Design and validation of tailored colour reference charts for monitoring cultural heritage degradation

Abstract

Colour changes of cultural heritage objects can be related with degradation of materials, thus a proper colour monitoring system can be used to detect conservation problems. With this purpose, a monitoring methodology for cultural heritage preventive conservation based on tailored colour reference charts and image analysis is proposed.Reference colour charts have been designed and tested for use in museums. Charts containing 64 colour patches have been printed using high-stability inks on 4 different substrates: Acid-free paper SkyLight, Acid-free paper covered with a propylene film, FOREX® and GlassPack. The stability has been studied by accelerated ageing in an UV chamber, and the harmlessness of the materials by Oddy Test. The final selection of material, laminated paper, is a balance between the colour change upon ageing and the performance in the Oddy Test. Using this material and the proposed design, colour change of copper and silver coupons has been assessed using images that are adjusted and calibrated by an adaptive calibration framework employing a given set of reference colours which homogenises the visual information in the supplied images. Thus, regardless of the camera of origin, any processed picture will deliver reliable information of the state of the colour in the metal surfaces at the moment it was taken.Results demonstrate the adequacy of the approach and the design for colour calibration, so these charts can be used to monitor colour change of sensitive materials –metal coupons– using photographs. As colour change of reference metals is a consequence of corrosion by environmental factors this may be used as a measure of air quality in museum environments. This methodology can be used to design a low-cost preventive conservation tool, where colour change of metal coupons –or other reference materials– can be followed through image analysis of pictures taken periodically by conservators or visitors, introducing citizen science in the conservation strategy.

Assessment of the Robustness of a Color Monitoring Chart Calibration Method for Crowdsourcing-Based Preventive Conservation

“…These inks are used for printing advertising posters intended for outdoor display, and they are known to resist solar radiation. Their lightfastness is rated 5 (yellow and magenta) and 8 (cyan and black) to blue wool standard according to the manufacturer [11].…”

“…Aside, in order to demonstrate the capabilities of the conceived charts, an experiment that involves the calibration scheme outlined in [11,13] over pictures taken with commercial brands of phones has been performed. With the material chosen with the best results (explained in the following sections), a chart has been printed and photographed.…”

“…The objective of this system, besides being a low-cost preventive conservation tool, is to increase social awareness in relation to heritage conservation. For this purpose, the system uses crowdsourced pictures taken by visitors with their cell phones [11] (Figure 1). The usage of crowdsourcing techniques is acknowledged as a useful resource, which has been used for long in monitoring of biodiversity, astronomy, and other fields [12].…”

“…The previous stages of this project, the conception of the color chart, the verification of the suitability of the materials for the monitoring system, and an adaptive calibration algorithm for image processing have already been presented [11,19]. The color chart is mounted on an inert substrate together with several metal coupons used as sensitive materials.…”

“…So, the materials employed to build the display and chart and the coupons need to be carefully selected so they do not undergo possible strong chromatic deviations from the initial state; and they should not produce alterations in the metal coupons and, more important, in the heritage artifacts [11]. It is utmost important that the reference color chart remains unaffected so the quality of the picture calibration does not lower when deployed for a prolonged time.…”