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Reworking the Rainbow

(March 1999) posted on Mon Dec 13, 1999

Learn about the issues in color control and proofing that printers face when producing ceramic decals with process-color separations.

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By Jere Williams

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.

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


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