CLAYS: FROM GEOLOGICAL CONTEXT TO CULTURAL HERITAGE

Clays are a fine-grained natural material that becomes plastic in contact with water, and hardens during drying and baking. In geological terms, clays are both minerals (phyllosilicates, main representatives: kaolinite, montmorillonite, illite) and rocks (clastic pelitic sedimentary rocks). The structure of phyllosilicates consists of TO or TOT layers. The layers are made up of tetrahedral (T) and octahedral (O) sheets. The tetrahedral sheet consists of SiO44-tetrahedron that are polymerized over three common oxygens. The octahedral sheet is connected to the tetrahedral sheet by a fourth oxygen. It consists of octahedron in the center of which can be Al, Mg, Fe, surrounded by six oxygens. Depending on the packaging of the sheets in TO or TOT layers, phyllosilicates can have different characteristics. For example, montmorillonite (TOT) can exchange cations with the environment and absorb water molecules, causing it to swell, while kaolinite (TO) lacks these characteristics. The use of clay dates back to the Neolithic and a number of unique artifacts have been found in Serbia. By mineralogical-petrological analysis, it is possible to determine the origin of clay, temperature and other firing conditions, on the basis of which the archaeological context is further interpreted (e.g. the technology of production and its change over time). Examples of such research in our country are the study of Roman bricks from Viminacium, early Byzantine (Caričin Grad) and medieval ceramics (Studenica) and others.