Buying on the Basis of Brightness

Coated and solid bleached sulfate cover stocks and packaging grades are sold primarily by brightness specification, in addition to dimension and quantity. Paper mills develop and produce these specifications with a combination of process control and the addition of performance chemicals.

On the process side, the mills screen, clean, and bleach wood fiber to a brightness spec of their own. At the paper machine, the paper maker has to decide what performance chemicals will work best to meet your specification: an optical brightening agent (OBA) in the paper slurry, high-brightness minerals in the coating, or temperature and pressure control on the calender stack.

The printer and designer then will select paper based on this brightness level, and pay for it.

Paper color, commonly referred to as shade, comprises two elements—blue-whiteness and red-green—that can be measured empirically. Mills using OBAs and other materials also will sell blue-whiteness not with empirical data, but with marketing and individual sight verification.

Says Dr. Bryan J. Ortman, MeadWestvaco's global graphics segment manager, "The paper industry defines paper shades using the CIE L*a*b three-dimensional model of the visual light spectrum." Ortman relates that the "L" value represents whiteness, or the total amount of light reflected off the paper's surface, while "a" represents the red-green axis of the space and "b" represents the blue-yellow axis.

"Paper manufacturers typically control the 'b' value to characterize the paper's style in marketing," he states. "A 'b' value of less than zero exhibits a blue shade ('blue-white'), while a value greater than zero exhibits a yellow shade ('cream' or 'warm')."

Blue-white sheets also will exhibit red hues, Ortman explains, while "warm-white" will exhibit shades of green. "On press, a printer must compensate for these shades to achieve accurate color reproduction, because most inks are transparent when applied," he says.

With each mill using a varying degree of control in process and product quality, shades vary greatly from mill to mill. While sheet brightness does not constitute a part of paper's color, it has become more of a marketing gimmick and a rather hollow pricing mechanism, blowing away the old numbered system of paper grading. Paper shade has become increasingly important to printers who expand multilaterally in region and product base.

Paper selection is a complicated process, as blue-yellow and red-green shading values are not always known nor reported in shipping documents.

Basically, paper makers do not regularly control the "a" value (letting it float) while they use process changes to control (or not control) the "b" value. Cream whites will run as high as +3.5, while blue-white will go as low as -1.5.

Remember that as you move in one direction, a compensation is required in the other. For example, for the reproduction of landscapes and outdoor images, blue-white looks great. But if a person (flesh tones) is added, more cream is needed.

Blue-white and high-brightness sheets tend to get print runs up to color faster, but, for the printer and print buyer, at what cost to image quality? Mills have the same problem in the trade-off. The use of OBAs is a cheap fix, but forces the paper maker to stay within the blue-white arrangement with no way to compensate for red-green (bias to red).

Mills currently are working on developing neutral-shade, premium-coated wood-free grades to eliminate this important aspect of paper selection. MeadWestvaco has emerged with a new version of enhancements to its Tango line, with what it calls absolute white styling, which seeks to control the shade to zero.

States Ortman emphatically, "To achieve superior color reproduction on press, the most desirable sheet is the one with no color value. The absolute whiteness of this enhanced Tango product results in more vivid colors and helps printers achieve greater precision in their color reproduction."

Paper companies have spent billions of dollars on their wood fiber processing plants to maintain the highest levels of environmental protection and fiber quality.

MeadWestvaco's Covington, Va. mill and Sappi's Cloquet, Minn. mill are two of the latest projects to come to completion. In MeadWestvaco's case, the project also included the development of process techniques designed for the elimination of OBAs to reach optical properties specifications. The project also entailed the development of new products that would act as superior stocking grades for versatile printing operations.

"When sheets containing OBAs are viewed under typical office conditions," says Ortman, "they don't seem to achieve the same brightness level as they do under natural light. Our work with Tango Absolute White's new enhancements strived to make the printed sheet look the same under different lights and with reduced coloration over time, preserving corporate color and image identities regardless of where they were viewed."

Ortman adds that Tango Absolute White is FDA approved, and offers high opacity and smoothness.

Sappi Fine Papers has also worked on its pulp quality to make McCoy coated cover stocks a success. The converting performance in the bindery and folding scoring have earned McCoy accolades in premium brand markets. Further, McCoy's propensity for cracking at the fold has been virtually eliminated with a proprietary coating technology.

Sappi also has added coating and sheeting equipment to its Muskegon mill to increase productivity within its Vintage and Lustro lines.

Finally, International Paper recently released a new coated cover grade in its Carolina line called Centennial Web Cover. Centennial, a 91-brightness grade available in four thicknesses, carries a U.S. Postal Service guarantee.

Much work at the Rochester (N.Y.) Institute of Technology and among favored printers has proven the new product's capability both on press and in converting applications.