Protecting metallic cultural heritage from corrosion is a major challenge for metal conservators. Electrochemical techniques offer a great potential for monitoring the conservation condition of this type of heritage, providing a powerful tool for designing conservation strategies. For this reason, the application of techniques such as electrochemical impedance spectroscopy (EIS) has aroused increasing interest in recent years, although some difficulties in application and interpretation of the results still keep it from being routine practice. This Review aims to picture the state-of-the-art of using electrochemical impedance spectroscopy (EIS) for the in situ monitoring of metallic cultural heritage, focusing on spectra interpretation and different approaches that have been proposed to perform field measurements since the 1990's. In the first part of the Review, different cell designs based on the use of liquid or solid electrolytes for in situ measurements are reviewed. In the second part, a few general equivalent circuits are proposed as a basis for interpreting results on different metal surfaces after discussing different models proposed in literature. This overview intends to summarize and clarify the key points in the use of EIS, and encourage the use of this and other electrochemical techniques in the field of conservation science, for a better preservation of our cultural heritage.

An electrochemical sensor for corrosion studies on cultural heritage has been designed.• The effects of the cell setup on EIS measurements are evaluated.• Cell contribution can be discriminated from the working electrode response.• Field measurements on outdoor sculpture are comparable to laboratory results.• The cell has proved to be a useful tool for in situ conservation assessment.

a b s t r a c tTarnishing is produced by reaction of silver with gaseous reduced sulphur compounds from atmospheric pollution. It induces the formation of Ag 2 S crystals which produce a dark appearance. To remove sulphur tarnishing, different mechanical, chemical and electrochemical methods have been traditionally used. To assess the effect of different cleaning treatments on tarnished silver, coupons of pure and sterling silver (92% Ag/8% Cu) were subjected to six cycles of induced tarnishing and cleaning. The cleaning methods evaluated were mechanical (soft abrasives and rubber point mounted on a rotary tool); chemical (chelating and acid solutions) and electrochemical (potentiostatic reductions). Surface morphology, composition, weight, color and luminosity were evaluated by optical microscopy, scanning electron microscopy/energy dispersive X-ray spectroscopy, atomic force microscopy, X-ray photoelectron spectroscopy and colorimetry. The cleaning impact on tarnished silver depended on the cleaning procedure and the composition of silver. Mechanical treatments restored the original visual appearance of silver although they produced a significant mass loss and a fast re-tarnishing. Chemical cleaning methods were fast; nevertheless the surface appeared completely attacked. Electrochemical cleaning did not yield good result for sterling silver, but was an effective cleaning method for pure silver coupons. Different cleaning methods have been used for many years to remove sulphur tarnishing from silver. However, scientific studies which assess the treatment's impact on silver and the re-tarnishing rate are very scarce. Costa [1] carried out a complete bibliographic study about the cleaning procedures and their applications, although the results of these different scientific works are difficult to compare since they have not been systematically evaluated in similar conditions and materials.The aim of this study is to quantitatively assess the efficiency, residues left on the metal and impact on the base metal of eleven mechanical, chemical and electrochemical cleaning methods to remove tarnishing on pure and sterling silver. To evaluate the effect in the long term, several cycles of tarnishing/cleaning are considered.

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The use of gelled electrolytes has revealed an interesting solution for in situ electrochemical measures on cultural heritage, where liquid electrolytes cannot be easily handled. After developing an agar gelled electrolyte cell, other gelling agents such as agarose are being considered to improve it. In this work, the performance of agarose gels in different concentrations has been studied and compared to agar and a liquid electrolyte. Measurements have been done on patinated quaternary bronze and weathering steel, as representative materials of outdoor monuments. Agarose gels have been validated for in situ corrosion studies on bronze and weathering steel sculptures.

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a b s t r a c tIn this study we evaluate the laser cleaning of tarnished pure and sterling silver substrates using a nanosecond Q-switched Nd:YAG laser at 1064, 532 and 266 nm. To assess the effects associated with cyclic laser cleaning treatments, several cycles of tarnishing followed by laser cleaning were applied on silver coupons that were characterized by gravimetry, colorimetry, scanning electron microscopy, X-ray photoelectron spectroscopy and micro-Raman spectroscopy. According to the obtained results, none of the three wavelengths is recommended for laser cleaning of pure silver objects, while for sterling silver artifacts, the visible laser wavelength of 532 nm seems the most appropriate.

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.