Ceramic materials

The famous Crater of Eufronio (6th century BC), preserved in the Archaeological Museum of Cerveteri (RM).

Ceramics, a term that derives from the Greek κέραμος and means "clay", understood as "potter's earth”. It is an inorganic material with insulating or semiconductive characteristics and a very ancient origins, widely diffused and produced in a disparate quantity of shapes and colours. Potteries are mainly made up of a plastic component (clay) and an inert material (quartz, straw, bones, obsidian, milled ceramics, etc.). 

Clay is a fine sediment rock (the granules does not exceed 2 micrometers of diameter, corresponding to 0.002 millimeters), not lithified (not transformed into a compact sedimentary rock), consisting of phyllosilicates and aluminosilicates, or rather of minerals composed mainly by oxygen and, respectively, silicon or aluminium. In case of hydration this material appears to be malleable and easily workable; it begins to harden and retreat when it naturally loses water due to dehydration but firing at high temperatures (from 600 °C onwards) lead to an irreversible transformation of its state. 

However, the hardness of the ceramic depends also by the inert substances which are not very sensitive to high temperatures and contrasts any deformations that occur during firing. 

The aggregate contrasts the clay withdrawal and constitutes the real skeleton of the ceramic object. It can be characterized by different nature: in the past marble dust, refractory material, obsidian, but also ground bones, straw and other organic materials have been widely used. 

The ceramic, after firing, becomes a hard material, resistant to heat and not combustible; on the other hand, it's sensitive to impacts and thermal shock. 

The color of the ceramic is also characteristic, because it changes according to the type of firing condition and based on the presence of oxides. Indeed, in association with the chemical composition of the mixture, the reducing environment (poor in oxygen) causes a dark colour that turns from gray to black; on the contrary, the oxidizing atmosphere favors lighter colours, in the range from red to white. Titanium oxides produce ceramics with shades that change from yellow to white, whereas ferric and ferrous oxides give a colour that can vary from red to black and red-brown to orange-yellow respectively. In addition, the surface can be coated with glaze and decorated with paints. 

Diagnostic investigations allow to discover a lot about these materials, which can be found in the form of vessels, porcelains, bricks, fireside, furnaces and so on. 

Multispectral imaging allows a preliminary non-invasive and non-destructive screening, useful to identify the presence of paints and the colorimetric characteristics in the different bands of pigments possibly present in the work. 

One of the most used diagnostic techniques for the study of ceramic objects is thermoluminescence, which allows to date and authenticate archaeological finds. The microscopy investigations allow the observation of the ceramic mixture and the pictorial decorations, when present, whereas the techniques of FT-IR spectroscopy, Raman spectroscopy, fluorescence and X-ray diffraction support the identification of both the substances constituting the matrix and/or the aggregates, and of the nature of the pigments and of the substances used in the pictorial decorations and/or in the glazes. Recently, also the electrochemical techniques (EIS and VIMP) have been employed for this purpose. 

Finally, with the aid of radiographic techniques, it's possible not only to observe the thickness of the ceramic body and identify the presence of metallic minerals in the mixture, but also to recognize the manufacturing methods (columbine, bands, lathe and so on) and to get different information about their conservation status. 

Tiziana 

Bibliography 

- C. Barry Carter, M. Grant Norton, Ceramic Materials: Science and Engineering, New York, 2013 

- H. Edwards, P. Vandenabeele, Analytical Archaeometry, selected topics, Croydon, 2012. 

- S. W. S. McKeever, M. Moscovitch, P. D. Townsend, Thermoluminescence Dosimetry Materials – Properties and Uses, Ashford, 1995.