Mineralogical transformations due to salt whitening agent in modern Hebron ceramics
Résumé
The aim of this research is to investigate the influence of NaCl on the colours and chemical composition of Carich ceramic bodies. The addition of salt to ceramics is a practice that has been observed in several potter communities where the addition of salt is explicitly intended to whiten ceramics. In order to conduct this research and characterize the physico-chemical properties induced by the presence of salt, raw clay material and pot sherds were collected from the modern production at Hebron. Experimental bricks were made using two clayed sediments: one with a low CaO content (<10%) and one with a higher CaO content (20–25%). Different proportions of NaCl were added (up to 5%) to clay pastes which were fired at different temperatures. Mineralogy (petrography, XRD), chemistry (SEM-EDX) and colour analyses were carried out on raw clay materials, pot sherds and experimental (lab-made) bricks. SEM imagery offered the possibility to monitor the evolution of the mineralogical transformations, the pore system with increasing firing temperature and salt content. Results confirm the role of salt as a catalyst in the transformation reaction of calcium silicates during the firing process and its influence on the colour of the finished object.