180 Chapter 7
(e.g. Fe and Sc). As an example, the most famous Roman clayware, charac-
terised by a dark red glossy surface, called terra sigillata, can be distinguished
from fine pottery produced in the Rhineland by looking at the content of As,
Zn, Rb and Sr. Cluster analysis of chemical compositions of sherds from one
production site may even allow discrimination of wares produced in adjacent
kilns (e.g.based on the Cr, Mg and Ca content).
Apart from chemical analysis, thermoluminescence (TL), isotope analysis,
Carbon 14 (^14 C) dating and several methods using thin sections should be
mentioned here as further techniques applied to ceramics.
Analytical studies of archaeological ceramics are still in progress to obtain
improvements, e.g.to include possible changes in composition during firing
of clay or post-depositional changes in ceramics. The outcome of provenance
work depends strongly on the collaboration between archaeologists and ana-
lytical chemists, since the reliability of data is based on the selection of sam-
ples for analysis and on the profound description of the historical context.
3.6 Deterioration
In general, ceramics are rated as durable materials, which persist even thou-
sands of years of burial in the soil. Nevertheless, for specific objects, ageing may
produce changes in colour, accumulation of dirt or under more severe exposure
conditions, crumbling and even disintegration.
The rate of degradation is a function of composition, pore structure, manu-
facturing procedure, structural design, surface finish and possible damage
deriving from the time of daily use. Deterioration of ceramics results from
several mechanisms, including freezing and thawing, salt crystallisation,
chemical attack by water and other substances, various expansion reactions
or due to a mismatch of various components within one object.
The porosity of archaeological ceramic is the key factor for post-depositional
deterioration, as it allows soil solutions to penetrate and attack the ceramicbody.
For one specific case, it was shown that a low pH of the burial environment
reduced the hardness of pottery. Furthermore, rehydration is postulated as a
degradation mechanism, reversing the loss of water during firing and leading
to disintegration. Low-fired ceramics (below 700°C) may undergo miner-
alogical change, leading to the formation of, for example, carbonates, hydrosil-
icates or gypsum. Water may also cause damage by removing soluble phases
from the body, such as calcite (CaCO 3 ), occurring in pottery originally tem-
pered with limestone. Calcareous ceramics are sensitive to deterioration, even
when exhibiting low porosity, which is due to the solubility of lime-rich silicates
and the low firing temperatures of this type of ceramics. Under extreme cir-
cumstances, weathering may lead to a mass loss of up to 20% of an object.
On the other hand, penetration with soil solutions may lead to precipitation of