Degradation of cultural heritage materials is commonly associated with colour change, e.g., tarnishing of silver, yellowing of varnishes or colour fading of certain pigments. Though these alterations cause relevant visual changes, as their progress is slow, it may take some time to draw attention to the problem. An adequate system to detect these changes would improve conservation of heritage collections.
Colour variations have been used in heritage studies for evaluating stone soiling and decay [1,2,3,4,5], degradation of paper [6], varnishes [7], pigments [8], to quantify metal corrosion [9, 10] and for detection of defects on fresco [11]. This relation between colour and decay can be used to design a preventive conservation tool based in a systematic monitoring of colour change to detect conservation problems in heritage objects in museums and outdoor monuments.
Colour change can be measured periodically on real objects to assess their condition as in the previous examples, or on reference materials that react when exposed to degradation agents (light, humidity, or volatile organic compounds) to detect possible threats. For instance, blue wool standards, metal coupons or acid-sensitive strips can be used as visual alerts for light-damage, sulphur compounds or acids [12, 13]. These colour changes can be measured using a colorimeter or a spectrophotometer, obtaining accurate data for colour. However, this requires access to this specific piece of equipment, direct contact with the object to be measured and is time consuming. With the popularization of digital cameras, these devices have been proposed as an alternative, low-cost system to measure the colour change [14]. These devices are not intended for colour measurement, so each sensor has different response. Therefore, in order to use digital images for colour measurements, a proper calibration of the image is required, which is usually done by comparison with results obtained from a spectrophotometer or a stable colour reference chart [2, 15, 16]. This methodology has been explored by Brigham et al. [16] to use crowdsourced images from mobile phones for colour measurements. Pictures of an X-Rite ColorChecker®chart were taken with different smartphones, adjusted with an open-source package for image analysis and compared with direct measurements made with a colorimeter.
Standard photographic colour charts as X-Rite have the advantage of being a known standard of validated stability, but they have not been tested for use in proximity of heritage objects -it is known that many materials can outgas harmful volatiles-, the selection of colours has been made with other purposes (similarity to typical colours in photographs, such as green of foliage, blue of the sky or skin tones) and they are quite expensive for a large-scale use of the system.
Here we propose a further development of this approach, which includes the design and validation of a tailored colour chart and the application of computer image analysis for colour evaluation.
The design and validation of our own colour chart has two major advantages over using standard commercial colour references. The first one is reducing the cost, so this methodology can be used as an affordable monitoring system, that can be deployed in many sites. The second one is that allows the selection of colour, number, and disposition of references in the colour chart, optimizing the references for the calibration of the images. It has been demonstrated, in other applications, that a colour chart specifically tailored to the colours to be measured can yield a better performance than a general-purpose one [17, 18].
Based on these ideas we propose a methodology to design a monitoring system for preventive conservation based on colour change. The system is based on a colour chart with an empty area in the middle, where the reference material for study is placed, e.g. metal coupons, to be used as a dosimeter for degradation agents in a museum environment. Colour change of metal coupons can be followed through image analysis of pictures adjusted and calibrated using the surrounding colour chart of chromatic coordinates. Source images can be acquired either by conservation professionals, other museum staff or, more interesting, by visitors, introducing citizen science in the conservation strategy.
As sensitive material to detect aggressive environments, we use metal coupons. Pictures of the reference charts taken periodically will allow monitoring the museum environment through colour change of metal coupons by computer analysis. For this purpose a methodology to calibrate the images has been developed [15], and an automatized system is being developed to automatically process a large quantity of images.
The aim of this paper is to present the design and testing of the colour chart and the validation of the methodology of image analysis for monitoring colour change, assessing the stability of the materials used (both substrate and inks) and their harmfulness to the metals used as colour sensors. This proof of concept may be used for other applications too.