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This technique produces blue lines on a white background. The drawings are also called blue-lines or bluelines. [12] [13] Other comparable dye-based prints were known as blacklines. Diazo prints remained in use until they were replaced by xerographic print processes. Xerography is standard copy machine technology using toner on copy paper.
The paper draws out the ink, and the letter B is printed in intense black. When the surface of a metal plate is sufficiently polished to be used for engraving, the slightest scratch upon it will print as a black line. An engraved plate from which visiting cards are printed is a good example of some elementary principles of engraving.
Initially, paper was ruled by hand, sometimes using templates. [1] Scribes could rule their paper using a "hard point," a sharp implement which left embossed lines on the paper without any ink or color, [2] or could use "metal point," an implement which left colored marks on the paper, much like a graphite pencil, though various other metals were used.
The deepest drypoint lines leave enough burr on either side of them that they prevent the paper from pushing down into the center of the stroke, creating a feathery black line with a fine, white center. A lighter line may have no burr at all, creating a very fine line in the final print by holding very little ink.
Before the development of photography and of halftones, line art was the standard format for illustrations to be used in print publications, using black ink on white paper. Using either stippling or hatching, shades of gray could also be simulated.
The difference between the non-photo blue and black ink is great enough that digital image manipulation can separate the two easily. If a black-and-white bitmap setting is scanned in, the exposure or threshold number can be set high enough to detect the black ink or dark images being scanned, but low enough to leave out the non-photo blue.
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Notations with leader lines at top and bottom cite statutory authority for not declassifying certain sections. Click on the image to enlarge. Printed documents which contain classified or sensitive information frequently contain a great deal of information which is less sensitive.