.This will help some of the watermarks to become visible.
With stamp tongs, hold up each stamp to allow the light to shine through them. More watermarks might be visible with this step.
Place 2 to 4 drops of watermark fluid into the watermark-detection tray. There should be just enough to get one stamp wet.
Lay the stamp facedown in the fluid, using the tongs. If a watermark is present, it will show up once the stamp comes into contact with the fluid. Some stamps will have partial watermarks, and others might have full letters or symbols.
Lay the stamps flat on a paper towel until dry. The fluid will evaporate quickly.
For serious collectors, Signoscope or Optic-Electric watermark detectors can reveal watermarks and other features of the stamps without using watermark fluid.
Use watermark fluid in a well-ventilated room. To avoid damage, do not overuse watermark fluid or keep your stamps in the fluid for long periods.