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.

This paper presents results on the protective properties of different coatings over metals representative of those found in scientific–technical heritage collections. An examination of several objects in the collection of the Spanish National Museum of Science and Technology have allowed the identification of brass and steel as the most representative metals, and the existence of coatings (mainly shellac and acrylic resins) applied for aesthetic and/or conservation purposes. Based on these findings, brass and steel coupons coated with Paraloid B-44, B-48, B-67, B-72, Incralac, Cosmolloid H80, B-72 + H80 double layer and shellac have been studied to carry out a first screening in order to select the most promising candidates and determine the most appropriate methodology and key factors for the study. Coatings have been aged up to 336 h using cycles of UV light and water condensation. The change in the aspect of the coupons has been assessed via visual examination, optical microscopy and colorimetric measurements, with B-44 and Incralac showing the least change. EIS has been used for a quantitative evaluation of the protective properties and FTIR to measure chemical changes experienced by some coatings, such as B-48 and B-67. These evaluations allowed us to follow, understand and compare the behaviour of the different coatings and substrates after artificial ageing.

The main component of the cement hydration, are both, the calcium silicate hydrate (C-S-H) and calcium silicate hydrate with Al (C-S(A)-H), whose composition is characterized by its calcium to silicon ratio (Ca/Si), which normally varies from 0.6 to 1.6. The theoretical Ca/Si ratios of the synthesized gels were compared with those of the experimental gels, which were determined by inductively coupled plasma atomic emission spectroscopy (ICP-OES). In addition, the microstructure of the gels was studied by spectroscopic techniques: Infrared and Raman spectroscopy and Nuclear Magnetic Resonance. By the double decomposition method used in this work (1 day at 25 ⁰C, inert atmosphere and pH = 12,3), only C-S-H and C-S(A)-H gels with a maximum Ca/Si ratio ranging from 0.8 to 1.0 were synthesized. However, the structures of the gels are slightly different as the Ca/Si ratio increases.