Reworking the Rainbow: Printing on Ceramics and Glass

WHILE CONTROLLING COLOR is challenging in any type of screen printing, printing designs for ceramic and glass decoration poses some unique obstacles. Among the most significant is that the image color at the time of printing is quite different from the fired color, which won’t be seen until several hours, or even days, later. In the following pages, I’ll take a closer look at this and other issues in color control and proofing that printers face when producing ceramic decals with process-color separations.

Fundamentals of ceramic color

Screen printing for ceramics is generally done in one of two ways: Colors are printed directly onto the ceramic or glassware, or they are printed onto decal release papers and then transferred onto the ware. In either case, after the decoration is applied, the product is fired at temperatures ranging from 950-2200°F (510-1205°C) to yield the final image.

Decorations on ceramic and glassware range from solid, single-color designs to continuous-tone spot-color images featuring upwards of twenty colors, and from basic process-color images to process colors with additional integrated “boost” colors. The final fired design is extremely durable and may include many special effects, such as bright or burnished gold and platinum, iridescent colors, and textures. All the fired colors are permanent.

The color fidelity of a fired decal depends on three major factors:

1. Pigments Ceramic pigments tend to be much less pure and vibrant than the corresponding pigments in conventional printing inks, such as those used for offset lithography and screen printing.

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2. Color space Because of pigment limitations in ceramic inks, the color information for the original image must be “remapped” from a standard color space, such as SWOP-CMYK, RGB, or L*a*b, into the unique color space that will be used to render that image with ceramic colors. However, even after a color has been remapped into ceramic color space, significant areas of the conventional-printing color space will remain unattainable.

3.  Firing Fired color is the result of chemical interactions that occur within the inks at high temperatures. Therefore, anything that influences these interactions will affect the final color.

Most ceramic pigments used in process-color decal printing can be classified as cadmium based or gold bearing. In most situations, these two groups are chemically incompatible with each other–when they’re both used in the same image, any area of overlap in printing between the two pigment types tends to turn brown when fired. However, a few colors, such as cyan and black, are cross-compatible.

<P>Cadmium-based pigments offer strong reds, yellows, and oranges, but are very weak in the rest of the color spectrum. The gold-bearing colors are used primarily for the remaining range of the spectrum: green, blue, purple, magenta, as well as yellow. The gold-bearing colors are not especially brilliant, but they are superior to what is available from the cadmium pigments in the green, blue, and magenta ranges.

A typical process-color separation might consist of six colors in order to approximate a broader range of final colors in the design. Generally, these six colors include cadmium red and cadmium yellow, gold-bearing magenta and gold-bearing yellow, plus cyan and black.

This six-color set will cover most artwork without undue compromise, but there are some notable exceptions. First, any artwork that has gentle transitions of color from red into magenta, such as a sunset, is problematic. The color range required by the image cannot be rendered fully with either the cadmium or gold palette. Furthermore, since a sharp mask in the sunset sky between cadmium and gold colors would be objectionable, it is sometimes necessary to print such images exclusively with one of the two systems, which can significantly compromise color fidelity.

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The second problematic issue is that cadmium colors tend to be more difficult to print than gold-bearing varieties. Inks made with cadmium pigments dry quickly on press in the smaller highlight-dot areas of screens. This rapid drying makes cadmium colors unsuitable for printing critical shades in the quarter tones, such as flesh tones. While much of the flesh-tone range can be rendered adequately with a combination of gold-bearing magenta and yellow, bright, rosy cheeks are a challenge. And when this rosy shade is used by an artist to give shape to a face, trying to recreate it with the gold palette will result in a dull color that may make the face look undefined and flat.

Translating color spaces

The methods used by ceramic printers to remap color from a standard color space into ceramic color space are varied and generally quite proprietary. Unlike conventional offset printing, which typically uses industry-accepted standards and methods for color separation, most ceramic screen printers develop their own color-separation systems. These systems are a combination of prepress techniques and screenmaking, ink, and printing parameters designed to support each facility’s unique environment.

Generally, it is not possible to use one set of unmodified films or digital separation files to support printing at multiple facilities. Rather, once the artwork or electronic file is received, each facility must develop its own process-color separation method to reflect the capabilities and limitations of the ceramic inks and production techniques it uses.

Today, almost all ceramic process-color separations are done on a computer. While some facilities use Adobe Photoshop to manipulate color, a few color-separation systems specifically for glass and ceramic decorating are also available.

Regardless of the software used, all separation approaches start with a file created in a known color space, which is then translated into ceramic color. The known color space could be a graphics-arts standard, such as SWOP-CMYK, RGB, or a similar color model. The best color space to begin with varies from facility to facility, so when preparing artwork for a project, make sure it complies with your printer’s preferences.

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Converting color between a standard space and a ceramic space can be quite time consuming. You can make the conversion manually by “borrowing” color from channels in a standard space to create a new ceramic color channel. For example, a cadmium red ceramic channel might be built from color borrowed from the magenta and yellow channels of a standard CMYK separation. Alternately, you can make the conversion automatically by simply changing the color “mode” or model of the graphic from a standard space, such as RGB, to a predefined ceramic color space. Both of these approaches have their merits and faults.

The borrowing approach offers artists the most latitude in generating the desired ceramic color, but it is a tedious process that requires much time and expertise. The automatic mode change yields initial separations very quickly, but due to the complexity of ceramic color, each separation may require extensive adjustment before the image is ready for press. With either approach, the quality of the final fired color is only as good as the skill of the people who generate the artwork, print the decal, and fire the decorated piece.

When it comes to building ceramic color separations, the primary challenges involve maximizing in-gamut color fidelity and minimizing out-of-gamut compromises. Those colors that are attainable with a given color model, such as a custom ceramic color space, are called “in gamut.” While establishing and controlling in-gamut color is challenging, the approach that works for one project can generally be applied to other similar projects, as well.

Colors that cannot be accurately rendered by a given separation set are called “out of gamut.” The rosy cheeks example mentioned previously is a good example of out-of-gamut color. Unfortunately, most conventionally printed colors–to which ceramic decorations are often matched–are out of gamut for ceramics. And to make matters worse, the most troublesome colors are those that give the original image much of its perceived depth, detail, and realism. Out-of-gamut color is the most challenging type to handle when producing separations for ceramic decorating because there are no right answers, only com-promises.

Any solution to an out-of-gamut color will involve compromising one or more of the following color characteristics: hue, saturation, or brightness. An example of such a compromise would be a brilliant blue sky.

Assume that the original art presents a scene representing midmorning on a perfectly clear day. Most likely, the color of the sky on such a day would be beyond the gamut of color you could attain by mixing ceramic magenta and cyan. An extra blue separation to boost the color might be helpful, but, for this example, assume that the cost of an additional printing pass is prohibitive. So the only solutions must come from varying the percentages of magenta and cyan you’ll use.

The blue sky would probably exceed the gamut of this ceramic-color combination in both saturation and brightness. If you increase the percentages of magenta and cyan equally to boost the saturation, the sky will darken and appear stormy. If you decrease the cyan and magenta equally to preserve the brightness, the sky will appear more white and, thus, less “clear.” If you move the hue of the sky closer to either magenta or cyan, you may be able to increase both the saturation and brightness. But an increase in cyan will make the sky appear more blue, as it would at midday, and an increase in magenta will move the sky toward the pinker cast of the morning sky.

Any of the previous compromises will limit the color range within the sky, thus reducing the sky’s apparent depth and the perceived separation between the sky and the foreground. Additionally, the gentle transition in saturation of blue from a richer appearance in the upper sky to a paler appearance at the horizon may also be lost. In the end, the compromises you settle on will depend on what is happening in the rest of the image and what is important to the client. Pitfalls in proofing

<P>While it would be easier and more economical to resolve color-reproduction questions with clients by using some form of prepress proof, rather than a proof run on press, currently no accurate prepress color-proofing system exists for glass and ceramic decorators. Again, the problem stems from the limited gamut ceramic inks provided and the fact that fired and unfired colors appear quite different.

With conventional printing, the same film or digital separations used for production can be used to render accurate prepress proofs, and the process colors used for each layer of the proof can be closely matched on press for hue, saturation, and density. When proofing conventional prints, the typical workflow generally begins with the digital artwork, which is output to a contract proofing system. The proof print is then submitted to the client for approval. If the contract proof is rejected, adjustments to the original artwork can be made, and a new proof can be quickly generated and presented to the client.

Once a proof is accepted, it can be closely replicated on press during the production run. Since most conventional printers run all of their colors at once, and since the printed color is also the final color, the accuracy of the press prints relative to the proof can be quickly verified as the first production sheets are completed.

Unfortunately, ceramic printing has yet to benefit from similar technology. Although technically feasible, no proofing system currently available will accurately represent fired ceramic colors. Thus, ceramic printers must rely either on their computer monitors or on conventional contract proofs with non-ceramic colors. With this latter option, color fidelity depends on the accuracy of the translation from ceramic to conventional color space and is subject to all the same translation problems described previously.

Due to the lack of a reliable, standardized contract-proofing system for ceramics, the ceramic proofing workflow is much more laborious than its conventional counterpart. Typically, clients will provide original art, a transparency, or a conventional contract proof as a color target. The ceramic printer then must translate the incoming color into the ceramic color space best suited to the project, using the color displayed on a computer monitor to verify separation accuracy. Film is then output, screens are made, and the job is sent to press.

Next, the press operator will print the work, focusing primarily on controlling dot gain. Since colors on the final print bear little resemblance to the colors that will develop with firing, assessing the printed but unfired decoration is unlikely to uncover color fidelity problems. Color assessment must wait several hours for the decoration and firing to be completed. Then, if color problems are detected, the entire process must be adjusted and repeated–from prepress through production.

Firing concerns

Correct firing parameters are also critical in maintaining color fidelity. In order for the printed pigments to attain their final colors, they must mature at a given temperature. Any variation from the desired temperature and atmosphere of the kiln can alter the color that develops during the firing process.

While the peak temperature of the kiln is important, it is also important that the pigments themselves reach a particular temperature. With heavy pieces of ware, longer exposure to peak kiln temperatures may be necessary for the ware and pigments to reach the correct temperature. If the ware is not exposed to peak temperature for a sufficient amount of time, the pigments in the decal may not mature, resulting in a decal that is underfired. The color of an underfired decal will tend to be dull and muddy, and the surface finish may lack gloss.

Conversely, if a decal is overfired (exposed to an excessively high temperature), colors may burn out and lose vibrancy. Differences of just 20-30°F (approximately 10-15°C) from the optimal firing temperature recommended by the color supplier can degrade final image quality.

The atmosphere in the kiln can also influence color, especially with cadmium colors. If the kiln provides insufficient air circulation, cadmium colors may fail to develop. Ceramic printers usually print cadmium inks as heavily as possible to minimize this problem, but the heavy pigment load alone won’t compensate for a lack of circulation. So it is important to conduct test firings of sample decals in order to identify and resolve problems as early as possible.

Hope on the horizon

While conventional offset printers can rely on a highly repeatable contract proof as a cornerstone around which to build a successful print run, ceramic screen printers have only the final fired result for guidance. And this final result is subject to a long list of variables, such as printed dot size, ink thickness, concentration and mixture of pigments in the ink, and the exact temperature and atmosphere within the kiln during firing. With such a broad range of variables, it is very difficult to yield subtle changes in one area of a print without introducing subtle and unwanted variations into other areas.

Recent technical advances offer hope for resolving some of these issues. As color separation technology and color management through ICC profiling are developed further, it may be possible to get fairly reliable translations into and out of ceramic color. These translations would enable ceramic printers to offer more economical separations, reduce the cost of art development, and make prepress proofs feasible.

Additionally, ceramic pigment manufacturers are continuing to develop the quality of their pigment ranges, which should reduce the amount of out-of-gamut color that printers must resolve in order to yield a color-accurate product. Until these solutions become available, however, printers must continue to rely on their knowledge of ceramic colors and careful control of the entire decorating process.

About the author
Jere Williams is the Prepress Manager at Heinrich Ceramic Decal, Worcester, MA. He has more than a decade of experience in the screen-printing industry, and has been involved in applications such as textile decorating, point-of-purchase graphics, and decal screen printing.