Authenticity screening of stained glass windows using optical spectroscopy.

Wendy Meulebroeck,Karin Nys,Hilde Wouters,Hugo Thienpont

doi:10.1038/srep37726

Abstract

Civilized societies should safeguard their heritage as it plays an important role in community building. Moreover, past technologies often inspire new technology. Authenticity is besides conservation and restoration a key aspect in preserving our past, for example in museums when exposing showpieces. The classification of being authentic relies on an interdisciplinary approach integrating art historical and archaeological research complemented with applied research. In recent decades analytical dating tools are based on determining the raw materials used. However, the traditional applied non-portable, chemical techniques are destructive and time-consuming. Since museums oftentimes only consent to research actions which are completely non-destructive, optical spectroscopy might offer a solution. As a case-study we apply this technique on two stained glass panels for which the 14th century dating is nowadays questioned. With this research we were able to identify how simultaneous mapping of spectral signatures measured with a low cost optical spectrum analyser unveils information regarding the production period. The significance of this research extends beyond the re-dating of these panels to the 19th century as it provides an instant tool enabling immediate answering authenticity questions during the conservation process of stained glass, thereby providing the necessary data for solving deontological questions about heritage preservation.

Highlights

Window glass composition changes throughout time[1]
Since the colouring agent signature depends on the composition of the glass, optical absorption spectroscopy can be used as a first-line technique to unveil information about the glass composition of coloured glasses
For the investigation of the decoration layers we follow an analogue approach; we investigate the spectral properties of the silver stained pieces and verify if the spectral properties match with those of well-dated historic samples described in literature

Summary

Introduction

Window glass composition changes throughout time[1]. Major compositional groups within the wood and plant tradition (~800–1800 AD) are defined based on differences in calcium, potassium and sodium concentration and include potash, high lime low alkali (HLLA) and mixed-alkali glass[2]. The procedure consists of coating the glass surface with a silver compound dispersed in a clay medium and firing at a temperature just above the glass transition temperature During this process silver ions are exchanged with the alkali ions from the glass (Na+or K+) and diffuse in the glass. In recent years attention focused on non-destructive and portable analysis methods for measuring glass composition This as a replacement for the electron- or X-ray based analytical techniques such as Wavelength-Dispersive X-ray spectrometry (SEM-WDS), Electron Probe Micro-Analysis (EPMA), or Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS) which require expensive laboratory equipment and sample pre-treatment. The purpose is to relate a specific glass composition to a characteristic spectral pattern consisting of one or several absorption bands located at well-defined spectral wavelengths These absorption bands are the spectral fingerprint of the transition metals. The exact date of invention of the mill remains unclear, written sources hint at the later 15–16th century

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