The raw materials

(ITA)

The raw material in the ceramic production is usually defined with the term “clay”.
From the geological point of view with the term clay is defined a sedimentary rock in which clay minerals prevail on other minerals and accidental materials.
Clay minerals are the main components of clay, they are composed by micrometric particles that are responsible of their plasticity.From the mineralogical point of view, clay minerals are hydrous aluminium phyllosilicates with a lamellar structure made by the association of two base-units: Tetrahedra [SiO4]4- and Octahedra[AlO6]9-

The overlapping of tetrahedral silicate sheets (T) and octahedral hydroxide sheets (O) originates different clay minerals structure. Among the clay minerals, the most used in the pottery production are:
-          Kaolin group(T-O)
-          Illite group(T-O-T)
-          Smectite group (T-O-T)

The most diffuse minerals present in the clay are:
-          Iron oxides and hydroxides (e.g., goethite and hematite) that influence the color of the clay
-          Quartz that decreases the clay minerals plasticity
-          Carbonates and feldspars (K-feldspar, albite and anorthite) that decrease the melting temperature

Along with clay minerals and other minerals, it is possible found also accidental materials as fragments of rocks, fossils and organic materials.

The composition of clay (clay minerals + other minerals + accidental materials) determines the typical features useful for the modelling and pottery production. The plasticity is the principal feature, indeed the amount of water in the clay minerals determine the possibility to model the clay. In order to create a suitable paste, the potters added materials defined tempers to modify the plasticity of the clay. Fragments of minerals, of rocks, grog (crushed ceramic) or sand were usually added to improve the resistance of the paste. On the other hand, when the clay was non-plastic, vegetable or animals residues were usually added to improve the plasticity due to the presence of the organic colloids.

 

Finally, fluxes agents (e.g., K-feldspars) could be added to decrease the melting point.

Shoval et al. (1993)[1]explained in a scientific way the reason because crystals of calcite were preferred in the production of Kitchenware. The works demonstrates that at high temperature crystals of calcite prevent the cracks due to the re-cooked typical of Kitchenware.


[1]Shoval S., Gaft M., Beck P., Kirsh Y. (1993). Thermal behaviour of limestone and monocrystalline calcite tempers during firing and their use in ancient vessels. Journal of thermal analysis40, 263-273. 

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