ISO 18936:2020

INTERNATIONAL ISO
STANDARD 18936
Second edition
2020-08
Imaging materials — Processed
colour photographs — Methods for
measuring thermal stability
Matériaux pour l'image — Photographies couleurs après traitement
— Méthodes de mesure de la stabilité thermique
Reference number
©
ISO 2020
© ISO 2020
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ii © ISO 2020 – All rights reserved

Contents Page
Foreword .iv
Introduction .v
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Requirements . 2
5 Sample preparation . 2
5.1 Target selection . 2
5.2 Use of replicates and reference samples. 3
6 Holding and measurement conditions . 3
7 Test methods — Thermal stability. 4
7.1 General . 4
7.1.1 “Free hanging” versus “sealed bag” methods . 4
7.1.2 Humidity effects . 5
7.1.3 Testing of low glass transition temperature products . 5
7.1.4 Concerns around the effects of atmospheric pollutants . 5
7.2 Test methods and equipment . 6
7.2.1 Temperature . 6
7.2.2 Relative humidity . 6
7.2.3 Number of specimens . 7
7.2.4 Free hanging method at constant relative humidity . 7
7.2.5 Sealed bag method (constant moisture content). 8
7.3 Computation of dark stability . 8
8 Test report . 8
8.1 General reporting requirements . 8
8.2 Test reporting . 9
Annex A (informative) Illustration of Arrhenius calculation for dark stability .10
Bibliography .13
Foreword
ISO (the International Organization for Standardization) is a worldwide federation of national standards
bodies (ISO member bodies). The work of preparing International Standards is normally carried out
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electrotechnical standardization.
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described in the ISO/IEC Directives, Part 1. In particular, the different approval criteria needed for the
different types of ISO documents should be noted. This document was drafted in accordance with the
editorial rules of the ISO/IEC Directives, Part 2 (see www .iso .org/ directives).
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iso/ foreword .html.
This document was prepared by Technical Committee 42, Photography.
This second edition cancels and replaces the first edition (ISO 18936:2012), which has been technically
revised.
The main changes compared to the previous edition are as follows:
— the test methods for environmental stress factors have been changed to align with ISO 18944;
— the calculations and computations section has been removed as they are now contained in ISO 18944;
— Annex A has been removed as the method for interpolation is now contained in ISO 18944:2018,
Annex B;
— the usage and reporting requirements have been updated to ensure consistency within the
documents of the ISO 189## family.
Any feedback or questions on this document should be directed to the user’s national standards body. A
complete listing of these bodies can be found at www .iso .org/ members .html.
iv © ISO 2020 – All rights reserved

Introduction
This document covers the methods and procedures for measuring the long-term, dark storage stability
(thermal stability) of colour photographs.
Today, the majority of photographs are made with colour dyes and pigments. The length of time that such
photographs are to be kept can vary from a few days to many hundreds of years, and the importance of
image stability can be correspondingly small or great. Often the ultimate use of a particular photograph
may not be known at the outset. Knowledge of the useful life of colour photographs is important to many
users, especially since stability requirements often vary depending upon the application. For museums,
archives and others responsible for the care of colour photographic materials, an understanding of the
behaviour of these materials under various storage and display conditions is essential if they are to be
preserved in good condition for long periods of time.
Any change in density, contrast or stain, whether due to colourant fading, changes in colourant
morphology or discolouration of residual substances will change the appearance of the photograph.
The most damaging change tends to be contrast balance distortions brought about by differential fading
of the three image colourants. These manifest themselves as shifts in colour balance from highlights to
shadows that are especially noticeable in a scale of neutrals, for example a shift from magenta to green
due to fading of the photograph’s magenta image colourant, or from yellow to blue or cyan to red due to
fading of the yellow or cyan colourant.
The second most consequential change is that caused by an increase in stain. The result may simply be
a discolouration of the D areas (unexposed processed media or unprinted substrate) or a change in
min
the D colour balance.
min
Cyan, magenta, yellow, and sometimes black, red, green and blue colourants that are dispersed in
transparent binder layers, or absorbed onto special receiver layers coated onto transparent or white
opaque supports, form the images of most modern colour photographs. Colour photographic images
typically fade during storage and display; they will usually also change in colour balance because the
image colourants seldom fade at the same rate. In addition, a yellowish (or occasionally another colour)
stain may form and physical degradation may occur, such as embrittlement and cracking of the support
and image layers. The rate of fading and staining is governed principally by the intrinsic stability of
the colour photographic material and by the conditions under which the photograph is stored and
displayed. The quality of chemical processing or post-processing is another important factor. Post-
processing treatments and, in the case of digitally generated photographs, post-production treatments,
such as application of lacquers, plastic laminates and retouching colours, also may affect the stability of
colour materials.
The three main factors that influence storage behaviour or dark stability are the temperature and
relative humidity (RH) of the air that has access to the photograph, as well as atmospheric pollutants
to which the photograph is exposed. High temperature, particularly in combination with high relative
humidity, will accelerate the chemical reactions that can lead to degradation of one or more of the
image colourants. Low-temperature, low-humidity storage, on the other hand, can greatly prolong the
life of photographic colour images for typical materials. Other potential causes of image degradation
are microorganisms and insects.
Most modern photographs degrade too slowly under normal room conditions to permit evaluation of
their dark storage stability within reasonable periods. However, it is possible to assess the probable, long-
term changes of some photographs under low and moderate storage conditions with accelerated, high-
temperature tests, because recognizable losses in image quality under high temperatures are apt to be
generated also under milder temperatures, if at a slower pace. The effects of relative humidity on thermal
degradation can also be evaluated with Arrhenius tests conducted at two or more humidity levels.
Long-term changes in image density, colour balance and stain level can be reasonably estimated only
when good correlation has been confirmed between accelerated tests and actual conditions of use.
Density changes induced by the test conditions and measured during and after incubations include
those in the support and in the various auxiliary layers that may be in a particular product. With most
materials, however, the major changes occur in the image-bearing layer. An exception to this is found in
some inkjet papers where the inks are thermally stable and substrate yellowing is the failure mode (see
Reference [9]).
The tests for predicting the stability of colour photographic images in dark storage are based on an
adaptation of the Arrhenius method described by Bard et al. (see References [2] and [3]) and earlier
references by Arrhenius, Steiger and others (see References [4], [5] and [6]). Although this method
is derived from well understood and proven theoretical precepts of chemistry, the validity of its
application to predicting changes of photographic images rests on empirical confirmation. Although
many chromogenic-type colour products yield image fading and staining data in both accelerated and
non-accelerate
...