By combining the information from environmental monitoring, modern
coupons and archaeological artefacts it is possible to give an overall evalu-
ation of the preservation conditions for iron in Nydam. The majority of dete-
rioration of the artefacts probably took place in the first few years after
sacrifice (Figure 7), and we consider the current conditions as conducive to
the in situ preservation of the artefacts. However, the water level and soil pH
have to be monitored also in the future. It seems that even a slight lowering
of the soil pH may cause a dissolution of the corrosion products covering the
artefacts (Figure 8), and from the modern coupons it seems that even a tem-
porary lowering of the water table may have an adverse effect on the preser-
vation (Figure 9).
5 Conclusions
In situpreservation is a form of preventive conservation, but absolute preser-
vation in situis not achievable. All sites are dynamic and deterioration of
archaeological materials will continue, albeitat slow, and often impercept-
ible rates. However, environmental conditions favourable to the preservation
of archaeological sites, and artefacts, can be assured by a “three pronged”
sequence of environmental monitoring, study of the deterioration of analo-
gous modern materials placed on a site, and the state of preservation of arte-
facts themselves.
326 Chapter 12
Figure 8Pourbaix diagram for iron in an environment with 10^2 M carbonate, and 2 10 ^4 M
dissolved iron at a temperature of 10°C, generated by the software HSC chemistry
5.0 from Outokumpu. The thermodynamic data for the different corrosion products
are compiled by the French Agency for Nuclear Waste Management (ANDRA).
Hatched area shows the pH values and corrosion potentials measured in situ at
Nydam. Dotted line shows the stability limit for water
FeOOH
Fe2+(aq)Fe 3 O 4FeCO 3Fe Fe(OH)^26810-0.8-0.6-0.4-0.20.00.2Eh (volt)pH-1.0
4 12