Materials such as wood, textiles, or plastics that are part of the exhibition system in museums are known to emit pollutants such as organic acids. Scientific and technical objects that include these materials in their composition can themselves be a potential source of emissions, which, together with inappropriate humidity and temperature conditions, can lead to corrosion of the metallic parts. In this work, we have studied the corrosivity of different locations in two venues of the Spanish National Museum of Science and Technology (MUNCYT). Coupons of the most representative metals from the collection were placed in different showcases and rooms for 9 months. The corrosion of the coupons has been evaluated in terms of the rate of mass gain, colour changes and characterisation of the corrosion products. The results were correlated to the relative humidity and concentration of gaseous pollutants to determine which metals are most susceptible to corrosion. The results show that metal artefacts exposed in showcases have a higher risk of corrosion than those exposed directly in the room, and that some pollutants are emitted by the artefacts. The corrosivity of the museum environment is low for copper, brass, and aluminium in most locations; however, some placements present a higher aggressivity for steel and lead, due to the high humidity and the presence of organic acids.

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 lack of a standardised methodology for the application of coatings on heritage metals can lead to non-comparable results. A careful and meaningful study requires considering the different application variables involved, especially in the preparation of the coupons. In this work, the effect that these application variables can have on the protective capacity of the coatings used has been studied. The influence of the thickness, number of layers and drying time (between layers and final) of Paraloid B-72, C80 microcrystalline wax and Incralac has been evaluated. Coatings have been applied on bare steel coupons, thickness measured with an induction probe, and subject to artificial ageing on UV light/water condensation cycles. The performance of the coatings has been studied by visual inspection and electrochemical impedance spectroscopy (EIS). Morphology of the layers has been characterised by optical microscopy and the composition of the corrosion products by Raman spectroscopy. Results show that the number of layers is not always proportional to the final thickness of the coating, and that drying time is a critical factor affecting the thickness and the protective properties of the coatings. After accelerated ageing, some coatings that had been left to dry for 14 days have a much better protective capacity than those that were left to dry for less time. Without taking into account these factors, performance measured can be wrong and comparison between materials misleading.

The development of new alternatives for the protection of metallic heritage aims to improve the performance of the coatings used in recent decades. For this purpose, the performance of three novel or poorly explored coatings in this field of study has been evaluated: Owatrol Oil, TiO₂-modified Paraloid B-72 and a hybrid of poly(2-ethyl-2-oxazoline), PEOX and poly(4-hydroxy styrene), PHS as a self-healing coating. These have been compared with coatings widely used in conservation such as: Paraloid B-72, Incralac and microcrystalline wax C80. The coatings were evaluated after accelerated ageing in terms of aesthetic changes (visual observation under stereomicroscope and colour measurements), protective ability (electrochemical impedance spectroscopy) and chemical stability (FTIR). After 500 h of exposure to Xenon-arc lamp, it was observed how the addition of TiO₂ to Paraloid B-72 partially reduces photo-oxidation of the polymer, but at the same time harms the protective ability due to the appearance of defects. The self-healing coating is quite stable and can also regenerates superficial scratches of about 100 µm if subjected to 90% RH. On the other side, Owatrol demonstrated a very poor performance. These advantages and disadvantages in their use have been compared with those of B-72, Incralac and C80 microcrystalline wax, identifying in which cases their application for protection on scientific-technical artefacts may be feasible.