Portugal has an extensive collection of historical stained-glass windows from different chronologies spread around the country. The most ancient stained-glass windows in Portugal are located in the Monastery of Batalha, built between around 1388 and 1533 (for what is to be seen in-situ), in the church aisle, south transept and west windows as well as in the Founder’s Chapel. Various panels of these ancient windows, preserved in the monastery, were designed by Luis Alemão (Louis the German) and present figurative and geometric motifs [1
]. Afterward, a more realistic style of stained-glass windows was produced. An example of this movement was observed in the church choir and chapter house glazings, produced between 1514 and around 1531. They were certainly designed (and partly painted) by Francisco Henriques and financed by the King Manuel I of Portugal.
During the 16th century, stained-glass windows were frequent in Portuguese monuments; however, the majority of them have been lost. A few examples of them are still preserved either as window panels in monuments or in museums. From these examples, it is important to highlight the fragments located in the Convent of Christ in Tomar, a small heraldic panel at the National Museum Soares dos Reis in Porto, a dated piece at the Museum of Setubal, and two figurative half-panels at the Church of Viana do Alentejo. From the 17th century, some panels have been preserved in the National Palace of Pena in Sintra, along with a few more ancient ones, brought to Portugal by King Ferninand II in the 19th century.
In the 18th century, the Portuguese stained-glass windows were severely damaged, with many gaps and bad-quality restorations, due to a change in the decorative movements together with the destructive consequences of the earthquake from 1755 and the Peninsular War (1807–1814) [3
]. However, in the second half of the 19th century, the stained-glass windows rose again in Portugal due to the renascence of this art as part of the architectonic context, not only in religious buildings but also in public and private constructions. They were complex works in which big uncolored pieces of glass were painted with enamels and grisailles, and the lead cames were concealed in the contours of the figures. Progressively, enamels were replaced by colored glasses in which the lead cames and the grisaille defined the contours and shadows [2
]. During this period, stained-glass windows competed with the panels of ceramic tiles, very frequent in Portugal, as decorative elements [2
Historical stained-glass windows are usually situated in buildings as part of their façade. This position permits to illuminate the inner part of the building, to serve as decoration with an instructive function, and also to protect the building from the external environmental conditions (rain, wind, extreme temperatures…). Nevertheless, this latter function is the most hazardous for the conservation of historical stained-glass windows.
The influence of environmental conditions has been an important field of study in recent years since many European medieval windows present a poor state of conservation [4
]. Nevertheless, few works have been done to compare the alteration mechanism of the same glass composition in different places. In the framework of the “International Co-operative Programme on Effects on Materials including Historic and Cultural Monuments” (ICP Materials) [16
], some glass samples with medieval and modern composition were spread around Europe in different atmospheres (industrial, urban, rural) [7
]. The exposure of these samples showed that the glasses exposed in polluted environments (industrial atmosphere) present a faster alteration due to the influence of the higher concentration of the pollution gases (CO2
). With a similar procedure, the glass-sensors developed by the Fraunhofer-Institut für Silicatforschung (Germany), were used as alteration sensors in European stained-glass windows by the assessment of environmental corrosive stresses in low durability glasses. For the same time of exposure, sensors with a more advanced degradation state indicate that the stained-glass windows are in a more hazardous environment [19
In spite of these works, there is not a systematic analysis of how Portuguese stained-glass windows can be affected by the environmental conditions. For this reason, the aim of this study is to determine the chemical stability of three glasses (soda-lime, potash-lime, and mixed-alkali silicate glasses) in different Portuguese monuments with historical stained-glass windows. The glass samples were exposed in the Monastery of Batalha (Batalha), the Monastery of Jerónimos (Lisbon), and the Cathedral of Évora (Évora). This research will be especially useful to evaluate the conservation strategies of Portuguese stained-glass windows.
The present study has proved that the preservation of historical stained-glass windows in Portugal depends on the type of glass used and on the meteorological conditions specific to each place. Medieval stained-glass windows, usually made with potash-lime silicate glass, are the most vulnerable because they have an open glass network which favors the hydrolytic attack of the glass. On the contrary, soda-lime silicate glass and mixed-alkali silicate glass have the highest chemical resistance to the environmental conditions because they present a better structural package.
Regarding the climatology, glasses located near the coast and in areas with high humidity were the most threatened because of the great impact of the environmental water on the glass surface. Meanwhile, those located in the inner part of the country with a drier climate, were better preserved. From the point of view of heritage conservation, it is recommended to conduct an individual study in each environment due to the specific characteristics of each place.