Laser-cleaning effects induced on different types of bronze archaeological corrosion products: chemical-physical surface characterisation

Highlights

• Assessment of laser-cleaning induced effects on different surface structures.

• Systematic study of corrosion products characterisation.

• Laser-cleaning of different Cu-based archaeological artefacts.

• Novelty in using a laser-scanning system to clean archaeological surfaces.

Abstract

Archaeological bronzes (Cu-Sn alloys) might be different in composition and these variabilities lead to a different interaction with the environment and, if they were buried, with several chemical elements present in soils. Generally, the result is the presence of complex corrosion products layers on the Cu-based artefacts which might be protective or reactive (and so dangerous) for the artefacts preservation itself. Moreover, the corrosion products can also be unwanted because hide the drawings present/imprinted on the original metal surfaces.

So, for several reasons, it can be necessary to remove the unwanted corrosion products: laser-cleaning procedure is one of the most selective cleaning technique used.

The main goal of this work was to assess possible laser-cleaning induced surface effects on different archaeological bronzes corrosion products structures through a systematic and multi-technique compositional study.

Chemical-physical characterisations were performed on laser-cleaned and non-cleaned types of Cu-Sn archaeological corrosion products structures. The results show the efficiency and the low-invasiveness of the treatment.

Introduction

Understanding the corrosion behaviour in Cu-Sn alloys have always aroused wide interest for those that try to reproduce these “patinas” for both aesthetic and protective purposes [1], [2]. However, the formation of corrosion layers on archaeological Cu-based artefacts is the results of complex interactions of the metal with the soil chemical-physical properties and with the environmental conditions: the corrosion mechanisms that take place lead to the complex stratified microstructures usually found [3], [4], [5], [6]. The final surface aspect is of extremely importance for both the already mentions aesthetic and protective purposes. In this perspective, the study of corrosion phenomena and surfaces gains importance for both improving knowledge in the field of long-term corrosion and, above all, for controlling and stopping deterioration processes of ancient and historic metals in storage conditions or in museums [1], [4], [7], [8], [9].

It is known that not only the corrosive environment but also the metallurgical technique employed, the artefact chemical composition and how the corrosion attacks the object (stratification, transgranular or intergranular corrosion) play an important role in the structural transformations of Cu-based artefacts during a long-term corrosion process that leads to a steady state [3], [4].

On bronzes, literature agrees on two generic models of corrosion (and corresponding deviation structures) firstly introduced and extensively studied by Robbiola and colleagues [3], [4]: Type 1 and Type 2, due to cationic or anionic migrations, respectively.

Type 1 structure is characterised by the presence of two layers: a thin protective corrosion layer (noble patina) grown in contact with the metal overlapped by a second thicker porous layer; in that case, it is possible to find a tin-enriched corrosion layer due to a decuprification process. The inner protective red-brown layer is mainly constituted by cuprous oxides (cuprite, Cu₂O) while the generally green outer layer is constituted by different chemical compounds, in dependence of the interaction with soil constituents, oxygen and humidity. The outer layer can be characterised by low copper contents and high tin contents. Furthermore, the aging of the corrosion products leads to soil elements incorporation (O, Si, Fe, Al, Ca, P, Cl, …) and, e.g., the possible transformation of cuprous compounds in cupric compounds. As a consequence, the surface can be constituted by soil elements and copper compounds as malachite (Cu₂(OH)₂CO₃), brochantite (Cu₄(OH)₆(SO)₄), etc.

Type 2 is characterised by a three-layer structure: in presence of Cl⁻ ions (aggressive environment), the outer porous corrosion layer, can let penetrate the Cl⁻ ions through the layer creating a high concentration of chloride ions, together with tin and copper compounds, at the metal-corrosion layer interface (of the inner layer). The outer layer is characterised by copper (II) compounds, such as hydroxychlorides (atacamite (Cu₂Cl(OH)₃ and its polymorphs), hydroxycarbonates (malachite), hydroxysilicates and hydroxyphosphates. Between the outer and the inner layer, an intermediate layer of cuprous oxides is present. In the inner layer, this might lead to the formation of reactive cuprous chloride (nantokite, CuCl), responsible for the cyclic copper corrosion processes, well known as bronze disease, where the dangerous corrosion layer grows at the expense of the bulk alloy [1], [3], [4], [6], [10], [11], [12].

As a consequence, the corrosion products (that are a multi-component material) can hide the drawings present/imprinted on the original metal surfaces or, in the worst case, they can compromise the conservation of the object itself. Therefore, the removal of the unwanted and/or dangerous corrosion products might become necessary. Since laser-cleaning (or laser-ablation, or removal) treatments are a low-invasive and highly-selective method, they have been studied for removing the unwanted and/or reactive corrosion products on several archaeological/historical materials [13], [14], [15], [16], [17], [18], [19], [20], [21]. On metals, the final aims are stopping possible dangerous processes and preserving the protective patina and the metal surface. Laser-ablation methods have been also preferred for the fact that they are eco-friendlier than the common chemical and mechanical methods generally used and they guarantee less risk for the operators [13], [15], [16], [17], [21], [22], [23].

Even so, there is a lack of sustained models explaining the laser-material interactions in the conservation field: they are usually more fundamental studies or special conservation cases more focused on the final cleaning results than on the discussion of the on going mechanisms. Only few authors reported considerations on the mechanism that occurred and on the chemical-physical characterisation of the laser effect on the surface of the artefacts [14], [16], [22], [24], [25], [26], [27].

This can be explained by the heterogeneity in composition of each unique cultural object: it is a complex problem to standardise cleaning procedures as a function of the interactions that occur among the laser parameters and the composition of the unwanted materials (e.g. dust or multi-material corrosion products) present on artefacts. In fact, the complex interactions, that allow laser-ablation process, are dependent from laser parameters (e.g. laser wavelength, continuous or pulsed laser, pulse duration, fluence, irradiance), optical and thermal properties of the material to remove (e.g. absorption coefficient, thermal conductivity) and surrounding environment (e.g. air, vacuum). Furthermore, the success of a laser-cleaning procedure is due to a proper combination of the laser parameters: they have to be set as a function of the properties of both the multi-component material to be removed and the underlying original surface. In that way, the interactions are confined, as much as possible, on the outmost layer occupied by the unwanted materials [28]. Recently, a more theoretical approach with experimental applications on coins has been published [29] with the final aim of assessing some laser conditions and parameters (e.g. penetration depth of the laser beam and laser fluence); however, the study was performed on modern coins.

Fig. 1 proposes a model of the laser-cleaning process on Cultural Heritage Cu-based metal artefacts. The laser, thanks to complex interactions within the plasma, is able to remove (high-selective) the unwanted corrosion products (green in the picture), which absorb the laser beam, leaving the protective layer of corrosion product (brown in the picture). The metallic surface, reflecting the laser beam, is not damaged (low-invasive). So, a properly set laser is able to discriminate between unwanted and protective corrosion products, which similarly absorb the laser beam. Moreover, it makes possible to focus the ablation on specific point of persistent unwanted corrosion products.

This research (part of a wider project [27], [30]) tries to overcome to this literature lack assessing possible laser-cleaning induced surface effects (similar or different) on different types of Cu-Sn archaeological corrosion products structures through a systematic and multi-technique compositional study on laser-cleaned and non-cleaned areas.

Regardless of whether the stable corrosion products are generally preserved by conservators and restorers, and knowing the high-selectivity of the laser and that archaeological objects might present both corrosion situation on the same surfaces, this study aims to determine the effects of the laser on both stable and dangerous corrosion products.

To do this, an infrared nano-pulsed laser was used. For this kind of used wavelength, thermal laser-materials interactions (e.g. heat propagation and dissipation) are the most important laser-induced process to be considered: they allow thermal decomposition, the formation of a plasma and finally the ablation of material [28]. Moreover, the research introduces the novelty of the use of a laser-scanning system that, thanks to a dedicated software, allows to remotely move the laser on the surfaces and to control not only the laser parameters but also the geometrical parameters (scanning parameters) as the pulse overlap on X and Y axes. Video 1 (Supplementary data) shows the XY scanning movement of the laser during a treatment process over a surface.

The effect of the laser-ablation parameters applied in this article were previously studied on Cu-based reference samples presenting different artificial corrosion products, in order to assess, and not test, the procedure on archaeological objects. The previous tests, partially published in [27], [31] highlighted that a shorter pulse duration is more efficient in the removal of unwanted Cu(II) corrosion layer preserving cuprous oxide layer. In addition to this, the diffuse reflectance of the corrosion crystals toward the laser wavelength, their grain size and the porosity of the layers play an important role on the laser-ablation effects.