Stained glass is one of the most fragile components of our cultural heritage. Since it was used for structural elements such as windows, it is exposed to weathering and consequently to deterioration. The concern regarding the decay of modernist enamelled glass has led to a long-term study and to a doctoral thesis by Martí Beltrán. “We are quite satisfied. We have obtained totally new information and, in particular, data that may help to better preserve the enamelled glass windows of this period”, highlights Trinitat Pradell, a researcher from the Department of Physics of the Universitat Politècnica de Catalunya · BarcelonaTech (UPC) and the supervisor of the thesis.

Synchrotron light has important applications for studying historical and artistic heritage, and the UPC’s Materials Characterisation Group (GCM) uses ALBA facilities for its research. In this case, the beamline used for performing the study, MSPD, uses the microdiffraction technique. Stained glass samples cut into very thin sections (100 microns) have been analysed with X-rays to obtain high resolution diffraction patterns that give information about the chemical composition of the materials and enable the identification of the pigments and colourants used, their microstructure and the products formed as a result of corrosion.

Modernist stained glass workshops in Barcelona are famous for their creativity and artistic value. Catalan modernist stained glass produced at the end of the 19th century and at the beginning of the 20th century used a new type of ready-to-use enamels (powder ready for painting), which was a novelty in those days. The study has revealed that many of these enamels were made of lead-zinc borosilicate glass with a wide range of pigments and colourants.

Studying the alteration degree of the enamels
In particular, researchers have studied a collection of ready-to-use enamels from one of the most important glass workshops in Barcelona back then—Rigalt, Granell i cia—belonging to J. M. Bonet’s private collection. From these pre-prepared powder products, 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 had been stored inside the original containers and had not been exposed to atmospheric corrosion. Analyses have been crucial in determining the alteration degree of the enamels, the mechanisms of corrosion and, particularly, the reasons why blue and green enamels are the most altered.

The collaboration of Vitralls Bonet has been absolutely fundamental for this research because they not only provided the materials for replicating the enamels, they offered their collection of small fragments of original enamelled glass, which had been collected over more than a hundred years restoring stained glass in Barcelona. The stained glass windows by Rigalt, Granell i cia that decorate the Barcelona Courthouse or the Sants-Montjuïc District Council headquarters stand out among those studied. Researchers also analysed samples of stained glass by other prestigious modernist workshops in Barcelona taken from private houses in the Barcelona Eixample (Bordalba workshop) and in Badalona (Buxeres i Codorniu workshop), Palma Cathedral and Sant Jaume de Calaf church (Hijos de Edualdo Ramon Amigó workshop) and the Estació del Nord in Barcelona (Maumejean workshop), among others. One of the samples belongs to one of the most important works of Catalan Modernism, Les Dames de Cerdanyola, displayed in Museu de Cerdanyola del Vallès. It was produced by Ludwig Dietrich, an Alsatian stained glass artist who moved to Barcelona around 1900.

Atmospheric corrosion is key to degradation
The results have revealed that the lead-zinc borosilicate glass of the enamels is defined by a low softening temperature and a good stability against chemical corrosion and especially water corrosion. However, the relatively narrow temperature range required to ensure the adherence of the enamels to the base glass without deforming it may have required the addition of flux, decreasing its stability. The historical enamels show a completely altered chemical composition due to atmospheric corrosion processes, which result in a reduction of lead, boron and zinc and the formation of an amorphous hydrated silicate and lead and calcium sulphates or carbonates, depending on the atmosphere. The blue and green enamels show a microstructure (heterogeneous layers with pigment particles on the surface) more prone to degradation. Moreover, researchers have observed that the infrared absorption of the colourants (cobalt and copper) used in blue and green enamels may lead to temperature differences between the enamels and the base glass and between pigment particles and the vitreous phase, which may be responsible for the formation of cracks and eventually the subsequent flaking off of the enamels.

Better preserving stained glass
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 protection against humidity and atmospheric gases should also include an infrared light filter, particularly important in Mediterranean climates. In addition, they conclude that, although humidity, pollutants and solar radiation are the main causes of corrosion, enamel microstructure also plays a key role. Therefore, the results indicate that the artistic and technical “creativity” of some of the Catalan modernist workshops resulted in stained glass with a reduced stability.

“Our study opens the door to scientific knowledge of such an important element of Catalonia’s artistic heritage as modernist stained glass. The historical documentation of all these pieces can now be completed with their structural characterisation”, explains Martí Beltrán. “Our work allows to tackle enamel conservation from an eminently scientific and not speculative point of view. Knowing the composition, structure and thermal properties of materials is essential to find out the reasons for their degradation and thus implement appropriate conservation strategies”, he concludes.

The research group stresses that conducting such studies is essential to understand the behaviour of materials used by artists over time and to optimise the conservation strategies adopted by Catalan museums.