Reacción a "Reactions to the study showing how corrosion has turned the Alhambra's gold leaf into purple nanospheres"

This article presents the study of a pathology or deterioration that is somewhat peculiar and, therefore, little studied: the corrosion of the gilding of the plasterwork of the monumental complex of the Alhambra. This deterioration manifests itself through a purple formation in the gilding. The researchers present a successful multi-analytical methodology to determine both the nature and the distribution of the corrosion products present in the gilding. In this case, the tin layer applied before gilding assisted the subsequent formation of gold nanospheres deposited on the surface from the original gold layer. The purple colour observed would be associated with what is technically known as surface plasmon resonance, which, through the presence of metallic nanoparticles (gold in this case), causes a tonal change in the surface where they are deposited. In this case, the addition of gold nanoparticles has formed spontaneously, but many researchers are currently taking advantage of this phenomenon to, for example, tune or modify the colour of materials to taste, detect nanoplastics in the environment by means of a colour change due to their interaction with metal nanoparticles, etc. 

This work demonstrates that the use of microscopic techniques and analysis at low lateral resolution is crucial for detecting and analysing particles at the micro and nanometric scale. It also highlights the coupling of different analytical techniques in order to determine the elemental and molecular nature. This coupling of instruments is crucial if one wants to determine the nature of randomly distributed phases or decay products at the microscopic scale. Therefore, this article shows that, although it is a good proposal for research in the conservation of cultural heritage, it is necessary to have state-of-the-art instrumentation in order to be able to respond to the problems that arise in this field.  

In addition, this work shows that, on many occasions, it is crucial to be able to have a sample available for study in the laboratory. Taking into account the historical-artistic value of heritage assets, it can sometimes happen that sampling is restricted or even unauthorised. Nowadays, there are numerous portable alternatives based on non-invasive analysis techniques, which allow direct analysis of surfaces or materials. In some cases, these techniques are sufficient to obtain conclusive results. However, when the material or object to be analysed is structured in different layers on a microscopic scale (as is the case here) and the aim is to study what happens in each of them and how they affect the rest, it is essential to take a sample. Without a stratigraphic study, the authors of this work would never have been able to propose the chemical deterioration route described to explain the pathology studied. 

This study also shows that, on many occasions, it is not necessarily anthropic factors or sources caused by humans that promote the deterioration of cultural heritage. In this case, as the authors point out, humidity itself and the diffuse impact of marine aerosol may be sufficient to assist the deterioration process described. 

For all these reasons, this article presents a clear example of how science applied to cultural heritage is crucial to understand the mechanisms of deterioration and the causes that give rise to them. This information is extremely valuable in order to subsequently develop appropriate conservation strategies to conserve the jewels of our heritage such as, in this case, the Alhambra.