Natural Science in Archaeology

62 3 Exploitation of Mineral and Rock Raw Materials
and are the result of high pressure metamorphism. Jadeites, commonly consisting
primarily of the pyroxene mineral jadeite, also are the result of high pressure metamorphism.
From the Neolithic through the Middle Ages basic and ultrabasic metamorphic
rocks of the greenschist facies have been quarried in the central Alps, in
Italy, for use in a variety of fire-resistant objects such as stoves and cooking pots.
These materials were called “pietra ollare”. They had good thermal stability up to
1200°C and regular linear expansion up to about 700°C. These rocks were soft and
easily workable with Mohs hardness ranging from 1 to 4. Also in Italy axes, adzes,
and chisels were made from high pressure meta-ophiolites from the Early Neolithic
to the Bronze Age (D’Amico and Starnini 2006).
Of more than 20,000 examples of stone axes recovered from prehistoric sites in
Ireland, the raw material that accounted for over half was porcellanite (Mandal et
al. 1997). Porcellanite axes are widely distributed throughout the Irish Neolithic
with lesser representation in Britain. This porcellanite is the product of thermal
metamorphism of a ferruginous bauxitic clay that was derived from weathering of
basalt. For a thorough summary of the use of metamorphic and igneous rocks as
stone axes and tools, see Cummins (1983).
3.6 Unconsolidated Deposits
3.6.1 Surface Deposits
Alluvium is a subaerial deposit of a river, both within the channel and beyond the
banks of the channel. Coarser materials such as sands and gravels characterize channel
deposits, whereas overbank deposits away from the channel are dominantly silt
and clay particles (Table 3.5).
Humans have not only used alluvial terraces as habitation sites, but they have
also exploited the local deposits for building materials and ceramic raw material.
Sands, silts, clays, and clastic sediments in general have been useful raw materials
for millennia. Silty clay is a common raw material used for pottery, tile, and other
ceramics. Sands are found and recoverable from river channels, beaches, and dunes.
Loose sand was used in antiquity as an abrasive and as a basic raw material for
faience, glass, glaze, and brick. Sand that is composed almost exclusively of quartz
has been the source of silica for the manufacture of glass since the beginning of
glass technology. Pliny noted the production of glass from the sand of the Campanean
littoral. Based on Pliny’s text, Silvestri et al. (2006) studied experimentally the
composition of the suggested raw materials and the resulting glass objects. Their
studies showed that Campanian littoral sand could produce “Roman glass”.
Sedimentary processes account for important concentrations of minerals in placer
deposits (e.g., gold, cassiterite) and in the formation of secondary clay deposits.
After the deposition of clastic or chemical sedimentary deposits, some elements are
dissolved, transported, and redeposited in useful concentrations. This is the origin
of many silicas (chert, chalcedony), manganese oxides, and hydrated iron oxides.