A watermark is a recognizable image or pattern in paper that appears as various shades of lightness/darkness when viewed by transmitted light (or when viewed by reflected light, atop a dark background), caused by thickness or density variations in the paper. There are two main ways of producing watermarks in paper; the dandy roll process, and the more complex cylinder mould process.
Watermarks vary greatly in their visibility; while some are obvious on casual inspection, others require some study to pick out. Various aids have been developed, such as watermark fluid that wets the paper without damaging it. Watermarks are often used as security features of banknotes, passports, postage stamps, and other documents to prevent counterfeiting.
A watermark is very useful in the examination of paper because it can be used for dating, identifying sizes, mill trademarks and locations, and the quality of a paper.
Encoding an identifying code into digitized music, video, picture, or other file is known as a digital watermark.
Dandy roll process
A watermark is made by impressing a water-coated metal stamp or dandy roll onto the paper during manufacturing. These watermarks were first introduced in Bologna, Italy, in 1282; however the dandy roll was invented in 1826 by John Marshall. Watermarks have been used by papermakers to identify their product, and also on postage stamps, currency, and other government documents to discourage counterfeiting. In France, they were introduced during World War II by the Vichy regime, and counterfeited by people such as Adolfo Kaminsky. The invention of the dandy roll revolutionised the watermark process and made it much easier for a company to watermark their paper.
The dandy roll is a light roller covered by material similar to window screen that is embossed with a pattern. Faint lines are made by laid wires that run parallel to the axis of the dandy roll, and the bold lines are made by chain wires that run around the circumference to secure the laid wires to the roll from the outside. Because the chain wires are located on the outside of the laid wires, they have a greater influence on the impression in the pulp, hence their bolder appearance than the laid wire lines.
This embossing is transferred to the pulp fibres, compressing and reducing their thickness in that area. Because the patterned portion of the page is thinner, it transmits more light through and therefore has a lighter appearance than the surrounding paper. If these lines are distinct and parallel, and/or there is a watermark, then the paper is termed laid paper. If the lines appear as a mesh or are indiscernible, and/or there is no watermark, then it is called wove paper. This method is called line drawing watermarks.
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