METROPOSTCARD.COM GUIDE TO PRINTING TECHNIQUES 4
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Tricolor and Process Printing


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OFFSET LITHOGRAPHY


The concept of offset printing dates back to 1875 with the invention of flexography. Ink was transferred to cardboard wrapped around a roller before being printed onto sheet metal. The similar Orloff process developed at the end of the 19th century uses a rubber impression cylinder to take ink from multiple line blocks and transfer it all simultaneously to a single sheet of paper. Around 1903 Ira Rubel noticed the same type of phenomenon when an impression cylinder accidentally passed over an inked printing plate without paper being present. The image was offset from the plate and onto the soft rubber cylinder coating, and when transferred again to a sheet of paper he noticed the image had more desirable qualities than the original print. The following year he began constructing presses where a lithographic image on a plate is offset from its surface and then transferred to paper by this indirect means. Shortly afterwards Charles Harris invented the first rotary offset press.

The general principals behind offset lithography are identical to that of traditional metal plate lithography except for the manner by which they print. Various offset printing presses may have different roller systems but all share three major components; a plate cylinder that holds the printing plate, a blanket cylinder wrapped in rubber that carries the image being transferred, and the impression cylinder which applies the pressure needed to print the image. A gear train attached to the press connects all three cylinders so they work in perfect synchrony with one another. Printing begins when a processed litho-plate containing an image is mounted on a cylinder, mechanically dampened with a wetting agent, and then rolled with ink. The oily ink is repelled from the damp areas and is attracted to the dry image areas. A blanket cylinder is then rolled over it, picking up the inky image onto its soft rubber surface. Paper then passed between this blanket cylinder and the hard impression cylinder, which presses (kisses) all three surfaces together, transferring the image to the paper. If a web press is used, an additional roller system will cut the paper. By using an intermediate soft roller to transfer the image, the delicate metal litho-plate never comes into contact with paper. Since the plate will suffer far less abrasion through this process, it can yield more impressions before wearing out. Color prints made from the offset process were also easier to keep in registration because the paper does not pick up any moisture from the plate that might cause shrinkage. Previously the printing plate and paper needed to make perfect contact but here the soft blanket can pick up and deposit ink much better than a hard surface, creating impressions on almost any material with greater fidelity.

Photograph

Rotary Offset Press: This photograph shows us a mirrored image in ink that has been offset onto the blanket cylinder from the printing plate on the rotary cylinder of an offset press.


The rotary offset process has been in use since the early 20th century but it did not immediately catch on because of the high quality images that could be pulled from litho-stones printed on a flatbed press. Even though offset production increased when litho-stones became scarce during the First World War, plates were still difficult to used and only played a minor roll in the production of postcards. It was only after 1951 when an easy to use, storable, photosensitive aluminum litho-plate was developed that the offset process became popular. The newest plates are made from steel, which are thiner and lighter than the plates traditionally used in lithography. They also have a much smoother grain since they suffer less abrasion when a blanket roller is used. This smoother surface allowed for the doubling of dots per inch (dpi) that could be printed. Sometimes the entire plate would be hardened (anodized) or the image would just be electroplated with copper. Press runs up to a half million could be achieved with hardened plates on presses printing 800 impressions per minute. Offset lithography quickly became the standard in photochrome postcard production that completely replaced their closest competitor, linen postcards. While larger web fed presses produce much commercial work, nearly all postcards produced today are made through process printing on small sheet fed offset lithography presses.

The popularity of offset printing has led to competing techniques like gravure and letterpress to be readopted to these principals rather than be completely eliminated. Letterpress now uses an impression cylinder as an intermediary step when transferring ink from the plates reliefed surface to the paper. This process is referred to as letterset or dry offset. Despite these efforts to modernize old technologies, new digital technologies of the 21st century are beginning to replace the very notion of offset printing.

Postcard

Offset Lithography: For most of postcard history this simple design would have been automatically typeset and printed as letterpress, but as a modern postcard the image has been reproduced by the most common contemporary printing method, which is as a halftone on an offset press. Unlike the early 20th century when there was a multitude of printing houses employing a number of techniques, there were far fewer printers by the end of the century and nearly all relied on offset printing.


Unlike chromolithographs, the white of the paper on process prints was allowed to show through an image to help create optical tone, but this exposure also cut down on color saturation. The addition of extra tinting plates of tan or grey that once helped reduce contrast and hide the dot patterns were largely abandoned on photochromes because they reduced color saturation even more. This effect was tolerated because the technology of process printing had steadily improved to the point where it could render the natural color captured in photographs with great fidelity. At the same time as color imagery became more commonplace, it began to draw less attention to itself forcing printers to seek out more saturated palettes. While this seeming betrayed the basic principals behind natural color, perception is largely a matter of interpretation, not optics, and postcards had to satisfy common taste.

Postcard

Offset Lithography: This postcard reproducing a much older multicolored chromolithograph was printed in only four CYMK colors on an offset press. Even though less ink is printed on the paper’s surface than typically found on a chromolithograph, more white of the paper shows through rendering the image much lighter and more vibrant than a traditional chromolithograph. At the same time its coloration lacks the richness and subtlety often created through older techniques.

Postcard

Offset Chromotypograph: The rivalry between letterpress and color lithography led to line block being adapted to multiple plate color printing in the 1860‘s under the name, chromotypography. While this postcard from 1953 is printed in offset lithograph, it uses the same principals and a red, yellow, and blue palette normally associated with chromotypographs. It does not try to imitate the look of older work as much as incorporate older methodology to create a unique card.

Printers had always retouched photochromes to pump up individual colors, but the results not only tended to look artificial, they often looked odd. After higher saturated dyes were added to film and the overall color of ink was also enhanced, very striking postcards began being produced with offset printing. The high color saturation of these cards were no longer viewed as a flaw in the printing method but as a marketing necessity to attract straying customers. The more modern postcards printed in offset lithography have a much smaller dot pattern allowing the black plate to provide more subtlety in tone. This in turn has lessened the need to have as much white paper show through, allowing for more color to cover its surface. Black also has the tendency to make the colors surround it appear brighter by contrast, while the exposure of white paper only dulls saturation. Modern photochromes have a denser and darker look to them but they carry far more saturated color because of it.

Postcard

Photochromes: Both of these postcards of colorful subjects are made through process printing in CYMK hues, but the postcard above dates from 1958 while the card below was printed sometime after 2000. Even though the more modern card printed in offset lithography has been given higher color saturation, it looks more natural than the older card because it has a more dense and nuanced distribution of dots.

Postcard


Postcard Detail

Photochrome Details: n the detail above from the older card further above, much of thewhite paper can be seen showing between individual dots. Even though many of these areas were never meant to appear white, they were left bright to help create optical tone. In the detail from the newer card shown below, there is almost no white paper to be found. Optical tone has been created here by the ability to infuse very small black dots into the image allowing the remaining hues to expand into higher concentrations of color.

Postcard Detail

Gelatin photosensitized with dichromate had been the base for most photomechanical transfer techniques, but most plates used in offset lithography are pre-sensitized with a coating of photosensitive diazo emulsion. After exposure, these plates are developed with a solution made up of lacquer and a gum etch that dissolves the diazo away. The lacquer, which will attract ink is deposited in the areas exposed to light while the etch reacts with the rest of the bare plate to make its grain hold moisture. It can then be dampened and rolled with ink for printing. In driography the litho-plate is first coated with an emulsion of diazo and silicon. Here the silicon will deposit itself on the non-image areas and its rubbery surface will repel ink without the need to use water. There have been a number of additional variations made to litho-plates to suit specific needs.

Postcard

Offset Lithography: Since the dots of halftone litho-plates are transferred to paper by means of a soft roller when printing in offset, they tend to enlarge (dot gain) as the ink is pressed outwards forming fuzzy borders. Sometimes dot gain increases to the point where nearly solid blocks of color are created. Each of the two details below were created with rotated halftones but they are from two different postcards printed more than 100 years apart. In the detail from the postcard above pictured immediately below, the higher dot gain and saturation of inks used in offset lithography are obvious.

Postcard Detail


Postcard Detail

A great deal of scientific thought in the 19th century went into discovering the means by which humans perceive color, which was followed up by putting this knowledge to practical use in reproducing color. Soon after photography was discovered, a new quest was born to find a way that it could reproduce color with the same fidelity that it captured an image. As photography grew to become an essential part of the printing industry, the quest for reproducing natural color in printed form came with it. The end of this search came into reach with the introduction of the modern photochrome, and digital technology’s ability to explore the way we see with much more precision has in turn only improved printing technology. Despite the importance of our newfound ability to render images in natural color, it is only sometimes used in the production of postcards today. It seems that most of us are really looking for something we like, not what is real. It seems that our understanding of color and theory has just given us more tools to use, but this use will always be a balance between production costs and public demand.

Postcard

Offset Lithography: While nature can provide us with colorful displays that defy the imagination, we as consumers have come to expect the spectacular far more than it appears in real life. As a result the reproduction of natural color is usually put aside to render images in more appealing exaggerated colors. Exaggeration is now needed for an image to pass for real.



Tonal Offset Lithography

In the early years of halftone reproduction a black & white image would simply be printed in black ink on white paper, but now it cannot be assumed that black is always black on postcards printed in offset. Printers have since come up with a variety of ways to create subtle shades and tones that can produce a much richer printed image. Tinting plates inked with some fairly neutral color were commonly added to older images to soften their contrast and render line screen patterns less visible. This method is also employed in modern offset printing, but a far more common practice is to create a duotone image where one plate prints in grey. The same negative is used to create both printing plates, but each receives a different exposure. The plate to print grey will be exposed for a short time to capture the details in the highlights while letting shadows fill in. The longer exposure on the black plate will bring out details in the shadows while bleaching out the highlights. When the black is printed over the grey it covers much of it, but the grey highlights shine through in areas that were the most bleached on the darker plate. This principal can be carried even further by printing a light and medium grey in addition to the black plate (tritone printing).

The duotone and tritone process largely compensated for an inherent problem in photographic reproduction. Light passes through the pale areas of a negative with a much greater intensity than through the dark areas so that the image gains contrast (highlight jump) and with it looses detail during every reproductive cycle. Even though high quality images can be created through tinting methods, most black & white postcards tend to employ the full CYMK process palette, though sometimes near composite blacks (process blacks) are created with CYM colors alone. Most shops are only set up for color printing and any deviation from standardized methodologies brought with it the risk of costly mistakes so printers did not often stray. While the same subtitles of tone cannot be produced with a CYMK palette, it can capture nuances of color cast on images that for the most part are rarely purely black or white. On the other hand these added colors can also cause their own shifts in color cast under changing light (metamerism).

Postcard

Offset Black & White: The postcard above only appears to be printed in black but there are two many subtleties in the grey to all come from black ink. All colors that read grey are actually an optical mix of the CYM primaries carefully balance out against the white of the paper to print in a near neutral range. In the detail below a full CYMK pallet can be observed.

Postcard Detail


Postcard

Offset Black & White: This black & white postcard was printed with a full CYMK pallet but these colors only appear within the darker tones because they were exposed to bleach out of the brightest highlights of the image. In the detail below they seem to fill in the center of dots within the black halftone but this is only because no color is printed in the lightest toned areas.

Postcard Detail

Offset Tints
While most neutral tones in offset were printed with a full range of CYMK colors, there were alternatives that eliminated the need of primary palette. A common method was to use a single negative to expose two different plates. The one exposed to create a high contrast image would be inked in black; the other exposed to leave more tone would be inked in a lighter color. This range of exposure could be extended to include three, four, or five plates each carrying different values of the same or similar colors.

Postcard

Offset Tinted Duotone: This tinted postcard reproducing an old photograph was printed with just two plates; black was used for the dark tones and brown for the light. The detail below shows that the dots are still rotated to form the rosette pattern typical of a process print.

Postcard Detail


Postcard

Facsimile Duotone Tint: This postcard is meant to look like a traditional duotone tint printed from a plate holding a black key and the other a solid color tint. The image is actually printed in a full CYMK palette rendered through halftone as revealed in the detail below.

Postcard Detail


Postcard

Offset Tritone Tint: This postcard printed with three plates is a hybrid between the black and grey duotone process with a traditional tinting method. In the detail below we can see that the base id formed by laying down a solid toned field of neutral tan that warms and softens the image. It is a subtle effect more clearly visible when comparing the image with its surrounding border. A medium grey halftone was then laid over it as bedding for the high contrast black halftone. The two tints lower the overall contrast even more than in a duotone tint, while still allowing the top layer to print a rich black.

Postcard Detail


Postcard Detail

Quadradot: Some images were not tinted with color but only with a neutral grey. This detail shows extremely fine halftone dot patterns laid down in varying sizes from an image printed in black and four different values of grey.



SCREENLESS OFFSET LITHOGRAPHY

In the 1960’s a new method of offset printing was developed in which dots were created without the need for halftone line screens. The results very similar to the look of photo-chromolithographs that were manufactured at the beginning of the 20th century. This process begins by mechanically graining anodized aluminum litho-plates with both high and low random peaks to resemble the surface of a lithography stone. After being coated with a photosensitive polymer emulsion the plate is exposed to a transparency. In this case the random peaks that receive the most exposure to light will print white and the lesser exposed specs between them will accept ink. Tonal range is created through the differing peak heights. Since the dot array is completely irregular, four process colors can still be used to produce a full gamut without the risk of forming interference patterns. While screenless offset printing is still being used, the advancement in finer line screens and stochastic screens combined with advancing digital technology has prevented this extraordinary process from dominating the printing industry.

Postcard

Screenless Offset Lithography: At first glance the bright colors and flat fields make this postcard appear to be printed in photo-chromolithography. Even in the detail below it looks as if its colors were made up of odd shaped blots of bitumen. On closer examination however we can see that the surface of this image is actually printed in very small dots only made up of CYMK colors. Without the use of a halftone screen many small dots of one color have a tendency to clump up, but as we can see in the even higher resolution detail further below the dots are still so small they are barely perceivable and form no set patterns that can be discerned.

Postcard Detail





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