Archaeological iron
Project leader: Quanyu
Wang
Department: Conservation, Documentation and
Science
Project start:
23/08/2004
End date: 21/07/2006
Other British Museum staff:
Fleur Shearman
Project funded by: The
British Museum
Description:
Archaeological iron objects have proved to be
problematic as a result of inherent instability. Corrosion often
causes surface flaking and even makes an object disintegrate (Figs.
2-3). No single conservation treatment works for every iron object.
15% has been reported to be the critical RH (relative humidity)
level above which further deterioration can occur if chloride ions
are present on the object. However, keeping iron in storage at such
a low level of RH may be an expensive option and practicably
difficult to achieve.
The aim of the project was to identify the
main causes of deterioration of archaeological iron in the Museum
collection and to identify a suitable storage environment. The
project was carried out both on material in the collections of the
British Museum and on iron coupons by investigation and
experiments.
The
investigation of deterioration of archaeological iron objects in
the Museum was carried out on archaeological iron from British
Roman and Anglo-Saxon sites. Both metal (if possible) and surface
corrosion were studied using a variety of analytical techniques,
e.g. microscopy, scanning electron microscope with
energy-dispersive X-ray analysis (SEM/EDX), Raman spectroscopy,
X-ray diffraction (XRD) and X-ray fluorescence spectroscopy (XRF).
Archaeological contexts, previous conservation treatments and
storage environments were also investigated to help understand the
mechanism of deterioration.
In the experiments Industrial powdery
FeCl2.4H2O, mixtures of iron
coupons with FeCl2.4H2O, and
archaeological nails were exposed to variable RH levels, including
75, 54, 44, 33 and 23%, at room temperature for over a year.
Changes in these tested materials were examined using the
analytical techniques mentioned above and ion chromatography.
Objectives:
- Survey of conditions of objects from
different sites to look into the correlation between object
condition and archaeological context, e.g. burial environment.
- Investigation of the effects of
microstructures and manufacture techniques on the degree and
mechanism of degradation of the objects.
- Investigation of the effects of chloride ions
on the degradation of the objects.
- Experimental tests on the effects of RH on
the corrosion of iron.
- Comparison of the effectiveness of past
desalination methods by comparing the current condition of the
objects.
- Comparison of the desalination effectiveness
using sodium hydroxide and alkaline sulphite by monitoring changes
in the treated objects in storage in relation to the environmental
condition, e.g. RH levels, of the storage
- Assessment of the level of risk to artefacts
and evidence preserved in the corrosion layers incurred by wet
treatment approaches to the stabilisation of archaeological
iron.
- Recommendation of an acceptable RH level for
long-term preservation of the archaeological iron.
- Presentation and publication of the research
results at academic conferences and publications.
Publications:
Q. Wang, ‘An investigation of deterioration of
archaeological iron’, Studies in Conservation, 52, (2007), pp.
125-134.
Q. Wang, 'Effect of relative humidity on the
corrosion of iron: an experimental view', The British Museum
Technical Research Bulletin, 1, (2007), pp. 65-73.
Images (from top):
- Investigative cleaning using a
microscope, with xray
- Photomicrograph of a Roman nail
showing pitting on the cross section caused by Cl containing
corrosion