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Written Artefact Profiling Guide
Written ArtefactProfiling Guide

Photo: CSMC

Written Artefact Profiling Guide

Clay and Pottery

Stylianos Aspiotis, Olivier Bonnerot, Samaneh Ehteram, Cécile Michel

According to the description of soil science, mineral soil components are divided into sand, silt and clay, going from the coarsest to the finest material. Technically, some of the objects described as made of clay should be described as silt materials instead. However, ‘clay’ is also used as a general term to describe a form of viscous soil.

The word ‘clay’ refers to very fine-grained sediments with an average grain size of less than 2 μm (see Santacreu, 2014), composed of clay minerals, non-clay mineral inclusions such as quartz and feldspars, as well as organic matter (e.g. hay or plant roots). Clay minerals are the most important constituents of clay, as they are giving the clay body its characteristic mechanical and physicochemical properties. They are hydrous layered silicates consisting of an octahedral sheet being sandwiched between two tetrahedral layers, while the interlayer space between two adjacent stacking sequences is filled with monovalent large cations (e.g. Na+, K+ and Ca2+) or larger structural units in the form of hydrated cations (Mg(H2O)6) (Deer et al., 2013;  Wang et al., 2015). The presence of constitutional water (in the form of OH groups) and loose absorbed and/or adsorbed H2O molecules in clay minerals provides the clay body, when wet, with its most notable mechanical property, namely plasticity and ability to be moulded, that can be enhanced by prolonged exposure to weathering and by increasing the clay mineral fraction. Simultaneously, when clay is dried and fired, it tends to harden due to the loss of water and the induced physicochemical changes in the structure of clay minerals.

Clay was used in various ancient civilisations as a medium for writing, being engraved before the drying and firing process and often modelled in rectangular, square or even round clay cushions known as ‘clay tablets’. In addition to tablets, clay was shaped into many different forms before being given a text: nails and cones, barrels, cylinders, prisms with a variable number of faces, livers and other organs, hands, bricks and various architectural elements (see Walls and Buildings). The bricks could be also stamped using a matrix.

Clay tablets were predominantly used for cuneiform script (Middle East from 3400 BCE to 100 CE) impressed in fresh clay, but also for Cretan hieroglyphs (Crete, 2100–1700 BCE), linear A (Aegean islands and mainland Greece, 1800–1450 BCE) and B scripts (Crete and mainland Greece, 1450–1200 BCE). Apart from these, a small number of cuneiform tablets discovered at Mesopotamian sites and dating to the first millennium BCE contain texts using the Aramaic alphabet. The letters were then incised into fresh clay like for linear A and B scripts, or written in ink on tablets that had already dried in the sun. Clay tablets can be almost entirely made of silt, in particular of magnesium aluminium layered silicate mineral (probably palygorskite), which would be considered low-quality clay by current pottery standards (Taylor 2012, 7).

When used for writing, clay is a constraining material concerning its quality, storage and manufacturing techniques. It was usually taken from the sediments of rivers and canals and stored in large jars or basins. In the containers, the clay decanted in water and was cleared by way of levigation: the impurities fell to the bottom of the container, and once the water had been removed, the fine clay was collected on the surface. Some tablets could have a core covered with a thin layer of pure clay, while others were made from a block of cleaned or uncleaned clay, and could then contain little stones, plants and animal remains.

From the fourth millennium onwards, clay was adopted in Mesopotamia as a medium for writing, which rapidly took on a cuneiform shape, meaning that each sign was made up of vertical, horizontal or oblique wedges, or even heads of wedges obtained by imprinting the right or acute angle of a reed stylus into the fresh clay. The wedges then appear as three-dimensional negatives. While writing, it was possible to erase signs, and once the tablet was dry, it was always possible to wet it before erasing some text, or recycling the clay.

Once the tablet had been written, the scribe left it to dry in the sun. As the water evaporated, the clay tablet became hard enough to be archived or sent away. When fire destroyed certain buildings, the tablets were accidentally baked, ensuring their preservation, sometimes for millennia. Tablets fired in antiquity are rare; they were intended to be preserved over the long term as scholarly or historical texts kept in libraries. During the twentieth century CE, many collections of clay tablets in museums around the world have been fired for their preservation, thus modifying their composition. Since the 1990s, new researches into the restoration and treatment of cuneiform tablets have been developed with the aim of respecting the material, using an inorganic consolidant, an ethyl silicate with catalysts.

Another writing medium, also made of clay, is represented by the ceramic shards, which are found in many Levantine, Egyptian, Greek and Roman archaeological sites; they are referred to by the Greek word ‘ostracon, ostraca’. 

Relevant analytic methods include:

From CSMC/UWA