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Deckle is a removable wooden frame or "fence" used in manual papermaking. In a related sense, it can also mean a deckle edge which is a type of rough cut edged paper used in the book trade.
In manual papermaking, a deckle is a removable wooden frame or "fence" placed into a mould to keep the paper slurry within bounds and to control the size of the sheet produced. After the mold is dipped into a vat of paper slurry, excess water is drained off and the deckle is removed and the mold shaken or "couched" to set the fibers of the paper. Some of the paper slurry passes under the deckle and forms an irregular, thin edge.:119, 177–178
Before the 19th century, the deckle edge was unavoidable, a natural artifact of the papermaking process in which sheets of paper were made individually on a deckle. The deckle could not make a perfect seal against the screen at the edges and the paper slurry would seep under creating a rough edge to the paper. The deckle edge could be trimmed off, but this extra step would add to the cost of the book. Beginning in the early 1800s with the invention of the Fourdrinier machine, paper was produced in long rolls and the deckle edge became mostly obsolete: although there was some deckle on the ends of the rolls, it was cut off, and the individual sheets cut out from the roll would have no deckle in any case.
With the appearance of smooth edges in the 19th century, the deckle edge slowly emerged as a status symbol. Many 19th-century presses advertised two versions of the same book with a smooth and a higher-priced deckle version, which suggested the book was made with higher-quality paper, or more refined methods. This tradition carried forward into the 20th and 21st centuries. Today modern deckle is created by a purpose-built machine to create the appearance of a true deckle edge by cutting a smooth edge into patterns. Many modern readers are unfamiliar with the deckle edge and may see it as a defect; for example, Amazon.com has left notes to buyers that the deckle is not a flaw in the product.
The deckle edge should not be confused with the practice of unopened pages, in which the edges needed to be cut open with a knife.
In film processing, deckles are die inserts that set the coating width of a slot die coater or the extrusion width of an extrusion die. They work by constraining the flow as the material exits the die. Since some materials have a tendency to neck in or spread out after leaving the die, deckle position may need to be compensated to achieve the target width.