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      Dampening in offset printing

  Acid or alkaline reactions of watery solutions are caused by their content of hydrogen ions.
  This reaction is described as pH value, which is expressed on a logarithmic scale in figures between 0 and 14. 
  Neutral water has a pH value of 7, figures below 7 indicate increasing acidity of the liquid, higher figures indicate increasing alkalinity. Contrary to the exact scientific explanations of the pH value, the offset printer should rather imagine that the pH value is comparable to a “ruler” with a division from 0 to 14.

  It should also be kept in mind, that an increase or decrease of the pH value by one numerical value means a multiplication of the acidity resp,alkalinity by the factor ten. In practical work a pH value of the damping medium of 5.5 to 5.8 has proven advantageous for offset printing. A slightly acid influence on the water-carrying parts of the plate and increased surface tension are achieved thereby.
The pH value is usually measured by means of specially treated paper, so-called indicator paper which changes its color upon contact with the liquid; this decoloration is compared with a given color scale and thus permits to determine the pH value. Unsuitable pH values of the dampening medium can be very detrimental to the printing result. By adding a buffer solution to the dampening medium, the pH value can be kept in the pH range of 5.5-5.8, which is favorable for offset printing.

Possible consequences when using an acid dampening medium with a pH value below 5:
      1.        Drying difficulties of the printing ink
      2.        Oxydation of metallic inks
      3.        Shorter operating life of the printing plate.

And when using alkaline dampening medium with a pH value above 7:
      1.  Reduction of surface tension between printing ink and dampening    
          medium; the ink emulsifies.
      2.  The plate tends to scum.

Water hardness
The quality of the water used is of extraordinary importance for trouble free printing.Tap water is not water in its chemically pure form. Its suitability for use as dampening medium is above all determined by the type and by the quantity of the salts dissolved in it. The water hardness is a measure for the content of dissolved salts of alkaline earth metals; it is expressed in “degrees German hardness” (0dH). 10 dH is the equivalent of 10 mg calcium oxide in one litre of water.

The following guide provides the classification of the water hardness:

          0–40 dH very soft                12–180 dH rather hard
          4–80 dH soft      18-300 dH hard
          8–120 dHmedium soft        > 300 dH very hard

10dH = 1,25 eH (English Standard) 
       = 1,79 fH (French standard)

  The water hardness varies geographically very much, it can even fluctuate temporarily. A water analysis, which can be done e.g. by the manufacturers of damping water additives, will give the information on the quality of the available water. The printer himself can also determine the hardness by means of small indicator rods or test solutions.
  For offset printing the water hardness should not be higher than 100 dH.  If the harder water is used, calcium and magnesium salts may settle on printing plates, blankets and rollers and disturb the ink respectively.  Moreover, the chemical reaction of calcium sales with fatty acids in the printing ink may cause lime soaps to develop. These then act as wetting agent and can also disturb the printing process by making ink-receptive areas water-receptive and vice versa.These difficulties can be avoided by installing a water softener. 
  If such deposits are already present on the surfaces, they can be removed by treating the surfaces with a solution of 50-g tartaric acid in 1 litre water.This method, however, is very time consuming and can by no means replace water softening.

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