Overview
ISO 18936:2020 - "Imaging materials - Processed colour photographs - Methods for measuring thermal stability" defines standardized test methods for assessing the long-term dark storage stability (thermal stability) of processed colour photographic images. Applicable to traditional photographic systems (chromogenic, silver dye-bleach, dye transfer, dye-diffusion-transfer “instant”) and to digital outputs (dry- and liquid-toner electrophotography, thermal dye transfer / dye sublimation, and inkjet), the standard provides procedures for accelerated ageing tests that support predictions of likely image change over time.
Key topics and technical requirements
- Scope and applicability: Test methods cover a broad range of photographic and digital print technologies for dark storage stability evaluation.
- Test principles: Uses accelerated thermal ageing (Arrhenius-type approach) to extrapolate long-term behaviour from elevated-temperature tests; empirical confirmation is required for validity.
- Sample preparation: Guidance on target selection, use of replicates and reference samples to ensure statistically meaningful results.
- Holding and measurement conditions: Specifies controlled temperature and relative humidity parameters, notes effects of humidity and atmospheric pollutants, and highlights sensitivity of low glass transition temperature products.
- Test methods: Two principal exposure configurations:
- Free-hanging method at constant relative humidity
- Sealed bag method to maintain constant moisture content
- Measurement endpoints: Monitor changes in density, colour balance (differential fading of cyan/magenta/yellow), stain formation and other visible degradations.
- Reporting requirements: Defines what must be recorded in a test report (general conditions, specimen details, equipment, measured results); computational details for Arrhenius predictions are now in ISO 18944.
- Caveats: Notes that some materials exhibit atypical behaviour (e.g., instant prints, some chromogenic and dye-bleach products) and that elevated conditions (above ~60% RH or very high temperatures) can produce non-representative degradation.
Applications and who uses it
ISO 18936:2020 is used by:
- Manufacturers of photographic papers, films and printing inks/toners for product development and quality assurance.
- Conservation scientists, museums and archives assessing storage strategies and expected lifetimes for collections.
- Testing laboratories performing accelerated ageing and permanence testing.
- Print service providers and color technologists evaluating output stability for client specifications.
Practical uses include validating materials for long-term preservation, setting storage recommendations (temperature/RH), and comparing product stability across media and processes.
Related standards
- ISO 18944 - Arrhenius-type predictions and computational methods (referenced for calculations)
- ISO 18924 - Test method for Arrhenius-type predictions
- ISO 18941 - Ozone gas fading stability for colour reflection prints
- ISO 18913 - Permanence - Vocabulary
- ISO 5-3 / ISO 5-4 / ISO 13655 - Density and colorimetric measurement methods
Keywords: ISO 18936:2020, thermal stability, dark storage stability, colour photographs, accelerated ageing, Arrhenius, inkjet permanence, chromogenic, photographic preservation, museum archives.
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