158 | wileyonlinelibrary.com/journal/jace J Am Ceram Soc. 2020;103:158–166. © 2019 The American Ceramic Society 1 | INTRODUCTION Despite the common belief that glass is an inert object, there are many types of glass and some are prone to deterioration due to their composition and method of manufacture. The en- vironmental conditions an object is subjected to can trigger active chemical deterioration in a seemingly stable object. 1 The ability to understand and detect early symptoms of chem- ical deterioration is critical to maintaining the longevity of precious glass objects in historical collections. This applies to a wide body of ancient to modern glass objects, with compo- sitions varying according to where, when, and by whom they were produced. Specifically, this research project focuses on 19th‐century glass objects and continues a study of 19th cen- tury glass flutes produced in Paris by Claude Laurent in the first half of the 19th century, 21 of which are housed at the Library of Congress. These flutes display a range of chemical compositions and levels of deterioration. 2 Historical glass objects within certain ranges of composi- tion may be moisture sensitive and prone to chemical deterio- ration, initiated by the attack of water at the surface (typically at atmospheric relative humidity levels above certain lim- its). 3‒8 Without the presence of moisture, most glasses are highly stable and can last for centuries, but atmospheric Received: 18 March 2019 | Revised: 16 June 2019 | Accepted: 21 July 2019 DOI: 10.1111/jace.16703 ORIGINAL ARTICLE Application of fiber optic reflectance spectroscopy for the detection of historical glass deterioration Stephanie Zaleski 1 | Elizabeth Montagnino 2 | Lynn Brostoff 3 | Isabelle Muller 2 | Andrew Buechele 2 | Carol Lynn Ward‐Bamford 3 | Fenella France 3 | Murray Loew 1 1 Department of Biomedical Engineering, The George Washington University, Washington, DC, USA 2 Vitreous State Laboratory, The Catholic University of America, Washington, DC, USA 3 Library of Congress, Washington, DC, USA Correspondence Stephanie Zaleski, Department of Biomedical Engineering, The George Washington University, Washington, DC 20052, USA. Email: szaleski@email.gwu.edu Present address Stephanie Zaleski, Northwestern University/Art Institute of Chicago Center for Scientific Studies in the Arts (NU‐ ACCESS), Evanston, IL, USA Abstract Historical alkali silicate glass is prone to deterioration over time due to the uptake of atmospheric water and subsequent hydrolysis of the silicate matrix. Recent studies of historical glass have provided insight into the mechanism of alteration; however, few techniques can assess early onset glass alteration noninvasively. Herein, we pre- sent fiber optic reflectance spectroscopy (FORS) as an invaluable tool to analyze historical glass alteration. We study a series of artificially aged model potash glasses and assess the nature of the alkali‐depleted alteration layer by microscopy, scanning electron microscopy (SEM) with energy dispersive spectroscopy, and FORS. We find that the model glass FORS spectra demonstrate peaks associated with hydrox- ide, liquid‐like water, and bound water. FORS was able to detect an alteration layer as thin as 0.66 µm. The model glass data were then used to generate a hydration thickness prediction curve in order to predict the alteration layer thickness of twenty‐ one 19th‐century glass flutes of similar composition. In the one case that an actual flute sample was available, the predicted value was in good agreement with previous SEM measurement. The results indicate the ability of FORS to noninvasively assess glass deterioration and to understand the nature of absorbed water in historical glass objects. KEYWORDS degradation, glass, silicates, spectroscopy