Control Procedures

Some of the quality control parameters used to monitor Hahnemühle papers are described below.

• Thickness
  Paper thickness or calibre significantly affects the feel of paper and thus the value of the work of art or the photograph. Thickness is measured with a thickness gauge and calculated at a pressure on a test area of 2 cm².

• Bursting strength
  Paper resistance to pressure applied vertically to its surface. This parameter defines paper resistance to burst. The test area of 10 cm², stretched on a rubber membrane, is subject to a steadily increasing loading pressure. On bursting, the loading pressure is measured in kp/cm².

• Tensile strength (break resistance)
  Paper resistance to tensile force. An increasing vertical load is applied to a 15 mm wide and 100 mm long test strip of paper. The amount of force on breaking is the tensile strength of paper. Tensile strength is measured both in the machine direction and in cross direction.

• Smoothness
  Paper smoothness is extremely important for paper quality, particularly for printing papers. The reproduction fidelity of screen dots and colour range depends on the surface evenness/porosity that is the smoothness of paper.

• Gurley
  Measurement of filtration time (air permeability) of 100 ml of air at a certain pressure on a test area of 1.56 cm²

• Cobb test (water absorption)
  Paper sizing degree is determined according to the water absorption ability of a test area of 100cm². The test area is weighed when dry. Then it is subject to 100 ml of distilled water, which is removed after 10 minutes. Testing consists in establishing the difference of the tested area before and after water absorption.

• Surface pH test
  The pH value is measured with a surface electrode after a precisely defined reaction time of 1 ml of distilled water. The reaction time, i.e. the time the tested surface is exposed to water, is between 5 and 10 minutes, depending on the type of paper.

• Ink absorbency
  Paper sizing/surface properties are tested with various inks/paints (e.g. oil paint, acrylic paint and watercolour). During the test both paper ability to absorb paint and paint bleed are tested.

• Wax-Pik Test (Wachsstift Test)
  Mittels Wachsstiften unterschiedlicher Härte wird die Oberflächenfestigkeit (Rupfen) von geleimten Papieren ermittelt. Die Wachsstifte werden nach dem Erwärmen auf die Prüffläche gesetzt und nach ca. 15 min wieder ruckartig entfernt.

• Ink limit
  The maximum absorbency of ink before ink dots start to become displaced.
  It is measured as a percentage of a particular colour (CMYK).
  The higher the ink limit, the bigger the ink area which can be reproduced on paper. The result is good colour saturation and diversity.