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Andreia Ruivo graduated in chemistry at the Faculty of Sciences of the University of Lisbon (FCUL) in 2005. In 2006 she started working at the Research Unit VICARTE (Glass and Ceramics for the Arts) at FCT-NOVA in the synthesis of luminescent glasses and enamels doped with lanthanide oxides and in the development of new methods for the synthesis of gold and copper ruby glass. From 2009 to 2013 she made her PhD in Sustainable Chemistry working on the synthesis and characterization of innovative luminescent glasses for artistic applications in collaboration with LAQV-Requimte at FCT-NOVA. During these years her research was focused in the production and characterization of innovative luminescent glasses using lanthanide oxides and using other elements as lead halides and copper.
Currently she is working as a Postdoctoral researcher at VICARTE developing new sustainable glass materials creating innovation in art and industry. More recently she has also been working on the reproduction and characterization of historical recipes of glasses, enamels and grisailles in the area of cultural heritage.

A. Ruivo2*1

M. Vilarigues 1,2, C. Machado1,2, A. Machado1,2, M. Costa1, L.C. Alves3,
I. Pombo Cardoso1,4, Department of Conservation and Restoration, Faculdade de Ciências e Tecnologia, Universidade NOVA de Lisboa, Campus da Caparica, 2829-516 Caparica, Portugal
2 R&D Unit VICARTE – Vidro e da Cerâmica para as Artes, Faculdade de Ciências e Tecnologia, Universidade NOVA de Lisboa, Campus da Caparica 2829-516, Caparica, Portugal
3 C2TN (Campus Tecnológico e Nuclear), IST/UL, Estrada Nacional 10 2695-066 Bobadela, Portugal
4LAQV-REQUIMTE, Faculdade de Ciências e Tecnologia, Universidade NOVA de Lisboa, Campus da Caparica 2829-516, Caparica, Portugal

Grisailles are brown or black glass-based paints generally applied on stained glass windows, that can be used as a thick opaque line to draw contours or as a watery and thin layer to add shadows and textures to the figures [1,2]. These paints are produced by mixing ground silica lead-rich glass (lead-base glass), with iron and copper, which are the colorant agents. This mixture is applied on a glass substrate, by using a vehicle, and fired afterwards (Figure 1).

Figure 1. Schematic representation of the grisaille layer on a glass substrate, in cross section, before and after firing.

The main conservation problems in historical stained glass windows are due to the chemical susceptibility of the glass and the detachment of the painting. Grisailles detachment is a well-known problem, being most often observed in stained glass produced in the first half of the nineteenth century.
This work aims to study the evolution of the composition and production technology of this paint and their influence on the grisailles detachment. Different grisailles from different historical periods were reproduced and their composition and morphology were characterized by means of optical and electron microscopy, ion beam analysis and X-ray diffraction. For the thermal compatibility tests three grisaille based on historical recipes from the treatises by Eraclius (10th-13th c.), Kunckel (17th c.) and Bontemps (19th c.) were selected and applied on glasses with different compositions: silica-potassium glass, silica-mixed alkaline glass and silica-sodium glass. The thermal behaviour of the produced grisailles and glasses was analysed by dilatometry and differential scanning calorimetry (DSC) techniques to better understand the thermal compatibility between the paint and the substrate.
The obtained results indicated that the raw materials used as colourants and for the lead-based glass remained the same until the 19th century, as well as the production methods. Differences were found in the chemical compositions, mainly due to variation of the composition of the lead-base glass, in particular of the ratio SiO2-PbO, which evolved towards a higher lead oxide content. This evolution has an impact on the thermal behaviour of the grisailles and compatibility with glass substrates. In particular 19th century recipes resulted in a grisaille with a lower coefficient of thermal expansion. The use of earth pigments instead of metal grains, the higher lead content and the removal of other raw materials as alumina gave rise to a more unstable grisaille, developing mechanical stress causing a higher detachment of the paint layer. The results reveal a low compatibility between these two materials, associated with the composition but also the morphology of the grisaille.
[1] Schalm, O., 2000. Characterization of paint layers in stained-glass windows: main causes of the degradation of nineteenth century grisaille paint layers. PhD dissertation. Antuérpia: Universitaire Instelling Antwerpen.
[2] Pradell, T. et al, 2015. Materials, Techniques, and Conservation of Historic Stained Glass “Grisailles.” International Journal of Applied Glass Science, 18: 33.