Glass - Back to the Future!

Presenting Author:
Inês Coutinho

article posted 22 March 2016

Inês Coutinho finished her PhD entitled “New insights into 17th and 18th century glass in Portugal: Study and Preservation”, in conservation and restoration at the Department of Conservation and Restoration, Faculty of Science and Technology at the University NOVA of Lisbon. She is currently working at VICARTE with an investigation scholarship in the fields of glass archaeometry and historical glass reproductions.

From black to blue: characterising a colour change phenomenon occurring in glass wine bottles dated between the 17th and 18th centuries

I. Coutinho a,b, J. Hormes c,d, W. Klysubun e, L. C. Alves f, B. Gratuze g, M. Vilarigues a,b

For the past few years, after observing more than 100 wine glass bottles excavated from archaeological sites located in Lisbon, Portugal, and dated between the 17th and 18th centuries, it was noticed that several fragments, both in olive green or black-appearing glass, presented areas where the glass was turquoise blue. Some of these fragments had different shades of turquoise blue, where some areas had become completely opaque, presenting a very light turquoise blue tonality (c.f. figure 1).

In general, this turquoise blue colouration appears in glass bottles or fragments distorted probably from the high temperatures of the great number of fires that enveloped Lisbon after the major earthquake of 1755.

The main purpose of this investigation is to try to identify the molecular changes responsible for this alteration in the glass colour, and if this phenomenon occurs during the glass bottles production process or if it is related with the high temperatures and atmospheres of the fires.

In a first step, the elemental composition of various samples presenting both the olive green or black-appearing colour and the turquoise blue colouration phenomenon, were investigated using three different techniques: LA-ICP-MS (laser ablation–inductively coupled plasma mass spectrometry), µ-PIXE (Particle induced X-ray emission) and SR-XRF (Synchrotron radiation excited X-ray fluorescence). All data show that the bottles were fabricated using high lime low alkali (HLLA) glass composition. No differences in the chemical composition were observed that could explain the observed different tonality in the glass.

Thus it is reasonable to assume that chemical reactions most likely induced by the high temperature of the fires are responsible for the colour changes. To explore these molecular modifications, some of the major elements were investigated using synchrotron radiation based X-ray absorption fine structure spectroscopy (XAFS) (here mainly XANES - X-ray absorption near edge structure spectroscopy) allowing the detailed chemical speciation of these elements. By comparing, for example, XANES spectra of an ‘unknown compound’ to those of appropriate reference compounds, the chemical valence state can be determined and conclusions can be drawn on the type of chemical neighbourhood, the local symmetry, and possible charge transfer between binding partners. One of the major advantages of XAFS is the fact that no long range order is required so that the technique is specifically suitable for “X-ray amorphous” materials such as glass. The XANES results indicate that the colour changes are caused most likely by high temperature chemical reactions modifying the chemical environment and the valency of some of the elements responsible for the coloration of the glass.

a)Unidade de Investigação VICARTE - Vidro e Cerâmica para as Artes, Faculdade de Ciências e Tecnologia da Universidade Nova de Lisboa, Campus de Caparica, 2829-516 Caparica, Portugal
b) Dep. de Conservação e Restauro, Faculdade de Ciências e Tecnologia da Universidad Nova de Lisboa, Campus de Caparica, 2829-516 Caparica, Portugal
c) Institute of Physics, Bonn University, Nussallee 12, D-53115 Bonn, Germany
d) Center for Advanced Microstructures and Devices, Louisiana State University, Baton Rouge, USA
e) Synchrotron Light Research Institute, 111 University Ave., Muang, Nakhorn Ratchasima 30000, Thailand
f) C2TN (Campus Tecnológico e Nuclear), IST/UL, Bobadela, Portugal
g) IRAMAT - Centre Ernest-Babelon, CNRS Université d’Orléans, 3D rue de la Ferollerie, 45071 Orléans, Cedex 2, France