What Is Halftone for Letterpress Printing?
Letterpress and other printing processes use the halftone method for the reproduction of photographs and other gradient color or black and white images. Halftones operate on an optical illusion principle, in which solid areas are broken up into dots of varying sizes and spacing.
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Function
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Traditional printing methods such as letterpress are only able to print solid color shades from printing plates. When a photo is required, the image must be broken up into many small solid areas for printing, giving the illusion of a "continuous tone" when seen at a distance. Close examination of halftones will reveal an elaborate series of dots.
Plates
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Modern letterpress plates are made from photo-polymer, which are processed directly from computer platemakers, or hand-developed using film negatives. Traditional letterpress plates are made from zinc or copper, and are acid-etched using a film negative photo process.
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Printing
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Letterpress halftones are quite difficult to print. Since the plate is pressed directly against the paper, the paper and underlying printing surface must be completely uniform for clean halftone prints. The press ink rollers must be in top condition and adjusted properly, in order to apply the right combination of ink to the plate without over or under-inking the tiny halftone dots.
Plate Care
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Zinc and copper letterpress plates are prone to oxidation, and should be stored in an airtight container or wrapped in cellophane. Polymer plates need no special storage, but all plates should be carefully handled to avoid surface damage from drops or contact with other items.
Halftone History
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The first printed halftones appeared in newspapers just before the turn of the 20th century. The process and plate materials have improved greatly since then, but the basic principle is still in use in all commercial ink printing.
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References
Resources
- Photo Credit Image by Flickr.com, courtesy of Sherrie Thai
