Different shades of red: The complexity of mineralogical and physicochemical factors influencing the colour of ceramics

Different techniques (X-ray diffraction, field emission scanning electron microscope, colorimetry, visible-nearinfrared reflectance spectroscopy) were carried out to investigate the cause of colour changes of traditionalceramic materials. Two clayey materials of different composition, collected in the Bay of Naples, were fired inoxidising atmosphere at different temperatures resulting in different shades of red colour.Hematite is responsible of the reddish hue of ceramics and its nucleation is strictly related to firingtemperature and chemical composition of the raw materials. A low CaO concentration allowed hematite to formin higher amounts providing a more intense reddish hue at high firing temperatures (over 950 °C). At thehighest temperature (1100 °C) all samples showed darker colour due to increased size of iron oxide particles.Black core developed in Ca-rich ceramics fired at low temperatures as the short time of firing is insufficient tocomplete iron oxidation within the matrix, except in those containing high temper amounts. Indeed,microstructural modification occurs due to the presence of discontinuities among temper grains and matrix,which improves the circulation of oxygen in the core of ceramics.