Colour development in porcelain stoneware tiles: the effect of a variation in the weight of engobe per unit area

In this thesis, the process of colour development in single-fired porcelain stoneware tiles produced in the Polis Manifatture Ceramiche S.p.A. plant was studied. In particular, the effect of a variation in the weight of engobe per unit area on the final appearance of the tiles was investigated, as this aspect has not been extensively studied in the literature yet. First, a systematic colour change as a function of the weight of the engobe per unit area was successfully demonstrated by colorimetry. To investigate the reasons for the observed colour alterations, a multi-analytical approach was adopted. Optical microscopy revealed larger pigment spots when the weight of engobe per unit area was increased. SEM-EDS analysis of tile surfaces and cross sections highlighted the presence of pigment elements inside the engobe and the glaze layers, thus suggesting a migration of elements from the pigments to the engobe, as previously reported in the literature for glazes (especially in the case of cyan). SEM observation of cross sections also pointed out that, for a given increase in the weight of engobe, the increase in the engobe thickness is more than proportional. This suggests a higher porosity in the samples with a higher weight of engobe, likely because degassing during firing becomes more difficult. A higher porosity might favour the migration of elements from the pigments to the engobe, thus contributing to the observed colour changes. Moreover, XRD highlighted that, when the weight of engobe is increased, the crystallinity index of the phases detected in the engobe layer was reduced (although some interference from the substrate might have affected the XRD results). A higher content of amorphous phases when the weight of engobe is increased could enhance the migration of pigment elements towards the glassy phases of the engobe, thus further contributing to the larger size of pigment spots observed in samples with an increased weight of engobe.