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

In 2011, the European Commission proposed a definition of "nanomaterial" as a material containing particles with one or more external dimensions between 1 and 100 nanometres. A nanometre is one million times smaller than a millimetre. 

Although talk of nanomaterials research has become more widespread in recent years, especially because of the interest in many of their applications, mankind has been using nanomaterials since ancient times, whether naturally or artificially produced. Scientific characterisation techniques, which have been refined and have gained in resolution in recent decades, have allowed us to gain in-depth knowledge of their composition and structure and thus understand the formation mechanisms of these historical nanomaterials, surprising us with the technology that was used centuries ago. For example, some research dates the first production of ruby glass, due to the presence of gold nanoparticles inside it, to the 7th century BCE, based on Assyrian tablets that speak of the production of "an artificial coral". 

The contribution made by this research on the alteration of the gold decorations of the Alhambra shows us, on the one hand, the complexity of the work carried out to create these extraordinary works of art and, on the other, provides arguments to explain how they degrade under certain conditions to produce gold nanoparticles. The study has been able to combine the use of several high-resolution analytical microscopy techniques, complementing each other, to gain an in-depth understanding of the result of the corrosion of the decorations and to propose possible formation mechanisms that explain the appearance of these "strange" purple colourations. Despite the "nobility" of gold, certain situations such as, in this case, humidity, the presence of a high chloride content and dirt lead to its slow decomposition, with oxidation and subsequent reduction phenomena that have seriously affected the work of art. The case of the colour we see in gold, like that of other metals, is very curious because it depends on the size of its particles, which can be red or purple if they are nanometres in size, or golden when they form sheets. 

The results of this research highlight the importance of proper conservation of our heritage, avoiding conditions that could alter it, especially at times like the present, when extreme temperatures or high levels of pollution can be common. They also provide, for the first time, the necessary keys to understand the appearance of these alterations and, therefore, to deal with possible restorations and proper conservation of this and other works of art with gold leaf decorations.