Wednesday 14 November 2012

Geoarchaeology with "terrazzo" tiles.

In this post I would like to describe a geoarchaeological analysis based on tiles built with the “terrazzo” technique. These tiles are made of sedimentary material, coming from the different layers of an archaeological excavation, poured with a cementitious binder (normal Portland cement) and then polished with a lapping machine.
This methodology has many advantages:
  1. It is relatively simple and inexpensive
  2. It allows a sistematic storage of the samples
  3. It allows analysis difficoult to achive (or not feasible) in other ways
Sample preparation

It is better to start with a copious sedimentary sample (at least 1 Kg) and sieve it to define a series of size ranges (16-8 mm, 8-4 mm, 4-2 mm, 2-0,06 mm), that will be used to build the tiles. Then we have to prepare some wooden molds with the preferred size (e.g. 30 x 20 x 5 cm) and to mix the sedimentary material with Portland cemenet, water and 1 dl of Vinavil glue. When the mixture is ready, we can put it in the wooden box and let it dry for a couple of days. After this time, if the cement is hard enough, we have to polish one of the larger faces of the tile with a lapping machine (for this operation we asked the help of a marble cutter). Now the sample preparation is complete (see the image below for an example).

An example of "geoarchaeological terrazzo tile"

Geoarchaeological analysis

The procedure described above allows to build a geoarchaeological archive, storing the sedimentary material of different excavations for future comparison.
The tiles built with the “terrazzo” technique expose a section of the components of the sedimentary material, in which it is possible to observe their genetic colour, their framework (internal structure) and their edge (on a random axis). The same parameters could not be visible on intact samples, due to the small size of the components and to their external surface, which is often dirty and altered.
On one hand, observing the colour and the framework of the sediment, it is possible to do some petrological analysis to determinate the rocks and minerals tipology, which could help in understanding their origin and the spatial distance they covered. On the other hand, the edge of the sediment gives morphologic and morphometric informations, which can explain the kind of transport (and the agent) the material was subject.
In the next days we will try to perform some of these analysis in a GIS to evaluate the potentialities of this kind of software also fo such a specific need.

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