Basic Structure of Passive Layers of Cu-Sn Alloys Applied to Patinas of Archaeological Bronzes

International audienceThe passive bronze corrosion structures have been characterized on numerous single-phase archaeological bronzes from Late to Early Bronze Age period (1500-900 BC), from metallographical examinations of cross-sections of artifacts. This paper focuses on the microstructure and the elementary composition of the passive patinas by SEM and EDX analysis (ultrathin window). A basic structure of the protective patinas is first described, then, deviations to the standard model are given. This model of passive structure and its deviations well agree with observations of archaeological bronzes

Characterisation of passive layers of bronze patinas (Cu-Sn alloys) in relation with the tin content of the alloy

International audienceIn order to develop our knowledge of the relations between the high degree of protectiveness of passive layers and their composition, corroded archaeological bronzes of Bronze Age (1500-950 BC) have been characterised by different methods of analysis. Results reveal that the passive layers have a bilayer structure and are due to the selective dissolution of copper from the copper solid solution of the alloy to the electrolyte. For all of the objects, the ratio [(Sn/Cu)p / (Sn/ Cu)a], with p for the outermost passive layer and a the alloy, has about the same value. Sn concentration within outermost passive layers is discussed in relation with the theoretical implications of the Kirchheim model

New model of outdoor bronze corrosion and its implications for conservation

International audienceA new model that takes into account both the tin and the copper in the alloy is proposed to improve the understanding of deterioration processes of outdoor bronze sculptures. The corrosion mechanism relates to the selective dissolution of copper. Two main processes occur: the process of tin oxidation with the formation of insoluble tin compounds which remain in the corrosion film, and the process of copper oxidation with the migration of copper ions to the surface. Considering this mechanism, the mechanical action coupled to acidity of the precipitations, two different evolutions at the surfaces are suggested. On the surfaces directly exposed to the rain, copper ions are washed out leaving a tin-rich external layer. A cyclic corrosion process is engaged. In the unexposed surfaces, the outermost layer is formed with well-known copper corrosion products. This model can be used to establish the degradation state of outdoor bronze sculptures for a new conservation approach

Characterization of naturally grown oxide layers on copper with and without benzotriazole by electrochemical and photoelectrochemical measurements

Autorisation éditeur okInternational audienceThe active corrosion of ancient artefacts (bronze disease) is linked to the presence of copper chloride underneath Cu(I) oxide formed at the surface of bronze. The properties of the Cu(I) oxide determine the mechanism of formation of the corrosion layer and their knowledge would to develop efficient stabilization treatments. The aim of our work is to characterize the structure and the electronic properties of Cu(I) oxide layers formed in different conditions (atmospheric and aqueous electrolyte) under opencircuit potential, using electrochemical and photoelectrochemical methods and SEM. Photoelectrochemistry allows the characterization the semiconducting properties of the passive layer and leads to a better knowledge of the nature of the electrode/solution interface and of the electrochemical reactions occurring at the oxide surface. Atmospheric Cu(I) oxide shows mainly p-type behaviour, whereas Cu(I) oxide layers formed at open-circuit potential by immersion in a NaCl solution were shown to exhibit p-type behaviour during the first hours of immersion and simultaneous n-p behaviour for longer immersion times. In the presence of benzotriazole added to the sodium chloride solution, the photoelectrochemical measu-rements revealed modifications of the semi-conducting properties of the Cu2O layer (duplex p-n layer from short immersion times) and showed that BTA makes the oxide layer more insulating. Results will be discussed in regards to the Cu(I) oxide role on the bronze disease

Apport de l'analyse statistique de la composition des produits de corrosion à la compréhension des processus de dégradation des bronzes archéologiques (Contribution of statistical analysis of corrosion products to the understanding of the degradation processes of archaeological bronzes)

Autorisation Dr P. LANOS (Rédacteur en chef) en date du 26/11/2010 - Archéosciences - Revue d'ArchéométrieInternational audienceThe corrosion products of three monophased bronzes, representative of Bronze Age objects, were analyzed by scanning electron microscopy (SEM). Contents of main chemical elements in the corrosion layers were identified. A model for the formation of the two principal corrosion structures in buried archaeological bronzes is proposed. Selective copper dissolution from the copper-tin alloy occurs via two transfer mechanisms: 1) by copper cations from the alloy corrosion; and 2) by anionic species of the soil electrolyte. A stationary state is reached after decades of burial, following which the corrosion process evolves only locally on the object.L'analyse élémentaire des composés de corrosion de trois bronzes monophasés, représentatifs d'un corpus d'objets de l'Age du Bronze, a été effectuée par microanalyse X sur microscope électronique à balayage. Les varations de teneurs en principaux éléments chimiques dans les couches de corrosion ont été caractérisés. Un modèle de formation des deux principales structures de corrosion des bronzes archéologiques enfouis est proposé. La dissolution sélective du cuivre de l'alliage peut être contrôlée par deux types de processus de transport : l'un, par les cations du cuivre issus de l'oxydation de l'alliage, l'autre par les espèces anioniques de l'électrolyte du sol. Un état stationnaire est probablement atteint après des durées d'enfouissement de quelques dizaines d'années. La corrosion n'évolue alors que localement sur l'objet

Isolation, structural assignment and insecticidal activity of (1S,2R,3R,4S)1,2-epoxy-1-methyl-4-(1-methyl)cyclohex-3-yl acetate, a natural product from Minthostachys tomentosa

[EN] (-)-(1S,2R,3R,4S)-1,2-Epoxy-1-methyl-4-(1-methylethyl)-cyclohex-3-yl acetate has previously been identified as the active compound of Minthostachys tomentosa responsible for the insecticidal activity against Oncopeltus fasciatus. Its structure was initially assigned on the basis of spectral data. In order to confirm the structure and to define the stereochemistry, stereoselective synthesis of its enantiomer, (+)-(1R,2S,3S,3R)-1,2-epoxy-1-methyl-4-(1-methylethyl)-cyclohex-3-yl acetate, starting from (R)-(-)piperitone, was carried out using a Sharpless reaction as the key step. The natural product is dextro-rotatory while the synthetic product is levo-rotatory. Measurements of insecticidal activities of the different steroisomers revealed that only the natural product is active.Cantin Sanz, A.; Lull, C.; Primo Millo, J.; Miranda Alonso, MÁ.; Primo Yufera, E. (2001). Isolation, structural assignment and insecticidal activity of (1S,2R,3R,4S)1,2-epoxy-1-methyl-4-(1-methyl)cyclohex-3-yl acetate, a natural product from Minthostachys tomentosa. Tetrahedron Asymmetry. 12(4):677-683. doi:10.1016/S0957-4166(01)00094-5S67768312