Spectrometric Characterization of Moroccan Architectural Glazed Tiles

Abstract The present work relates to a multi-analytic characterization of glazed tiles consisting of green monochrome glazed ceramics used in Moroccan architecture to protect ceilings, walls and roofs from rainwater. These tiles originate from five sites and date back to different historical periods: Bou-Inania Madrasa in Meknes (14th century), Prison of Qara in Meknes (18th century), Dar El-Beida Palace in Meknes (18th century) and Al-Hibous Cemetery of Mdaghra in Errachidia (19th century). Different analysis techniques were used in view to go back to the ancient technological processes adopted (materials, coloring pigments, firing temperatures, etc..). Optical absorption spectrometry revealed two different types of chromogenic ions in green glazes, chromium Cr3+ in the case of the tiles from Dar El-Beida Palace and Prison of Qara, and copper Cu2+ in the case of the tiles from Bou-Inania Madrasa and Al-Hibous Cemetery. Raman microspectroscopy identified different coloring phases with two types of green glazes, escolaite (Cr2O3) in the case of the glazes of the Prison of Qara and copper phthalocyanine mixed with a chromium-based pigment in the case of the glazes of the Dar El-Beida Palace. However, the origin of the green color in the glazes from Bou-Inania Medersa in Meknes and Al-Hibous cemetery of Errachidia may be due to the dissolution of copper in the vitreous glazes. X-ray diffraction, supported by Raman microspectrometry, revealed the mineralogical compositions of the terracotta tiles. Quartz and calcite are the main phases, while hematite and "high temperature" phases (anorthite, gehlenite and diopside) appear as minority ones. These identified phases permit to estimate the firing temperature of the tiles at around 950 °C in an oxidizing atmosphere. The chromatic coordinates of all glazes, represented in the Lab CIE color space, made it possible to discriminate objectively all green colors. The present investigation of glazes from different historical sites allowed the exploration of the coloring materials, revealed differences in the adopted technological protocols and permitted the establishment of a color reference database to follow glazes degradation and to help while replacing missing or degraded tile pieces.