Offset processing surfaces

The importance of letterpress printing today is very much reduced by the cold-set offset process, whereby most of the newspapers are printed. Conventional newspapers are uncoated papers, based on recovered and deinked printing papers and/ or mechanical pulps. However, a coated surface of such a newspaper or another base paper made from a relatively cheap furnish opens new applications for the cold-set web-offset process with significant improved print quality and cost-performance (Fig. 7.7). 

Blistering in web offset process (due to high temperatures in the drying section of the printing machine, the humidity of the paper web will evaporate instantaneously. Unless the coating has a high porosity, this will lead to blisters on the printed paper surface)... 

However, since in practice, the paper passes through not one but several printing presses (4—8 in the offset process), a further test can be performed to examine the effects of this sequential stressing of the paper surface. This is the so-called offset test... 

P.Y.12 and considerably more so in comparison with P.Y.13. A number of the P.Y.14 types are appreciably greener than the standard yellow on the European Scale. P.Y.14 is not only weaker than comparable P.Y.13 varieties with similar physical characteristics, such as specific surface area but it is also less lightfast by 1 to 2 steps on the Blue Scale. Its resistance to solvents is also comparatively poor. This somewhat limits its use for process inks in offset and letterpress application to special cases, which is equally true for P.Y.14 blends with reddish pigments. Types with fine particle sizes, which match highly transparent versions of P.Y.12 and 13, are not available in Europe. 

The primary objective of preprocessing treatments is to remove the nonchemical biases from the spectral information. Scattering effects induced by particle size or surface roughness may lead to offsets or other more complex baseline distortions. Differences in sample density or the angle of presentation may induce overall intensity variations as well as additional baseline distortions. Most samples are not perfectly uniform on the microscopic scale, and these effects may dominate the initial contrast observed in an un-processed chemical image. In some cases this contrast may provide useful information to the analyst however, it is generally removed in order to focus on the chemical information. 

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