The Printing Press and the Halftone Process
Large scale book production requires the use of a printing press. Printing presses use custom-made static plates that bear the image or text to be reproduced. There are several types of plates and presses, the most common of which is known as offset lithography. In this type of printing the plates were made by an engraver using a photographic process that transferred a copy of the original onto a metal surface.
These plates work like giant rubber stamps. They are inked and the image is transferred to paper by means of pressure. Each plate is inked with a single color of ink. A page with more than one color is created using multiple plates. In offset lithographic printing the plates are wrapped around cylinders that rotate as paper is fed through the press. A piece of full color artwork is created by passing a sheet of paper along multiple spinning plates in succession. Each plate can very rapidly print on thousands or even hundreds of thousands of sheets.1
Paul Zelinsky explains the printing press
The artwork or text to be reproduced on a press is referred to as copy. Copy is commonly divided into two categories: line copy and continuous tone copy. Line copy refers to any material that is formed by solid lines or areas of color with no gradations in tone. This includes text as well as illustrations that use cross-hatching or stippling. Continuous tone copy refers to any material in which variations in tones occur in an unbroken flow. This includes photographs, pencil illustrations, paintings, and more.2
It is important to understand that a printing press cannot produce an exact replica of a piece of artwork. Instead, printing should be thought of as a process of translation that creates a version of the original in a state that can be reproduced on a press. In order to make an offset printing plate a piece of art was first photographed and transferred onto film. Any required manipulation or correction of the image was performed on this film. A chemical process was used to etch the final image on the film to the metal printing plate. Sometimes the camera person who created the film image and the platemaker who transferred that image onto metal were different people working in separate facilities.3
Line copy is fairly easy to reproduce; printing plates excel at printing solid areas of color. Continuous tone copy is more difficult and therefore more expensive to reproduce. A pencil illustration is not made up of areas of black and areas of white but rather many different shades of grey. But a printing plate cannot be inked with different shades of grey to form an image. Offset printing applies applies a single, solid color of ink to each plate. Continuous tone copy could not be reproduced without the use of a clever workaround.
This workaround is known as halftone. During the process of photographically transferring an image onto film, the camera person would place a fine screen of perpendicular intersecting lines between the film and the image. This screen breaks up the continuous tone of the image into thousands of tiny dots of varying sizes, larger and closer together where the image is dark and smaller and farther apart where the image is light.4 These dots were retained when the film was transferred onto the printing plate.
In book printing these dots are usually so small that they require a magnifying glass to see. The contrast between the dots and the white paper they are printed on creates an optical illusion; the eye does not perceive the dots but rather a continuous image of varying tones. In this way a single plate can print one color of ink in a way that appears as a variety of shades and tints.5
Take, for example, this pencil illustration by Stephen Gammell from the book Stonewall, written by Jean Fritz. A halftone screen was used to translate the original pencil artwork into a form that could be reproduced on a press. If you look very closely at the printed version you will see that it is composed of thousands of tiny dots. In this way a plate printing black ink has created an image that appears to be composed of shades of grey.
1. Craig, James. Production for the Graphic Designer. New York: Watson-Guptill Publications, 1974. Print. 86-88 ↩
2. Stone, Bernard, and Arthur Eckstein. Preparing Art for Printing. New York: Reinhold, 1965. Print. 35-36 ↩
3. Zelinsky, Paul. "RE: EJK experience." Message to Paul Mostrom. 24 January 2016. Email. ↩
4. Pitz, Henry C. Illustrating Children's Books: History, Technique, Production. New York: Watson-Guptill Publications, 1963. Print. 160-161 ↩
5. Verfasser, Julius. The Half-tone Process: A Practical Manual of Photo-engraving in Half-tone on Zinc, Copper, and Brass, with Chapters on Three-colour Work and Photo-lithography for Offset Printing. 5th Ed., Fully Rev. ed. London: Iliffe & Sons, Limited, 1912. Print. ↩