Different shades of red: the complexity of mineralogical and physic-chemical factors influencing the colour of ceramics

Abstract

Different techniques (X-ray diffraction, field emission scanning electron microscope, colorimetry, visible-near infrared reflectance spectroscopy) were carried out to investigate the cause of colour changes of traditional ceramic materials. Two clayey materials of different composition, collected in the Bay of Naples, were fired in oxidising 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 firing temperature and chemical composition of the raw materials. A low CaO concentration allowed hematite to form in higher amounts providing a more intense reddish hue at high firing temperatures (over 950 °C). At the highest 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 to complete 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.