# Spectroscopic and Microscopic Characterization of Flashed Glasses from Stained Glass Windows
## 1 Introduction  
Flashed glasses are constituted by two layers of different thicknesses; a thick one of clear or light glass, and a thin colored layer applied on top of the first one. The production of flashed glasses usually involves dipping a colorless base glass into a colored bath, producing a multi-layered glass pane in which the thickness of the colored layers controls the color tone [1]. By sandblasting or etching the flashed (thinner, brighter) layer, a striking two-tone effect can be achieved [2]. Flashed glasses have been used since medieval times in stained glass windows, especially for obtaining those of ruby-red color. The procedure to obtain this type of flashed glass is very complex as it involves the nucleation of copper nanoparticles of a suitable size in the glass matrix. If the nanoparticles are too small, light could pass through the material without interaction, while if they are too large, the glass would appear dark brown or black [3, 4]. Additionally, the thickness of the colored glass layer also affects the color hue, being more intense when the glass is thicker [1]. This complexity, added to the expensive raw materials, has favored the development of these two-layer glasses, with the suitable color and the appropriate thickness to be installed in stained glass windows. Although red-flashed glass is the most common [5, 6, 7, 8, 9, 10], other colors have also been used in historical stained glass windows, including pink-purple [7, 11, 12], green [1, 7], orblue [7]. Nowadays, flashed glasses are elaborated in almost all chromatic palettes and are commonly used for the creation and restoration of historical stained glass windows. The characterization of glasses from stained glass windows has quickly improved in the last years. Laboratory-based techniques, such as field emission scanning electron microscopy/energy-dispersive X-ray spectroscopy (FESEM-EDS) and X-ray fluorescence spectrometry (XRF), are usually destructive, as sampling is usually necessary [13; 14; 15; 16]. On the other hand, portable XRF instruments are becoming more and more available for in situ measurements. This technique allows one to distinguish different glass types within the same panel, identifying chromophores, enamels, grisailles, and other decoration layers; however, it is also sensible for unpolished and weathered glass surfaces because the analysis can only be undertaken very near to the glass surface due to low penetration. Therefore, the real bulk glass composition cannot be properly determined. Additionally, low \(Z\) elements, such as magnesium and sodium, elements normally used for glass type differentiation, are not detected by XRF. Another analytical technique frequently used for the study of stained glass windows is UV-Vis-IR spectroscopy, which working in transmission or absorption modes allows us to identify the glass chromophores [5; 17; 18]. UV-Vis-IR spectroscopy is a point analysis technique, and recent developments in in situ visible hyperspectral imaging offer alternatives for the overall analysis of whole glass windows in a few hours using sunlight as the illumination source [19]. Nowadays, valuable glass objects benefit from the availability of non-invasive or micro-invasive techniques, which allow distinguishing different glass types within the same glass panel and identifying chromophores, enamels, grisailles, and other decoration layers [20; 21; 22; 23]. Laser spectroscopic techniques, such as laser-induced breakdown spectroscopy (LIBS) and laser-induced fluorescence (LIF), have been combined in several studies to provide complementary elemental and molecular composition in a non- or micro-invasive way [24; 25; 26; 27; 28; 29; 30; 31; 32]. LIBS is a micro-invasive technique based on the spectral analysis of the luminous plume generated by the pulsed laser ablation of a small amount of material from the surface of the sample and has the capacity for quantitative determination.