Left: image of a Modernist stained glass from Museu d’Art de Cerdanyola (Les Dames de Cerdanyola) by L. Dietrich, 1888–1910, showing the characteristic green and blue enamels decay (co. Jordi Bonet). Right: diffraction pattern obtained at MSPD beamline (its experimental hutch is shown).
22nd January 2021, Cerdanyola del Vallès Stained glass is a fragile component of our Cultural Heritage since was used for the windows of buildings, and a large part of it is exposed to weathering and consequently to deterioration. The concern raised regarding the decay shown by the modernist enamelled glass has led the path to a long-term study and to the thesis presented today by Martí Beltrán González. "We are satisfied because totally new information have been obtained and, in particular, data that may help to better preserve the enamelled glass windows of this period ”, highlights Trinitat Pradell, director of the thesis.
Synchrotron light has important applications in the field of historical and artistic heritage and the UPC group has been an ALBA user for years to carry out analyses for its research. In this case, the beamline where the experiments have been performed, MSPD, provides the use of microdiffraction technique. Stained glass samples cut into very thin sections (100 microns) have been analysed through X-rays to obtain high resolution diffraction patterns that give information about the chemical composition of the materials and enables the identification of the pigments and colorants used. The microstructure of the materials and the products formed as a result of corrosion can be detected too thanks to this synchrotron light technique.
Modernist glazier workshops from Barcelona are known for its creativity and artistic value. The stained glass produced during the last decades of the 19th century and first decades of the 20th century is characterised by the use of new type ready‐to‐use enamels (powder ready to be painted), a novelty in those times. The study has revealed that most part of the enamels are made of a lead-zinc borosilicate incorporating pigment particles and colourants.
In particular, researchers have studied a collection of enamels used in one of the most important stained glass Modernist workshops in Barcelona, Rigalt, Granell & cia, belonging to the private collection of J.M. Bonet. From these pre-prepared products in powder, purchased by Vitralls Bonet from Rigalt i Granell in 1931, replica enamels were obtained to compare them with those found in buildings. The main advantage is that these products have been stored inside the original cans and have not been subjected to the effects of atmospheric corrosion. In this way, the analyzes have been crucial in determining the degree of alteration of the enamels as well as the corrosion mechanisms, and in particular the reasons why blue and green enamels are more altered than other coloured ones
The collaboration of Vitralls Bonet has been absolutely fundamental for this research since, apart from the materials to replicate the enamels, they have also offered their private collection of original enamelled glass resulted from its 100 years of restoration activities. Among the stained glass windows studied, it can be highlighted the ones produced by Rigalt, Granell & cia which decorate buildings such as the High Court Palace of Barcelona and the Town Hall of the Sants-Montjuïc District. Moreover, researchers analysed stained glass samples from other prestigious workshops in Modernist Barcelona that can be found, for example, in private houses from l’Eixample (Bordalba workshop) and from Badalona (Buxeres i Codorniu workshop), in Palma de Mallorca Cathedral or Sant Jaume de Calaf church (Hijos de Edualdo Ramon Amigó workshop) and in North Station of Barcelona (Maumejean workshop). One of the studied pieces belongs to one of the most important work of Catalan Modernism: Les Dames de Cerdanyola, displayed in Museu de Cerdanyola del Vallès and produced by Ludwig Dietrich, Alsatian stained glass maker established in Barcelona in 1900.
The current work shows that the lead-zinc borosilicate glass of the enamels is characterised by its low sintering temperatures and high stability against chemical corrosion, in particular to water corrosion. However, the relatively narrow range of firing temperatures necessary for correct adherence of the enamels to the contemporary glass base without deforming it may have required the addition of a high flux, decreasing its stability. The historical enamels show a completely altered chemical composition due to atmospheric corrosion processes, which results in a reduction of lead, boron and zinc and the formation of silica rich amorphous glass, with precipitated lead and calcium sulphates or carbonates. On the other hand, the blue and green enamels show a heterogeneous layered microstructure more prone to degradation. Moreover, it has been observed that the infrared absorbance of colourants (cobalt and copper) used in blue and green enamels may lead to temperature differences between enamel and substrate glass, which would be responsible for the formation of cracks and the subsequent flaking off of the enamels.
In conclusion, the extensive work done by the UPC research group provides new relevant information on the causes of deterioration of stained glass. To improve the conservation of enamels, scientists propose that a protection from humidity and atmospheric gasses should also be accompanied by a filter of infrared light, of particular importance in Mediterranean climates In addition, although the exposure to humidity, pollutant gases and solar irradiation is the main cause of corrosion of the painted surfaces, the study concludes that the chemical composition and microstructure of the enamels plays a key role. Hence, the results confirm that the artistic and technical “creativity” of some of the Modernist Catalan workshops resulted in stained glass with a reduced stability.
“Our studies open the door to scientific knowledge of a very important material of the Catalan artistic heritage, such as Modernist stained glass. To the historical documentation of all these pieces we can now add their structural characterization. "commented Martí Beltrán. “Our work allows the approach of the conservation of enamels from an eminently scientific and not speculative point of view. Knowledge of the composition, structure and thermal properties of materials are essential tools to find out the reasons for their degradation and thus establish appropriate conservation strategies. "adds Beltrán.
The UPC group stresses that going on with this kind of studies is essential to understand the behavior of the materials used by artists over time and necessary for the conservation strategies launched by the art museums.
Referencias:
- Analysis and degradation mechanisms of enamels, grisailles and silver stains on modernist stained glass. Martí Beltrán González.
- M. Beltrán, F. Brock, T. Pradell, Thermal properties and stability of Catalan Modernist blue and green enamels, Int. J. Appl. Glass Sci. 10 (2019) 414–425. DOI: 10.1111/ijag.13098
- M. Beltrán, N. Schibille, F. Brock, B. Gratuze, O. Vallcorba, T. Pradell, Modernist enamels: Composition, microstructure and stability, J. Europ. Ceram. Soc. 40 (2020) 1753-1766. DOI:/10.1016/j.jeurceramsoc.2019.11.038
- M. Beltrán, N. Schibille, B. Gratuze, O. Vallcorba, J. Bonet, T. Pradell, Composition, microstructure and corrosion mechanisms of Catalan Modernist enamelled glass, J.Europ. Ceram. Soc. (in press). DOI: 10.1016/j.jeurceramsoc.2020.10.041
