# Towards a Glass New World: The Role of Ion-Exchange in Modern Technology
## 1 Introduction  
To celebrate, over time, the wedding anniversary between ion-exchange and glass, all noble metals (e.g., silver, gold, platinum, etc.) and gemstones (e.g., sapphire, emerald, diamond, etc.) offered by our generous planet would not be enough! In fact, the origin of this lucky marriage is lost in the dawn of time. The art of ancient glassmakers can be traced back to the 3 rd millennium BC among the peoples of Egypt and the Middle East where the main raw material, the silica rich sand, was widely available. From there, this knowledge was transferred to the peoples of the Mediterranean Sea, including the Greeks and the Romans (see, for instance, the natron glass used during the Roman Empire period). Over these three millenia, the colouring of glass was given directly during its manufacture by introducing additive metal oxides, such as iron, copper, manganese, etc., to the raw materials before firing the whole system in a kil and then cooling it in order to obtain the final product [1, 2]. Only between the 6 th and 7 th centuries AD, the Egyptians began to use silver or copper pigments--in the form of powders--to decorate and colour the surface of their artefacts, such as dishes, vessels and pots, by means of a thermal annealing process in a furnace. Through this technique, the pigment became part of the atomic structure of the products, colouring them in correspondence of their surface. However, due to the low temperature of the firing process and the difficulty of controlling it, the final artefacts remained rather opaque [3, 4]. Later, around the 9 th century AD, this staining method--improved in terms of technological process (i.e., higher operating temperature; better kilperformance) and now capable of giving glass lustre to ceramic artefacts--found great diffusion among the Mesopotamian peoples before arriving in Spain, in the first centuries of the millennium, following the expansion of Islamic culture [5,6]. In Europe, ion-exchange experienced a notable application during the outbreak of Gothic architecture (between 12 th and 14 th century) contributing to the realisation of the wonderful multi-coloured stained-glass windows of the cathedrals of that period. A paste, composed of clay/ochre and silver chloride/sulphide, was spread on the glass to be treated in order to produce a thin layer on its surface. Subsequently, the whole system was fired in a reducing atmosphere--induced in the furnace by introducing smoking substances--at a temperature close to the softening point of the glass. In these conditions, the ion-exchange process was triggered: the silver ions diffused inside the glass replacing the alkaline ions, such as sodium, widely present in the pristine glass. Finally, the simultaneous presence of reducing elements, both external (i.e., the smoking elements in the atmosphere) and internal to the glass (i.e., impurities, such as iron or arsenic), favoured the formation of silver metal nanoparticles (NPs) inside the ion-exchanged specimen, leading to a yellow-amber colour whose tonality and intensity depending on the process parameters (i.e., time and temperature) and the size of the NPs thus formed (from a few to one hundred nanometres), respectively. Other colours, such as red or green, were obtained using iron and copper salts, respectively. In the following renaissance period, this decorative technique became particularly popular in Italy in the creation of polychrome and lustre pottery artefacts, among which those of [PERSON] stood out par excellence [5,7]. It was only around the beginning of the 20 th century that the foundations for a possible application of the ion-exchange process in the technical-industrial field were laid. In 1913 [PERSON] was the first to study the diffusion of silver ions into the glass using silver nitrate salt (AgNO\({}_{3}\)) as ion source, starting a whole series of studies aimed to understand the chemical-physical nature of the phenomenon and its effects on some physical properties of the glass so treated.