Innovative materials and methods for the conservation of archaeological metallic artefacts
M.P. Casaletto1,*, G.M. Ingo1, C. Riccucci1, P. Bernardini2, S. Agostini3, I. Pierigè3
1
Istituto per lo Studio dei Materiali Nanostrutturati del Consiglio Nazionale delle Ricerche (ISMN-CNR),
Rome (Italy)
2
Soprintendenza Archeologica alle province di Cagliari e Oristano, Cagliari (Italy)
3
Soprintendenza Archeologica dell’Abruzzo, Chieti (Italy)
An extensive and up-to-date research of the needs and expectations of the European and Mediterranean basin
museums and archaeological-historical sites was carried out in the framework of several European projects: V FP
EFESTUS (INCO-MED CONTRACT N. ICA3-CT-2002-10030); VI FP PROMET (INCO-MED contract N.
509126) and MEDAL (INCO-CT-2007-043849) projects. The assessment of the projects results clearly evidenced
that in the Mediterranean basin, where the wealth of cultural property has a major impact on tourism, a growing
need of Conservation Institutions and Museums is to develop and use alternatives, safe and reliable long-term
conservation methods and strategies for bronze archaeological artefacts that could replace the presently used
hazardous materials and processes. Conservation materials and process should be used following standardized,
reliable and reproducible procedures for removing soil traces, for stabilising the bronze archaeological artefact and
for avoiding further degradation during exhibition or storage.
Degradation phenomena occurring during the archaeological burial and post-excavation reactions occurring during
exhibition and/or storage are the major problems encountered for Cu-based artefacts. A relevant danger for the
preservation of archaeological bronze artefacts is represented by the so-called “bronze disease”, i.e. a series of
autocatalytic reactions involving cuprous chloride derived from the interaction of the artefact with the soil and
humid air. This dangerous form of archaeological bronze degradation, if not prevented or adequately cured, leads to
the complete disruption of the metallic artefacts.
Starting from its initial assessment as pure copper corrosion inhibitor, nowadays 1H-Benzotriazole (BTA) is
widespread used in the conservation of copper-based artefacts. Unfortunately, although quite inexpensive and
easily applicable, BTA is harmful for animals and plants, toxic in aquatic environments, non-biodegradable and a
suspected carcinogenic agent.. Contact with the skin should be avoided and the powder should not be inhaled. The
synthesis of new environmentally friendly corrosion inhibitors and that of reliable reference materials is now
becoming a pressing question to face. The molecular design of new and safe protection systems should offer
alternative choices to conservation-restoration professionals for using effective products to stabilize and protect
metal artefacts that are safe to use and to dispose of.
Organic inhibitors, such as 4-methyl-imidazole, 2-mercaptobenzothiazole, 1-phenyl-tetrazole-5-thiol, 3-amino1,2,4-triazole and 3-ethyl-4-methyl-5-imidazole-caboxylate and tailored R-substituted compounds were selected on
the base of their potential activity on a nano-scale dimension to inhibit the cyclic copper corrosion. The main
application of corrosion inhibitors in conservation is in the form of coatings or “dry-films”. Since the
archaeological artefacts are unique and not easily available objects, reference materials are required to evaluate the
anticorrosive properties of innovative compounds toward bronze corrosion. For this purpose modern reference Cubased alloys were intentionally produced by ISMN-CNR with a chemical composition and micro-chemical
structure similar to that of ancient alloys, to be used as substrates. The efficiency of these alternative materials and
methods was preliminarily tested on artificially degraded Cu-based alloys produced by ISMN-CNR. The inhibition
properties of these organic films were investigated by advanced surface techniques, such as XPS, AFM, FEG-SEM,
XRD and by measures of contact angle (AC) and optical microscopy reflection (OM).
Then, in collaboration with museum conservators, some selected archaeological bronze artefacts with different
degradation phenomena, structure and chemical composition were treated by using the more promising materials
and methods in terms of toxicity, economical aspects (cost/effectiveness ratio) and marketability.
Results showed that the influence on the inhibition efficiency is often related to the chemical nature of the
inhibiting material, the application method, the composition and structure of the metal object, as well as to the
nature of the corrosion products and their layered structures.
Some of these materials, which are lower toxic and more efficient compounds than the popular and commonly used
BTA, were able to stop the cyclic copper corrosion, thus offering reliable and alternative solutions in terms of safe
and efficient materials to the conservation community. The possibility to ensure a long and stable life to ancient
metal artefacts offers also a safer exhibition and a better valorisation and fruition of these important objects, as well
as new market and industrial opportunities.
* Dr. Maria Pia Casaletto
ISMN-CNR, Via U. La Malfa 153 – 90146 Palermo (Italy)
Tel: + 39 091 6809378 Fax: +39 091 6809399 E-mail: mariapia.casaletto@ismn.cnr.it