ON THE OCCURRENCE OF BROMINE IN CORRODED SILVER

Abstract

Attention is drawn to the finding of silver bromide as a regular corrosion product on buried ancient silver. The chemical processes leading to this, and their relation to the burial environment, are discussed.     

THE CONSERVATION AND TECHNIQUE OF SILVER BANGLES DISCOVERED AT ROJDI,INDIA

Abstract

The treatment of two silver bangles found at Rojdi and dated c. 2000 b.c. is described. The bangles were separated by removing the hard incrustations with a 20% sodium hexametaphosphate solution and 15% acetic acid. Silver chloride was removed with a 25% solution of 0.88 ammonia and a 20% aqueous solution of ammonium thiosulphate.

Silver sulphide was removed in a 20% formic acid bath. Polyvinyl acetate was used for consolidation, and a coating of polymethyl methacrylate was applied. The silver was found to contain copper and lead. The causes of embrittlement and the manufacturing technique are discussed.     

Altération et nettoyage de pièces en argent doré

Abstract

By studying the mechanism of sulphation of silver-gilt objects in relation to the method used for gilding, its thickness and its appearance, it has been possible to develop a procedure for electrolytic treatment. The two-step treatment is carried out in a buffered solution of sodium nitrate and comprises cathodic reduction of the silver-based corrosion products followed by anodic dissolution of the reduced silver. The behaviour of the different materials is described, as well as certain limiting factors.     

THE USE OF THE AIRBRASIVE PROCESS FOR CLEANING ETHNOLOGICAL MATERIALS

Abstract

Ethnological specimens of many different materials can be dry cleaned mechanically by means of a grit-spraying unit. Plain and painted leather, corroded metal, beadwork, metallic embroidery, wood, ivory, bone, basketry, some paper, pottery, shell, some textiles and stone are items which can be cleaned with less disturbance to the surface than is often occasioned by the use of other methods. A specimen constructed of several materials may be cleaned by this one method rather than by several, thereby also avoiding the use of water, solvents or other chemicals. The powdered abrasives used are calcium magnesium carbonate, sodium bicarbonate, glass beads, and several grades of aluminium oxide. Silicone carbide powder is used occasionally for very hard corrosion products on metal.     

Influence of crucial parameters on the dechlorination treatments of ferrous objects from seawater

Abstract

This article compares chemical dechlorination treatments (immersion in sodium hydroxide or alkaline sulphite) and electrochemical treatments of iron bars from the Gallo-Roman period excavated from a marine environment. Some important parameters, such as storage before treatment, temperature, solution composition, and drying after treatment, were varied during the study to assess their influence on the chloride extraction process. The kinetics of these treatments depend mainly on chloride diffusion through the corrosion layers. The kinetics are promoted by high temperatures and, in the case of electrolysis, by the electric field effect. The reduction of corrosion products during electrolysis occurs only for objects previously stored in air. In fact, the manner in which the objects are stored before treatment is critical in the dechlorination processes. The sooner the objects are treated after excavation (with water storage), the better the removal of chloride ions, in both chemical immersion and electrolysis treatments. But if the object is stored in air, material losses occur, and only electrolysis results in complete extraction of the chlorides. These differences are due to modifications in the corrosion products during storage. Drying after treatment also has a significant impact on the composition of the corrosion layers. If the objects are dried too quickly, Fe(OH)₂ oxidizes into FeOOH, which thus forms a layer with low cohesion.     

Models for Chloride Ion Diffusion in Archaeological Iron

Summary

The rate at which chloride ions diffuse from archaeological iron into a treatment solution depends on how the chloride ions are initially distributed in the corrosion layer. This paper compares solutions of the diffusion equation for two limiting cases: (1) where the chloride ions are initially spread uniformly through the corrosion layer; and (2) where the chloride ions are initially concentrated at the interface between the iron and the corrosion layer. Although the first model has been used in the past to describe chloride ions diffusing from marine iron, the second is more appropriate in cases where corrosion has drawn chloride ions toward the iron surface. Because diffusion processes in archaeological iron are complicated, the limitations of both these models are discussed.     

Stress corrosion cracking in ancient silver

Abstract

Six cases of ancient silver embrittlement by corrosion are discussed with respect to evidence for stress corrosion cracking. Both intergranular and transgranular cracking can occur, depending on whether there is retained cold-work in the silver. The severity of cracking is influenced by a combination of factors, including the amount of cold-work in the silver and the burial environment. The burial environment is important for two reasons. Firstly, the soil weight can induce high stresses, particularly in hollow, thin-walled, and incompletely filled-in artefacts. Secondly, grave burials can result in more aggressive soil environments owing to high salinity from body decomposition.     

The impact of chloride desalination on the corrosion rate of archaeological iron

Abstract

Although desalination of archaeological iron reduces its chloride concentration and enhances object stability, the reduction in corrosion rate that this produces has never been quantified. This study measures post-treatment corrosion rates in accelerated corrosion environments to identify the impact of removing chloride ions on corrosion rate. Thirty-five archaeological iron nails, treated individually in either alkaline sulphite or nitrogen-deoxygenated sodium hydroxide, were exposed to 75°C and 75% relative humidity together with 31 untreated objects from the same archaeological sites. Object weight change and visual examination of physical change before and after the test period were used to monitor corrosion. 77% of treated objects showed no weight gain and no visible signs of corrosion, while 90% of untreated objects did corrode. The impact of chloride on corrosion of untreated objects was clearly established by a significant linear correlation between chloride content and weight gain. Treated objects with <400 ppm chloride content showed no corrosion behaviour. Corrosion of treated objects was attributed to incomplete treatment: 93% of objects treated to <5 mg/l Cl^? in the final solution bath displayed no corrosion behaviour. Based on these results, desalination of iron objects to enhance their stability offers a valuable option for reducing corrosion rates of archaeological iron, which should increase object lifespan. The results also raise the question of whether low levels of post-treatment residual chloride produce corrosion of any significance. Answering this will be an important step forward for managing the preservation of archaeological iron.     

The Surface pH Measurement and Deacidification of Prints and Drawings in Tropical Climates

Abstract

The interrelationship between the various chloride ion containing compounds associated with the corrosion of iron in a marine environment is reviewed, and their role in corrosion promotion is evaluated as a basis for the selection of conservation techniques with a high probability of success.     

The Influence of Relative Humidity and Intrinsic Chloride on Post-excavation Corrosion Rates of Archaeological Wrought Iron

ABSTRACT

This study examined the impact of relative humidity (RH) on the corrosion rate of 129 archaeological iron nails from two sites. Oxygen consumption of individual nails in controlled RH was used as a corrosion rate proxy to deliver quantitative data on corrosion rate as a function of RH. This was negligible at 20% RH, slow up to 40% RH for both sites, and increased rapidly at 60% RH for Roman nails from Caerleon (Wales) and at 70% RH for medieval nails from Billingsgate (London). The nails were digested and their chloride content was determined and related to their oxygen consumption at specific RH values. While a generic pattern of corrosion as a function of chloride was identified, for any single concentration of chloride corrosion rate was not predictable. Desiccation is in common use to control post-excavation corrosion of archaeological iron; quantifying how differing levels of desiccation changed corrosion rate provided a scaled tool for identifying corrosion risk, estimating object longevity, and calculating cost benefit for storage options.     

ON THE CONSERVATION OF THE BAPTISTERY DOORS IN FLORENCE

Abstract

The causes of deterioration of the gilded bronze doors of the Baptistery in Florence have been examined and the mechanism of corrosion discussed. An apparatus for thermal conditioning has been proposed and tested either on the north door of the Baptistery, or on samples exposed to the urban atmosphere in Milan and by laboratory tests on an electrochemical model. The effectiveness of the proposed apparatus has been verified by weight losses, by assessing soluble salts, by visual observations, by local chemical microanalyses, by morphological examinations with a scanning electron microscope equipped with an X-spectrometer and by X-ray diffraction.     

Zinc dust treatment—an effective method for the control of bronze disease on excavated objects

Abstract

Silver oxide is currently used to treat isolated spats of bronze disease, forming a seal of silver chloride. The technique is not infallible: an analytical study showed that the conductive properties of the silver chloride seal, together with the comparatively poor moisture barrier formed, are the factors responsible for failure. In the new zinc dust treatment proposed, tough, adherent and relatively stable zinc compounds form an improved barrier, effectively sealing the area of bronze disease. Objects treated with zinc dust were subjected to high humidity for several months and afterwards exposed to a tropical humid environment for four to five years. These objects showed no re-emergence of bronze disease whereas repeated use of silver oxide failed to stabilize the objects. The method developed is simple, less time-consuming, and relatively easy to employ.     

THE RESTORATION OF TWO MARBLE STATUES BY ANTONIO CORRADINI

Abstract

The restoration of two early eighteenth-century marble statues by Antonio Corradini is described. Methylene chloride was used to remove plant stains. Sepiolite (magnesium silicate) and water drew out the dirt, and thick incrustations were removed with an ultrasonic dental tool. Incorrectly restored parts were remodelled according to contemporary prints, not in marble but using a casting procedure.     

Technical Analysis of an Early Painting by Botticelli

Abstract

A study is presented of the growth of sulphidecontaining corrosion products on antiquities in museums. Analysis of the corrosion products suggests that they are dependent on the mode of formation. Techniques for conservation have also been investigated and l-amido-l-cyanoethylene, 2,2-di(sodium thiolate) is recommended for dissolving the sulphide layers, while a treatment with benzotriazole followed by lacquering is recommended for long-term protection.     

A New Methodology for Accelerated Corrosion Testing

Abstract

Accelerated corrosion tests have been used for almost 30 years to exclude corrosive materials from museum display and storage. The method reported by Green and Thickett in 1995 has been developed to allow the evaluation of three metals (silver, copper and lead) simultaneously in a single test. Comparison of a wide range of materials has shown that the differences in results introduced by this modification are less than the differences encountered with replicate tests using the same material employing the previous method. Problems encountered with corrosion in lead control tests have been investigated and the method has been modified to remove this effect. The cleaning method for the test vessels has been modified to exclude the hazardous chemical, chromic acid.     

An investigation into the cause of corrosion on indoor bronze sculpture

Abstract

Corrosion noted on a number of nineteenth and twentieth century indoor bronze sculptures in the collection of the National Gallery of Canada was the cause for a major condition survey of its Canadian, American, and European bronzes. The results led to a comprehensive study, which included material analysis. This article outlines the steps undertaken to determine the extent and type of corrosion, its possible cause(s), and potential remediation. The survey included 136 bronzes, of which 40 were selected for an in-depth study. Indoor air quality tests of the current building, housing the collection since 1988, were carried out. The dossiers and database records of the selected sculptures were reviewed to investigate their condition, treatment, and storage history. The metal, patina, and corrosion products were subjected to analysis. More areas of corrosion were generally associated with predominantly green patinas and chloride-containing compounds identified in either patina or corrosion samples. The most frequently identified compounds in the patina samples were atacamite, a copper(II) chloride hydroxide, and cuprite, a copper(I) oxide. The most frequently identified compounds in the corrosion samples were atacamite and rouaite, a copper(II) hydroxide nitrate. The analysis determined that the current environment is not contributing to the surface alterations of the sculptures; however, past storage conditions, chemical residues from artificial patination, and likely casting residues are the main causes of the current condition.     

ABSTRACTS

Abstract

The deterioration of Bronze Age objects from the archaeological site at Fort-Harrouard (France) has been studied, firstly, in relation to their external appearance and the elemental composition of the sound metal. A relationship between the composition of the corroded surface and the underlying sound metal was deduced. In addition, examination of metallographic sections of one of the bronzes allowed the differentiation of corrosion processes which are related to the shape of the object: the stratification of the corrosion products and the intergranular and transgranular corrosion processes. The existence of a pseudomorphic microstructure suggests that the mechanism of deterioration may be stratigraphic, which may be explained by the growth of the corrosion products in fissures which do not involve periodic precipitation.     

The Freer Gallery Laboratory for Technical Studies in Oriental Art and Archaeology

Abstract

A simple, low cost alternative to X-ray diffraction for the identification of corrosion products on marine iron artifacts is based on the differential solubility of the minerals in pure acetyl acetone and on the color of the product formed.     

A Note on the Products of the Reaction of AMT with Bronze and with Three Corrosion Products of Bronze

Abstract

The compound 5-amino-2-mercapto-1,3,4-thiadiazole (AMT) is utilized in the conservation of archaeological and artistic bronze artifacts, for their protection and for the removal of corrosion products. Many authors have discussed the utility and efficacy of this, but few have studied the products resulting from the reaction of AMT with bronze or with the corrosion products of bronze; this leaves the restorer with many doubts concerning the acceptability of this method. This paper discusses the study of products obtained from the reaction of AMT with synthesized corrosion products of bronze (malachite, paratacamite and brochantite) and with the commercial bronze alloy TM 23 (Cu 72%, Pb 15%, Zn 8% and Sn 5%). It was found that the film obtained from the direct reaction of AMT with the bronze alloy can be AMT itself, or the metallic complex Cu(C₂H₂N₃S₂)₂·H₂O if AMT is applied in a solution of an oxidizing acid. These results are expected to be of value to conservation scientists and restorers/ conservators concerned with the use of AMT for the conservation of bronze artifacts.     

Surface Investigation of Metal Threads and Solid Metals of Ottoman Textiles in the Topkapi Palace Museum

ABSTRACT

We analyzed 53 historical textile samples from a collection of caftans and brocades dating between the sixteenth and nineteenth century in the Topkapi Palace Museum (TPM) by scanning electron microscopy with energy dispersive X-ray spectroscopy (SEM-EDX). Data were collected on the type, dimensions, elemental composition, and level of corrosion. The technique is presented in this work as an efficient and micro-destructive tool for the characterization of the structure of historical metal threads. SEM-EDX analysis for different energies was performed on a selection of gold and silver threads belonging to the historical textiles. The results were interpreted via laboratory prepared samples of gilt Ag and sulfur reacted Ag at different thicknesses for Au and Ag₂S layers respectively. A series of Au–Ag alloys for different Au/Ag ratios were prepared for comparative analysis. The threads were made of metal strips wound around a silk core. Metal wires and flat strips were also used. The use of differential energies allowed for differentiating strips and wires made of homogeneous silver and gold alloys from the ones coated with a thin gold layer. Analysis of EDX data revealed that the Au film thickness was very thin (10–60?nm). Formation of Ag₂S layers over the thin Au film as thick as 1.5?µm was taking place. Formation of thinner Ag₂S layers over non-gilt Ag suggested different reaction mechanisms for gilt and non-gilt cases.