Rubylith, Amberlith - a Dying Art Form

Figure 1
Figure 1

This essay on Emulsion Mask work uses terms that are explained in a previous HubPages article: “Offset Printing Explained.”

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There is a certain art that has been a “behind-the-scenes” contributor during its life. This art is called “Rubylith” or “Amberlith” masking in the Pre-press area of offset printing. Here, I’m calling it Emulsion Mask Work, so to not ignore or downplay either product by not mentioning it. Mostly, I’ll refer to the product or process as “Litho Mask” or “Litho Masking.” The reason it is considered as being behind the scenes is because the process itself is not seen, nor the elements of it; only the results, and that through a printing process which cloaks the method of production. And now, the art is exhaling its last breath of life as desktop publishing replaces it.

The illustration above is an example of what Litho mask work can produce. Normally, a scene like this is used in an ad, with large words that pull in the reader, then smaller text to push the product. Litho masking would help to mask the text area and any insets used, but those are left out here to show the entire illustration. Only occasionally has “four-color” Litho mask work such as this one been done to produce an illustration.Litho mask is an acetate/solid emulsion combination, designed for the purpose of masking light that is burned onto a light-sensitive offset plate. The emulsion is a deep red, as shown in Figs. 2-6. An image in the emulsion is cut out with an X-acto or similar knife, then lifted off the acetate. The artist soon learns how hard to push, so to not cut through the acetate. Litho mask emulsion cutting is most effectively done with a brand-new blade. V-shaped X-acto blades are so finely pointed that it is easy to break off the tip. (When that has happened - often within the first few seconds of cutting - I’ve replaced the blade with another new one, and tried applying a lighter touch.) The remaining Litho mask sheet is put over a light-sensitive offset plate, and the part that was peeled off the acetate becomes the image burned onto the plate.

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Besides masking areas for ads, Litho masking was used for large related areas of a photograph, to produce an effect. For example, the whole foreground of a photo was highlighted with the same color, or with a shiny coating by using a Litho mask window. Or, a certain product - to accent it above other areas in the same photo - was given a color splash by using a Litho mask window and double-burned onto an offset plate. Litho masking has also been used to create starbursts, shiny lines around a product, and for a number of other convenient uses. One use was to help expose an array of photos that touch each other. Such photos cannot be taped down without cutting corners off and putting tape there. So photos are put down in alternate spaces, like the dark areas of a checkerboard. This way it is easy to tape down the photos on the edges, because a margins of the photos don’t need to be cut off. This would be one burn. Another burn would be photos put down in the left over spaces like the white areas of a checkerboad. One Litho mask would mask the areas not being exposed, and another would mask the first set burned while allowing a window for the second set.

When a particular shade or tint of a color is desired, then a piece of pre-processed film containing millions of tiny dots is placed next to the plate before a Litho mask film is put down for the burn. These are called “screens.” They come in density gradations from 5% to 95%. Screens, as exposed through Litho mask windows, have typically been used to fill in backgrounds behind a photo, or to provide color blocks in which to put text. Both the plate and overlaying masks are always put down on guide pins to assure proper registration of the images that will be printed. The screens are usually not pinned for registration, but are in fact turned a few degrees for each color, to avoid a moire pattern - if multiple are printed on top of each other.

Some form of Litho masking still exists today, but it’s basically used by silkscreen printers. But most of those use a light-sensitive ink resister emulsion where the unexposed parts dissolve away in the developer bath (see http://www.zverina.com/2003/0319.htm).

To save time, different screen values will be found on the same plate for a given color of ink. This is done automatically on one negative when separating colors out of a color photograph. But when using Litho mask, the artist has to determine which screen values will be used. In Fig. 7, different screen values are shown on a plate, as burned using the masks in Figs. 2-6, for the color Cyan. (For this article, I’m only showing the masks for Cyan, and for the illustration in Fig. 1. If all colors were represented, then there would be up to twenty or more samples.)

Figure 8
Figure 8

To get different values on the same plate, it would require multiple sheets of Litho mask, and different screens with different size dots to let the light through. This would therefore require multiple burns. One Litho mask sheet would expose an area where 30% Cyan is required, while masking off the rest of the area. The next Litho mask sheet would close off that area, and open up another that required, say, 60% dots.

Another Litho mask sheet for Cyan could expose two areas for both the grass and for a piece of wood painted brown. These two images would be the same dot percentage - let’s say 50% - and therefore appear equal. But when the Magenta ink is added later, the grass will receive 10% dots, and the wood will get 60%. At this point, the two areas would be two different colors. When yellow comes through, the grass might get 60%, and the wood will probably get 80%. Now the colors are more distinct and recognizable for the items they represent. When black comes through, 20% for grass and 30% for wood, then a more clear distinction for each area will happen. Fig. 8 shows this progression.

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Figure 9
Figure 9

In Figure 9, I showed what it might look like if a little bit of yellow and some magenta were added to the cyan family. Fig. 1 shows the completed project.

I don’t think you’ll ever see, these days, ongoing production of some of the intricate work described here. One exception could be if the artist uses a computerized plotter outfitted with an X-acto knife blade. But the art I’m talking about is done by hand, by people cutting out one patch at a time on Litho mask, while at the same time determining which colors or tints of colors are to be produced from each finished mask, and combinations of masks as described above. Figures 2 through 7 are samples of what kind of work has been done in the past. Anyone who possesses actual sheets of this kind of work, should hang on to them; they’re probably worth gold, by now.

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Comments 2 comments

Simone Smith profile image

Simone Smith 5 years ago from San Francisco

I've never heard of Rubylith - how interesting! You've explained and illustrated it wonderfully. Voted up!


Peggy W profile image

Peggy W 5 years ago from Houston, Texas

Very interesting even if it is a dying art form. Nice to know how it was done. Voted up!

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