The longevity of digitally printed images

 
The issue of longevity is very complex and in the early days of this technology, it is not possible to be categorical about results. The papers used for digital ink jet printing fall into two categories, coated and uncoated. The coatings applied are specially formulated to give good ink hold out and low dot gain, ensuring a sharper image. The formulations of the coatings are a closely guarded commercial secret by all manufacturers.

When considering the longevity of the images there are a few certainties:
  • Uncoated papers will give better longevity than equivalent papers which have been coated, but will not give as good a printed result.
  • Pigmented inks will give better longevity than dye based inks, but tend not to give such a good colour gamut.
  • Longevity is improved when ink density is increased. - This is part of the reason uncoated papers give better image life, because they require greater ink saturation to provide the same colour density.
  • Since the inks sit more on the surface of coated papers it is subjected to more environmental degradation and radiation.
  • Chemical contamination from the atmosphere or other chemicals such as the organic solvents in glues will seriously affect paper coating, potentially causing discolouration.
  • Some glue in adhesive tape does not even require direct contact with the paper to cause discolouration. Tests indicate that there are instances when a particular tape does not cause a problem when in direct contact with the paper. However, the same tape with an acidic ‘catalyst’, such as packaging board adjacent to the paper, will result in discolouration. Archival, pH neutral tapes are available and tests suggest they are less likely to cause a problem. See www.ukindustrialtapes.co.uk for more.
  • The atmosphere in London is approximately pH 4.5. The effect of this on coatings, inks or the reactions between the two is unknown.
  • All organic matter, including paper, is bio-degradable and as such should be treated with care.
  • Papers, which are bright white, contain varying degrees of Optical Brightening Agents (OBA). The OBA converts the longer wavelength ultra violet light into the blue end of the visible spectrum. This "blue-whiteness" reduces as the unstable OBA degrades. This results in paper looking slightly more yellow than originally. This is quite separate from a paper being Neutral pH or Acid Free.
  • If the whiteness of paper is achieved by the use of stable compounds such as Titanium Dioxide, their colour will better withstand the adverse effects of light radiation.

 

There are also some uncertainties:
  • The “guarantees” associated with many products are dependent on compliance with certain conditions. It is invariably the case that the end user would be unable to subsequently prove that they have complied with any of them for the whole life of a print. e.g. constant temperature, constant relative humidity, fixed light intensity, within certain spectral parameters and with specific shielding.
  • Changes in relative humidity affect life expectancy, though like many of the variables, the extent and nature of the effect is not certain.
  • The life expectancy tests conducted do not seem to give figures for standard deviation of expected result errors, despite extrapolation of limited data over a short period to expectations many years into the future. This may explain why tests conducted on the same paper sold under different brand names, with the same ink sets, has provided widely varying results.
  • Many tests involving high levels of radiation result in raised temperatures, which may affect the accuracy of the extrapolation.

 
It often seems that the questions asked about the longevity of prints are new. This is not the case. The issue has been a subject of discussion for all types of printing methods for years.

Many consider that one of the great advantages of digital prints is that they can be reproduced at any time, with confidence that they will resemble the original. The Guild of Fine Art has always insisted that a set of prints had to be produced in one run, in order to be faithful reproductions, because there were assumptions made about how they would be printed. This view now seems a little out-dated.

In conclusion, it can be said that when considering longevity; to use pigmented inks is better than dye based inks, uncoated papers are better than coated papers and the careful handling and storage of a print is necessary.

 

Clifford Burt - R. K. Burt & Company Ltd.

 
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