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.     

CORROSION AND CONSERVATION OF A SILVER VISCERAL VESSEL FROM THE BEGINNING OF THE SEVENTEENTH CENTURY

Abstract

Two different cleaning methods were found necessary to remove silver chloride corrosion products from different areas of a silver canopic vessel containing about 6% of copper. Two possible mechanisms of silver corrosion which could have led to the differing silver chloride corrosion products are put forward.     

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.     

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.     

Formaldehyde Oxidation and Lead Corrosion

Abstract

Carbonyl compounds are of concern in museums because of their potential to damage artifacts. The mode of damage for aldehydes is far from clear and seems to require oxidation. Lead coupons were exposed to gaseous formaldehyde (methanal) and formic acid (methanoic acid) in various humid oxidizing atmospheres. The resulting corrosion products were weighed and identified using X-ray diffraction analysis after as much as 56 days exposure. When fomaldehyde was in more oxidizing environments (atmospheres containing hydrogen peroxide) it caused heavy corrosion but in the dark, with no oxidant, there was only very slight tarnish, even at high humidities. This suggests that oxidation is an important control on corrosion by formaldehyde. Thermodynamic calculations indicate that at low formic acid concentrations (below about 1 ppb) the corrosion products should be carbonates (plumbonacrite and hydrocerussite) while at higher concentrations lead formate would be expected. The mineralogical complexity and the importance of oxidants make it very difficult to set standards for formaldehyde in the museum environment.     

The Radiography of Panel Paintings by Electron Emission

Abstract

A straightforward technique for the restoration of metallic objects is described. With this technique the total immersion of the artifacts in the electrolytic cell is unnecessary. It can also be used for objects with localized corrosion.     

Characterization of the corrosion layer on iron archaeological artefacts from K2 (825–1220 AD), an archaeological site in South Africa

Abstract

A study of the composition and phase distribution of the corrosion layers on three ferrous objects, excavated at K2 (Bambandyanalo), an archaeological site in South Africa, was conducted. The objective of the study was to obtain information that can contribute to conservation procedures to be performed on the iron artefacts from this site. Examination of cross sections by means of energy-dispersive X-ray spectroscopy coupled to a scanning electron microscope (SEM–EDX), X-ray powder diffraction (XRD), and micro-Raman spectroscopy revealed the same corrosion composition and structure for all the objects under study, namely an internal layer adjacent to the metal surface with ghost inclusions and an external layer containing quartz grains. The study also revealed that the presence of magnetite (Fe₃O₄), maghemite (γFe₂O₃), and lepidocrocite (γ-FeOOH) within the internal layer is the only difference between the chemical compositions of iron corrosion products within the two layers. The results also made it possible to retrace the corrosion history during burial and long-term storage.     

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.     

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.     

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 removal of metal soaps from brass beads on a leather belt

Abstract

Severe corrosion was found covering the brass beads on several beaded belts. The corrosion was a mixture of copper soaps and zinc soaps, and was so thick in some places that it completely buried the brass beads. One of these belts was chosen as a test case for cleaning. The details of the cleaning are described, as are the effects of the corrosion process on the beads and surrounding organic materials. Two supporting studies are also reported: tests of the solubility of the corrosion in a variety of organic solvents, including mixtures of ethanol and petroleum-based solvents; and tests of leather dressings and other coatings on brass, copper, and zinc coupons in various relative humidities (RH), to determine what values of RH could be recommended for storage.     

Metal corrosion rates in aqueous treatments for waterlogged wood-metal composites

Abstract

Corrosion rates are presented for metals in the aqueous solutions intended for treating both the wood and the metal components of a waterlogged wood-metal composite object. The metals include lead, mild steel, cast iron and the copper alloys of bronze, cast brass and wrought brass. The corrosion rates were determined using a standard weight-loss method in which clean metal surfaces were immersed in the solutions for two months at room temperature. The treatment solutions included Acrysol® G-11O for lead, Witcamine® RAD 1100 for copper alloys and Pluracol® 824 or the corrosion inhibitor Hostacor® KS 1 in PEG 400 for iron. These treatment solutions were prepared in distilled water as well as in a synthetic seawater. For reference, metal corrosion rates were also determined in distilled water, synthetic seawater and PEG 400. The results demonstrate that, in general, clean metal surfaces corrode more slowly in the treatment solutions than in PEG 400 solutions. However, before recommending these solutions for the treatment of composite objects, further testing is needed to determine how effective they are in reducing the corrosion rates of metals covered with a thick corrosion crust, and to see if Acrysol G-110 is capable of bulking and consolidating waterlogged wood.     

Method for Stabilization of Leaded Bronzes Affected by Corrosion of Lead

Abstract

To inhibit the corrosion of lead objects and lead in high-lead bronzes, particularly in the presence of organic emanations from wood, paint, etc., and/or high humidity, is a difficult conservation problem. A method to prevent corrosion of lead in copper objects was therefore developed using benzotriazole (BTA). The most appropriate treatment method for the conservation of the objects was to immerse them in neutral BTA solution, for which a 0.5% (w/v) concentration of BTA was enough. For bigger objects, three or four coats of 1.0% solution applied by brushing were equally effective. The method was used successfully for the conservation of some ancient copper coins and bronze sculptures containing high amounts of lead; it can also be used to inhibit corrosion of lead objects.     

After Eighty Years: Experimental Study of Atmospheric Corrosion in the Metallic Dome of Hafez’s tomb,Shiraz, Iran

ABSTRACT

Hafez’s tomb was constructed on the grave of the well-known Iranian poet in Shiraz, in the 1930s. The dome of this monument is made of copper metal and has been exposed to the urban environment for 80 years. In this study, corrosion mechanisms and conservation conditions of the metallic dome of the Hafez’s tomb are investigated by analytical methods including optical microscopy, ICP-MS, SEM-EDX, and X-ray diffraction. The results showed that the dome is constructed from many copper sheets that are hammered to shape the dome. The corrosion products formed on the surface of copper sheets include copper oxide and copper trihydroxichlorides. In fact, the corrosion mechanism occurring in the metallic dome consists of copper oxidation and redeposition of copper II products because of reaction with air pollutants.     

Evaluation of sodium nitrite as a corrosion inhibitor for USS Monitor artifacts

Abstract

The use of sodium nitrite (NaNO₂) as a corrosion inhibitor for wet archeological metal objects presents potential advantages of near neutral pH, low concentration, effectiveness on several metals, and compatibility with organic materials. The effectiveness of NaNO₂ as a corrosion inhibitor for storage of chloride-containing marine archeological metal objects from the wreck of the USS Monitor was evaluated using marine-corroded carbon steel analogs. The samples were tested in varying concentrations of NaNO₂ and evaluated visually and by monitoring solution chemistry using ion chromatography (IC). It was found that a concentration of 1000 ppm NaNO₂, replaced four times, was effective at protecting corroded carbon steel in the presence of chlorides. Nitrite solutions were no more rapid than sodium hydroxide (NaOH) at extracting chlorides from marine steel at equal concentrations and were considerably slower than 2% NaOH. IC analyses indicated that NO₂ does not easily oxidize to NO₃ under normal conditions, but does so readily when a polarizing current is applied, making nitrites unsuitable for electrolytic reduction treatments. Sodium nitrite does show promise as a storage solution prior to desalination of marine metals or after desalination to prevent flash corrosion during rinsing baths.     

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.     

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.     

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.