abstract
Portugal is well known for its facades decorated entirely with ornamented glazed ceramic wall tiles called azulejos. On ageing, the tiles may detach and fall off, or deteriorate to such an extent that it becomes necessary to replicate them. Hence tile replication is a common practice in Portugal for facade restoration, but very often these new tiles do not have the same physical and/or chemical properties as the original ones. Such differences might be a factor in differential deterioration of the facades after restoration. One step toward an improvement in compatibility is to make new ceramic bodies with the same characteristics as the original ones. This study focuses on two types of glazed ceramic wall tiles from the Oporto region in Portugal: 'calcic faience' and po de pedra. A total of 25 samples from the late nineteenth to early twentieth centuries were studied systematically as an attempt to improve knowledge of these materials and to create a basis for their replication. All samples were collected from facades that were under conservation/restoration at the time when sampling was performed. The original traditional ceramic bodies were analyzed by X-ray diffraction and X-ray fluorescence. Total open porosity, capillary absorption coefficient, and mercury intrusion porosimetry were also measured in order to gain knowledge on their physical characteristics in addition to their chemical and mineralogical compositions. High-temperature crystalline phases such as diopside, gehlenite, and mullite were found in the calcic faience, suggesting that the firing temperature of calcic faience bodies was within the range of 1100-1150 degrees C. Calcination trials were also performed in order to determine the most probable firing temperatures of the ceramic bodies. Collected data led to the assumption that the raw materials used for the ceramic bodies were kaolinitic clay, quartz sand, limestone, and talc. The raw materials for po de pedra tiles were found to include kaolinite clay and quartz with firing temperatures estimated within the temperature range of 1150-1360 degrees C. Technical replicas made on the basis of these investigations were found to have the same mineralogical and capillary properties as the original tiles.
keywords
FIRING TEMPERATURE; KAOLINITE; POROSITY; SEQUENCE; MORTARS; CLAYS
subject category
Archaeology; Art; Chemistry; Spectroscopy
authors
Costa, M; Cachim, P; Coroado, J; Velosa, AL
our authors
Marisa Célia da Silva Resende da Costa
Post-Doc Fellowship
Paulo Barreto Cachim
Invited Full Professoracknowledgements
Authors are thankful for the financial support to this research under research project AZULEJAR - Conservation of tile facades (PTDC/ECM/101000/2008) financed by the Fundacao para a Ciencia e a Tecnologia (FCT) and COMPETE Program. Authors are also grateful for FCT financial support to the research units LABEST and GEOBIOTEC.