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.     

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 occurrence of brochantite on archaeological bronzes: a case study from Lofkënd,Albania

A technical study was conducted on a group of copper alloy artifacts excavated from the burial tumulus of Lofkënd (fourteenth–ninth century BCE) to identify the alloy compositions and methods of manufacture. The surface corrosion was also examined in order to understand the diagenetic processes affecting the preservation of the finds and their condition. Portable X-ray fluorescence spectroscopy, metallographic examination, and X-ray powder diffraction (XRD) analysis were used to characterize the alloy composition and identify the corrosion products present. XRD analysis showed the presence of brochantite (Cu₄SO₄(OH)₆) on five of the metallic artifacts. Brochantite is not commonly reported on archaeological bronzes from terrestrial sites, but is more readily found on copper alloy objects exposed to sulfur pollutants in the air or soil. The possible conditions that could have led to the formation of this corrosion on the bronzes from Lofkënd are discussed in the context of the particular burial environment found at the tumulus.     

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.     

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.     

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.     

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.     

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.     

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.     

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.     

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.     

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.     

Action Mechanism of Sticky Rice-Paste-Modified Site Soil: A Traditional Chinese Cementitious Material

ABSTRACT

Sticky rice paste is a viscous material commonly used in traditional Chinese construction. It has the basic properties of high viscosity and viscoelasticity, good corrosion and weather resistance, and a small coefficient of expansion. Sticky rice paste is usually added to site soils to modify their basic physical and mechanical properties. This study found that gelatinized sticky rice-paste-modified site soils have clearly improved physical and mechanical properties compared to 20 and 100°C water-modified soils when 1% (wt%) sticky rice paste is added as the soil modifier. The liquid and plastic limits, and the plastic index of the soil are improved upon modification, to 27.4, 17.7, and 9.7%, respectively. The unconfined compressive strength and hydraulic conductivity were significantly increased and reduced, respectively. In addition, there was a reduction in the rate of disintegration by water, while the water resistance of the soil was improved. X-ray diffraction, Fourier transform infrared, and thermogravimetry/differential scanning calorimetry studies reveal that the improved properties arise through physical processes between the sticky rice paste and the soil. Scanning electron microscopy reveals that dispersed branched-chain sticky rice-paste molecules extensively infiltrate the surfaces of the soil particles on the nanoscale and become attached to them to form a good adhesive colloid that solidifies the soil particles after water evaporation, resulting in a compact structure.     

VERMILION AND CINNABAR

Abstract

A glow discharge in hydrogen gas can reduce silver tarnish back to silver. When a Daguerreotype is made an electrode of such a system, reduction takes only a few minutes. Many of the disadvantages of wet chemical methods are avoided.     

A Review of Lining Methods for Paper-based Photographic Prints from the Nineteenth and Twentieth Centuries

Abstract

A photograph may be lined when its physical condition is so poor that an additional support is necessary to ensure its survival. Photographic prints may comprise a number of layers, often composed of dissimilar materials that react differently to moisture. As most traditional lining methods involve the use of water, lining can thus cause stresses within and between the various layers. These stresses are thought to exhibit as increased cracking of binder layers and increased curling of the photograph itself. This paper reviews the literature to date regarding the lining of photographs and examines possible future treatment pathways. From the literature, it would appear that the question of how best to line traditional photographic formats, such as albumen, collodion and silver gelatin prints, has not yet been answered. Standard water-based lining adhesives can cause damage to various types of traditional photographs. Heat and solvent-activated adhesives can also cause damage; however, they may allow better control over the physical changes that occur within some types of photographs during treatment. Further investigation of non-traditional adhesives for paper (e.g. waxes and synthetic polymers) is recommended.     

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.     

New insights on the corrosion of ancient bronzes using X-ray powder diffraction: The importance of paratacamite,sampleite, and connellite

Research employing X-ray powder diffraction shows that clinoatacamite as the phase which should be predominant in the chloride-induced corrosion of ancient bronzes is not necessarily the dominant copper trihydroxychloride species at all, since mixtures are common, and other phases such as paratacamite or the dubious mineral phase anarakite have relevance to the corrosion products or patina of bronze antiquities. The potential reasons for this are discussed and related to the compositional parameters of ancient bronzes as well as the burial environment. Paratacamite is still identified, even in the absence of zinc, which is supposed to be the major stabilizing influence on the paratacamite lattice. X-ray power diffraction has been employed in the examination of a range of ancient bronzes from Chinese, Egyptian, Ecuadorian, Lydian, Albanian, Palestinian, and Greek contexts. This study has revealed that, far from being exotic novelties, the occurrence of both sampleite and connellite as important corrosion products of ancient bronzes has been underestimated in the published literature. Many occurrences of sampleite are related to the decomposition of contiguous human remains rather than to arid environmental factors which is the explanation for the occurrence in a geological environment, but in a burial environment other factors come into consideration.     

The Use of Moisture Barriers on Panel Paintings

Abstract

Deterioration of paintings is effected by chemical, physical and biological agents. Activity of these agents generates mechanical stresses which result in cracking, cupping, cleavage and flaking of the paint film. Internal mechanical stress appears to be primarily responsible for premature or drying cracks in paint. Stresses external to the paint cause age cracks which penetrate both paint and ground. The literature explaining the origin and characteristics of these cracks is reviewed. Although premature cracking can only be avoided by instructing the artist in proper use of materials, age or mechanical cracking can presently be prevented by environmental controls and careful handling. Present methods of reinforcement of paintings in general only partially stabilize them against environmentally generated stresses. Further research appears necessary for more complete understanding of the stresses old paint can withstand and of the forces exerted by the shrinking and swelling of textile and wooden supports. Search for and testing of more physically and chemically inert support materials are suggested.     

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.     

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.