Conservation and Restoration, [Metals Programme]
Amelia Hammond 13248960
University of Amsterdam, Amsterdam
Thesis Advisor: Tamar Davidowitz, MA, PDRes Second Reader: Sophie Glerum, MA
T r e a t e d C u p r e o u s W r e c k F i n d s f r o m t h e H o l l a n d i a
C o l l e c t i o n
M S c . T h e s i s
TABLE OF CONTENTS
ACKNOWLEDGMENTS ... 0
SUMMARY AND ABSTRACT ... 1
CHAPTERS 1. INTRODUCTION ... 2
2. HISTORICAL CONTEXT ... 5
2.1. The Hollandia Excavation... 5
2.2. The Conservation of the Hollandia collection ... 8
2.2.1. Mechanical Cleaning ... 9
2.2.2. Ultrasonic Cleaning ... 9
2.2.3. Desalination ... 10
2.2.4. Electrolytic Reduction... 11
2.2.5. Chemical Treatments ... 12
2.2.6. Wood Treatments ... 13
2.2.7. A Lack of Treatment ... 13
2.3. Understanding Past Treatments ... 14
2.4. Summary... 15
3. THEORETICAL CONTEXT ... 16
3.1. Past Approaches ... 16
3.2. Present Approach ... 17
3.3. Summary... 18
4. CASE STUDY OBJECTS ... 20
4.1. White Excrescences ... 22
4.2. Blue Excrescences ... 23
4.3. Pitting Corrosion ... 24
4.4. Summary... 25
5. EXPERIMENTAL DESIGN ... 26
5.1. Sample Collection ... 26
5.1. Optical Microscopy... 26
5.2. X-Ray Fluorescence Spectroscopy ... 27
5.3. Scanning Electron Microscopy ... 29
5.4. Fourier-Transform Infrared Spectroscopy... 29
6. RESULTS ... 31
6.1. White Excrescences ... 35
6.2. Blue Excrescences ... 36
6.3. Pitting Corrosion ... 37
7. DISCUSSION ... 38
7.1. Findings ... 38
7.1.1. Impact of Alloy ... 38
7.1.2. Impact of Composite Material ... 41
7.1.3. Markers of Burial Environment ... 41
7.1.4. Markers of Burial Environment ... 43
7.2. Further Results ... 45
7.3. Confounding Factors ... 46
7.4. Identification Tool ... 48
7.5. Identification Tool Interpretation ... 49
8. CONCLUSION ... 52
WORKS CITED ... 54
APPENDIX ... 57 All tables and figures by author unless otherwise stated.
I would like to thank the team of people who helped to make this research possible. First and foremost, my thesis supervisor, Tamar Davidowitz, MA PDRes (UVA/RMA) for her insight, academic support, and shared enthusiasm for this research.
In addition, I would like to offer many thanks to the following:
Dr. Tonny Beentjes (UVA), for sharing his great depth of knowledge of metals and corrosion over the course of the master’s program.
The team at the Rijksmusem Amsterdam, conservators Joosje van Bennekom, MSc for sharing her expertise, time, and space with me, Ellen van Bork, for her advice, and Sarah Crenage, MSc for her instruction on using the Hirox microscope. Also, to curator Jeroen ter Brugge, for sharing his research and insights on the importance of the collection.
Those who performed analysis of case study objects; Suzan de Groot MA for fourier transform infrared spectroscopy, Dr. Inneke Joosten (RCE) for scanning electron microscopy, Arie Pappot MA (RMA) for X-Ray fluorescence. Their analysis and interpretations of results were essential to this research.
My former colleagues at Texas A&M University’s Conservation Research Laboratory, particularly Dr. Chris Dostal, Karen Martindale, MA, and John Hamilton for sharing their extensive experience with treated cupreous wreck finds, and for fostering my interest in the field.
Dr. Christian Degrigny for his lectures on electrochemistry, discussion of wreck finds and their corrosion products, and instruction on the Micorr application.
My classmates - Terri Costello (UVA), for her peer review and sharing literature on submerged archaeological finds. Owen Ooievaar for sharing his research on microbially induced corrosion.
Maxx Folmer (UVA) for sharing his research and Mair Trueman (UVA) for her peer review and sharing her expertise and sources on waterlogged wood, and Kirby Martino (University of Amsterdam), for the shared study sessions that made this thesis possible. Paula Ogayar Oroz (UVA) for her peer review, and willingness to share conversations about our work. The rusters, for their genuine and infectious enthusiasm about all things metal. To Emma van Duin (UVA) and Rosemie Coppens (UVA) for their peer reviews, Dagmar Beyaard (UVA) for giving valuable advice on interpreting analytic results, and Meghan Parker (UVA) for always being willing to discuss our projects and ideas.
Xiao Yumi for her presence with me in all my endeavors.
My parents, and sister Jessica, for supporting me, and encouraging my academic interests.
Finally, Penelope, for being by my side.
The recovery of material from shipwrecks contributes significantly to a well-formed understanding of history, as these sites act as time capsules from the exact moment of their wreckage. The Hollandia was a Dutch East India Company (VOC) ship that sank off the coast of the United Kingdom, near the Scilly Isles in 1743 (Figure 1.1). Modern investigators of the wreck took extensive documentation of the ship, and there is a strong line of evidence that illustrates the excavation of the Hollandia wreck site. While the archaeological documentation is extensive, the conservation records are sparse. Information about the conservation treatments conducted on objects such as those collected from the Hollandia are vital, as cupreous objects in the Hollandia collection exhibit corrosion phenomena that have the potential to destroy the objects altogether if not treated. Without more information about the mechanisms that drive the formation of potentially destructive corrosion products, it is impossible to form an effective treatment regimen for the collection.
Due to the complexity of the development of corrosion on treated archaeological objects, it is challenging to understand their corrosion based on modern short-term studies. This brings the urgency of investigation into treated wreck finds to the forefront because as these objects age in collections they will likely require treatment, necessitating the development of identification and treatment protocols for them.
The analysis revealed correlations between corrosion appearance and the objects’ burial environments, previous treatments, and alloys. A visual identification protocol was created to assess corrosion phenomena via visual inspection within the Hollandia collection. Conservators can match the corrosion seen on objects in the collection to images of case study objects in the identification protocol concept map, which describes the elemental composition and, at times, specific components of corrosion products.
This tool was created for the conservators who will be working with the Hollandia collection, but it also lays the groundwork for a broader tool or publication that can utilize advanced analytic techniques to create visual identification tools for treated wreck find collections. A tool of this kind has the potential to be useful to other conservators and custodians of treated wreck finds, many of whom do not have extensive analytical tools at their disposal.
This thesis was designed to investigate the corrosion products present on previously treated cupreous wreck finds from the Rijksmuseum’s Hollandia collection. Through collaboration with the Rijksdienst voor het Cultureel Erfgoed, RCE, conservation scientists analyzed selected corrosion products. This analysis detected the elemental composition, and, at times, specific compounds present in corrosion products. The results of the study produced visual markers of corrosion products and chemical residue from previous conservation treatment. These visual markers were coalesced into an identification tool. Though this tool was developed for this collection, it has the potential for expansion.
KEYWORDS:copper corrosion, nautical archaeology, archaeometry, visual identification
CHAPTER 1 INTRODUCTION
For as long as ships have traversed waters, wreck sites have existed, and humans have been drawn to them. Goals ranged between the urge to salvage and to investigate, and it is only in the last thirty years that these actions have begun to separate. This was made possible with the development of underwater archaeology, which emerged as a field in the late 19th Century, moving from more speculative expeditions to a systematic discipline.1 Material heritage objects recovered from underwater environments have the potential to be extremely well preserved, acting as time capsules from the exact moment of their wreckage. They hold unique clues that can aid interpretations of the past, so the recovery of material from shipwrecks contributes significantly to a well-informed understanding of history.
The Hollandia was a Dutch East India Company (VOC) ship that wrecked off the Isles of Scilly in the waters off the United Kingdom in 1743 (Figure 1.1). This vessel was a rare example of one of the less than three percent of lost VOC ships that sailed from Europe to Asia, making the material recovered from this wreck invaluable to the field of nautical archaeology.2 The wreck was located in 1971 and excavated up until the early 1980’s.3 Some of the objects recovered from this excavation are now a part of the Rijksmuseum’s wreckage collection. The collection holds more than 4,500 inventory numbers, many of which contain between one and ten objects.4
Artifacts from the Hollandia itself account for more than half of these inventory numbers, with hundreds of cupreous objects at various stages of vital conservation treatment as the corrosion phenomena they present has the potential to destroy them completely over time.
Submerged copper alloys can present significant challenges for conservators, as they form aggressive, complex corrosion products as a result of interacting with seawater. These corrosion products can cause an object to disintegrate into nothing more than powder. The Hollandia objects were treated directly after excavation, but the objects and their treatment records have since become dissociated. The unknown treatments used mean that the mechanisms causing
1 Ian Oxley, “The Investigation of the Factors That Affect the Preservation of Underwater Archaeological Sites,” in Maritime Archaeology: A Reader of Substantive and Theoretical Contributions, ed. Lawrence Babits and Hans Van Tilburg, 1st ed., The Plenum Series in Underwater Archaeology, I (College Station, United States of America: Institute of Nautical Archaeology, 1998), 523.
2 Jerzy Gawronski, “Ships and Cities in Maritime Archaeology. The VOC-Ship Amsterdam and a Biographical Archaeology of Eighteenth-Century Amsterdam” (Symposium ter gelegenheid van het 75-jarig bestaan van de stichting Museum voor Anthropologie en Praehistorie in het kader van de zesendertigste kroon-voordracht., Amsterdam: Stichting Nederlands Museum voor Anthropologie en Praehistorie, 2014), 79–108.
3 Jerzy Gawronski, “Hollandia,” in Encyclopaedia of Underwater and Maritime Archaeology (London, United Kingdom: Yale University Press, 1998), 17.
4 Jeroen ter Brugge, “Wreck Finds in the Rijksmuseum: Valuation and Direction for the Future,” Discussed/established in departmental history meeting on 1 October 2018 (Amsterdam, The Netherlands: Rijksmuseum, September 2018). 1.
these objects to deteriorate are unknown, rendering identification of potentially harmful corrosion products impossible. The objects are awaiting further research before they can be conserved in the Rijksmuseum’s metals conservation studio, as an understanding of the corrosion phenomena is essential to the formulation of a treatment plan that addresses the causes of corrosion and prevents adverse outcomes as a product of the interaction between unknown substances and treatment solutions.5
Figure 1.1: Kannewet, Johannes. The warship named Hollandia, 1725-1780. Woodcut, colored in blue, yellow and orange;
letterpress text, 309 mm × w 412 mm. Rijksmuseum, Amsterdam. Credit: Image courtesy of the Rijksmuseum, Amsterdam.
There is extensive information on the historical and archaeological context of finds from the wreck site, however there are few associated treatment records. The treatment records that do exist were found in auction catalogues, handwritten notes, and magazine publications, but this information is not sufficient to form an understanding of the condition of each object. The corrosion products present on the surface of objects, along with their physical and chemical properties, therefore require investigation.
5 Personal communication by Tamar Davidowitz (Rijksmuseum conservator) in discussion with the author, March 2022.
Prior to this study, all cupreous objects from the Rijksmuseum’s wreck finds collection were reviewed to determine the extent and severity of corrosion phenomena. Study objects were selected that exhibit corrosion phenomena representative of other objects in this collection. The objective of this research is to investigate the question of what can be said about the condition of cupreous objects from the Rijksmuseum’s Hollandia collection based on an analysis of their corrosion products. In addition, subquestions that investigate the influence of the following factors on the condition of the objects will be considered:
• Do alloying elements influence the development of active corrosion in the Hollandia collection?
• Do composite materials influence the development of active corrosion in the Hollandia collection?
• Is it possible to distinguish chemical and visual markers of the burial environment of objects?
• Is it possible to distinguish chemical and visual markers of the previous treatment of objects?
In answering this research question through its subquestions, corrosion phenomena throughout the collection can be identified, facilitating the formulation of object treatment plans.
In addition to an analysis of the case study objects, a visual concept map was created to assist conservators in correlating corrosion phenomena throughout the collection to the analysis in this study.
The Hollandia is a prime candidate for a study of this kind both because it was excavated in the nascency of underwater archaeological conservation, but also because there are extensive archaeological records available on the burial environment of objects within the collection. This allows for an informed analysis of treatments on wreck finds fifty years after the first object was recovered.
It is impossible to replicate corrosion processes of wreck finds on test coupons. This can be attributed to the complex, slow-forming corrosion each object presents.6 This slow-forming corrosion, such as chloride contamination, can be so deep-set in archaeological materials that the same phenomena can be difficult to replicate on test coupons.7 Therefore, these objects offer a look into the real-time impact of conservation treatments on wreck finds.
This investigation was carried out in multiple stages. First, research was done into the historical and theoretical contexts that inform the study. This is summarized in the Historical Context and Theoretical Context chapters of the thesis. Information about the treatment of the Hollandia collection was limited to a few hints scattered throughout auction catalogues, scrap
6 David Scott, Copper and Bronze in Art: Corrosion, Colorants, Conservation (Los Angeles, United States of America: Getty Publications, 2002). 323.
7 Scott, 323.
paper, and popular publications, and so treatment information is buttressed by research into conservation conventions at the time of the wreck.
Then, the objects and the degree to which they represent specific corrosion issues are presented in the chapter Case Study Objects. The analyses utilized in the attempt to answer the research questions and subquestions, and their results, are presented in the following chapters:
Experimental Design, Results, and Discussion before the research and its findings are presented in a conclusion. This scientific investigation falls under the umbrella of archaeometry, which is defined as the application of analysis to answer archaeological research questions, relying on interdisciplinary cooperation between archeologists, conservators, and conservation scientists.8
In the case of the Hollandia collection of cupreous wreck finds, archaeometry was utilized to investigate the relationship between visual manifestation of corrosion products, which can assist conservators in forming an effective treatment protocol for the collection. Objects selected for this study and their corrosion products were studied through the use of X-ray fluorescence spectroscopy (XRF), scanning electron microscopy (SEM), energy dispersive spectroscopy (EDX), and fourier transform infrared spectroscopy (FTIR). X-ray fluorescence was conducted first due to the non-destructive and straightforward nature of this analytic technique. It helped to identify objects and areas to be studied further through SEM-EDX and FTIR analysis. The SEM-EDX analysis provided information on the elemental composition of study objects corrosion products, while FTIR conclusively identified some of their components.
The intention of this analysis was to reveal correlations between corrosion appearance and object burial environment, previous treatments, and alloys. Furthermore, there was an intention to create a visual identification tool to assess corrosion phenomena via visual inspection within the Hollandia collection, which may be applicable to other collections within and outside of the Rijksmuseum collection. Through this tool, conservators can match the corrosion they see on objects in the collection to images of case study objects. For this thesis, the identification tool developed is expressed through a flow chart, which describes the elemental composition and, where possible, specific components of corrosion products.
Many institutions hold similar collections of treated wreck finds that have been dissociated from their treatment records which, like the Hollandia collection, have the potential to disintegrate if left untreated. This brings the applicability of this project to a wider scope, as this identification protocol may be used by others with similar complex, yet undefined corrosion issues present in their collections. Through tools of this kind, the analysis done at large institutions such as the Rijksmuseum can help other conservators in identifying corrosion phenomena which may be destructive to their collection. Additionally, this tool can provide conservators with a peek into the future of the wreck finds being treated utilizing the same methodologies used to conserve the Hollandia collection.
8 Howell Edwards and Peter Vandenabeele, eds., “Preface,” in Analytical Archaeometry (The Royal Society of Chemistry, 2012), v.
CHAPTER II HISTORICAL CONTEXT
The Hollandia collection has a remarkable amount of associated archaeological records, which can be found in the Hollandia Compendium and the Rijksmuseum archives. Though most of this material does not directly discuss the objects and their treatments, it holds clues as to their original burial context and some aspects of their handling. Conservation treatment records themselves are sparse, and include brief treatment cards, a few comments on auction catalogues, popular magazine articles, correspondence, and a scrap of paper of unspecified authorship and origin. Further, information recovered from sparse notes available in the archives of the Rijksmuseum is insufficient to form an understanding of the treatments carried out on the Hollandia collection. In this chapter, the scarce treatment records on the Hollandia is bolstered by the standard operating procedure of the conservation of other wreck finds that were treated contemporaneously to the time of the Hollandia excavation.
In the years between the first 1968 survey of the site and the conclusion of excavation in the early 1980s, there was an explosion in research on the conservation of archaeological metals. The archival documentation of the conservation of the Hollandia is supplemented by publications on wreck finds in the same years as the excavation in Western Europe. These sources form the bulk of the established knowledge of treatments that were potentially carried out on the Hollandia collection.
2.1. The Hollandia Excavation
The Hollandia Compendium is the largest publication on the wreck, and describes the archaeological findings made at the site. Archival research was done at the Rijksmuseum to supplement this text with information found in daily logs taken by divers, clippings from newspapers, popular magazine articles, and letters written by the original director of the excavation. From these sources an understanding was formed of the history of the ship, its seabed environment, and the objects which make up the Hollandia collection. Understanding these factors is essential to direct and subsequently interpret the results of the archaeometric investigation.
The excavation initiated in 1971 was not the first attempt to recover the treasure of the Hollandia, it has been sought after since it first came to rest on the bottom of the ocean. Gerard Bolwerk an agent of the VOC, who worked with English diver John Lethbridge, made a salvage attempt on the wreck site in the 18th Century using diving apparatus consisting of a reinforced, weighted barrel with a glass window and leather gloves that allowed the diver to reach forth from the contraption, but it could not delve deep enough to reach the wreck (Figure 2.1.1).9 The
9 The Department of Dutch History of the Rijksmuseum, Prijs Der Zee: Vondsten Uit Wrakken van OostIndievaarders (Amsterdam, The Netherlands: Rijksmuseum, 1980), 4.
actual excavation of the Hollandia shipwreck was only made possible through the development of the SCUBA technology hundreds of years later. Excavation of the wreck began in 1971 and continued throughout the eighties, with some intermittent work in the early 1990s.10 This excavation was initiated in the nascent years of nautical archaeology, a field that only emerged after the commercialization of the sale of underwater diving apparatus. This period produced unique conservation treatments because there were no legal or institutional standards for the excavation and conservation of archaeological material from submerged sites.
In the early years of recovery of material from submerged sites, treasure hunting was a practice that often ran parallel to and was closely intertwined with underwater archaeology. The treasure hunting conducted on wrecks such as the Hollandia and Amsterdam in the Scilly Isles in part precipitated the Protection of Wrecks act in the United Kingdom.11 Some archaeologists have criticized the excavation of the Hollandia, saying that the lack of a methodological approach to the expedition clouded excavation records.12While this is true, it should also be
10 Gawronski, “Hollandia,” 17.
11 Matthew Harpster, “Shipwreck Identity, Methodology, and Nautical Archaeology,” Journal of Archaeological Method and Theory 20, no. 4 (2013): 588.
12 Harpster, 588.
Figure 2.1.1. Amery, John. Lethbridge in his diving barrel. 1880. In John Amery, John Lethbridge and his Diving Machine, 524.
Devonshire: Trans. Devonshire Association, 1880.
noted that the documentation of the wreck was extensive, including detailed diving logs, maps, and frequent correspondence. At the same time the most extensive treatment information on objects prior to acquisition by the Rijksmuseum is a few lines on a scrap piece of paper.
Rijksmuseum curator Jeroen ter Brugge expanded on the research of Garownski by compiling much of the research done on the Hollandia collection into an unpublished document titled
“Wreck finds in the Rijksmuseum: Valuation and direction for the future.” Despite this extensive research, to date there has not been an analytical study that takes an in-depth look at the effect of previous treatments on the collection.
There was, however, extensive documentation on the distribution of wreck finds on the seabed. At a depth of 30 meters, historic material on the site is distributed between three areas , likely lost as the vessel descended.13 When the vessel sank, archaeological records indicate that it drifted for a time, losing material from the lower portion of the hull.14 This material from the ship’s hull constitutes the “south site” (Figure 2.1.2.). The bulk of the ship and its contents settled on the “main site” and “north site.” In letters sent by Rex Cowan, the director of the expedition, it was noted that the seabed environment differed between these three areas.
Notably, Cowan mentions the material recovered from the south site was in “fine condition, often [exhibiting] less damage or corrosion than the material recovered on the main site.”.15
Figure 2.1.2. Hoekstra, A.F. 2018. Edited by author, based on Cowan 1975, Kist & Gawronski 1981, and original survey data by R. Graham and T. J. Hiron. The Hollandia wreck site. Rijksmuseum, Amsterdam. In Gawronski, Jerzy Hollandia, in Encyclopaedia of Underwater and Maritime Archaeology (London, United Kingdom: Yale University Press, 1998), 17.
13 Gawronski, “Hollandia,” 259.
14 Gawronski, “Hollandia.” 259.
15 Rex Cowan to Bas Kist, February 11, 1976.
South site Main site North site
2.2. The Conservation of the Hollandia Collection
Dispatching corrosion has been described as being analogous to grappling with a multi- headed beast, with new heads sprouting as others are defeated.16 This rings true in the case of cupreous objects treated in the early years of underwater archaeology. Within this field, there are many factors that influence corrosion processes, and these factors can be mysterious owing to a lack of documentation for conservation treatments from that period.17 Within the Hollandia collection, the sparse treatment documentation available is insufficient to understand the corrosion phenomena present on the surface of objects. The only official treatment records of the Hollandia collection are single-sheet treatment cards, which often have one or two words to describe previous treatments.
The few conservation records of objects recovered from the Hollandia excavation may be expanded on as a result of ongoing research on wreck finds from the site. Though incomplete, this information provides a partial understanding of the treatments that may have been conducted on objects selected for this study. Treatments described in the documentation associated with Hollandia objects seem to have been conducted in two stages: treatments done in England prior to Rijksmuseum acquisition, and treatments likely done afterwards by the Central Research Laboratory (now the RCE) in The Netherlands (Table 2.2.1.).
Table 2.2.1. Documented Treatments of Copper Alloy and Copper Alloy Composite Objects Within the Hollandia Collection
Desalination in demineralized water
Electrolytic reduction in sodium carbonate and/or sodium hydroxide Ultrasonic treatment in solvent
Benzotriazole Phosphoric Acid
Table 2.2.1. Documented Treatments of Copper Alloy and Copper Alloy Composite Objects Within the Hollandia Collection based on archival research.
Much of the documentation of treatments conducted on Hollandia objects prior to their acquisition by the Rijksmuseum was recovered from a single handwritten note that briefly outlined the treatments. Additionally, some treatment cards exist for most objects that were treated once in the collection. Based on the dates on these cards, and their Dutch headings, it is
16 Scott, Copper and Bronze in Art. 353.
17 Scott, 353.
likely that this treatment was conducted by the Central Research Laboratory. Descriptions are brief and include mention of benzotriazole and phosphoric acid treatments by name, though there are some cards that say, “Treated in England,” with no further details.
In the following pages, the details of each of these treatments in the archives will be discussed and elaborated upon using literature contemporary to the excavation. Some treatments described in the documentation are not well researched in the field of conservation, such as the treatment of marine finds using ultrasonic machines.18 Other treatments, such as desalination, electrolytic reduction, and corrosion inhibition are well represented in the literature. It is also notable that those who conserved the objects in England had the goal of keeping the “natural toning” of the objects intact, so there may have been additional unlisted treatments in service of that aim. 19t
2.2.1. Mechanical Cleaning
In the available literature, there was no mention of mechanical cleaning of objects selected for this study. Though no records of mechanical cleaning are present in the Rijksmuseum archive, it is mentioned here because mechanical cleaning is often the first step of a conservation treatment. Therefore, mechanical cleaning is likely to have been carried out on Hollandia objects.
In the treatment of similar cupreous wreck finds contemporary to the Hollandia excavation, small finds were recorded as being cleaned both mechanically and chemically.20 Throughout the history of conservation, mechanical cleaning has been carried out through the use of brushes, metallic instruments, abrasive particles, pneumatic chisels, and many unique improvised tools. While careful mechanical cleaning by a trained hand can help to remove aggressive corrosion products, it also, at best, irreversibly alters the surface of an object.21 At worst, mechanical cleaning can result in harming the metallic surface of an object, or completely destroying it.
2.2.2. Ultrasonic Cleaning
The W.H. Lane & Son auction house held multiple auctions that included objects from the Hollandia shipwreck. It notes that Cowan held exhibitions in concert with W. H. Lane in September 1972. The catalogue notes that cleaning of the coins was undertaken by Industrial Instruments Ltd of Bromley, Kent, United Kingdom. It is noted that silver coins were cleaned primarily through the use of an ultrasonic tank “combined with the use of solvents.”
18 Leo Caldararo, “Some Effects of the Use of Ultrasonic Devices in Conservation and the Question of Standards for Cleaning Objects,” North American Archaeologist 14, no. 4 (n.d.): 289–303.
19 Rex Cowan to Bas Kist, February 11, 1976.
20 Gawronski, “Ships and Cities,” 79.
21 Scott, Copper and Bronze in Art, 359.
This method was still in use in some laboratories between 1960 and 2000.22 In past conservation treatments ultrasonic tanks were utilized to mechanically remove corrosion products owing to the impression that removing visible corrosion would put a stop to harmful corrosion processes. In reality, this technique does nothing to initiate processes which would have any impact on chloride concentration in marine archaeological objects.23
This treatment is not often mentioned in the literature pertaining to the conservation of treated wreck finds because it does not derail harmful corrosion processes and can cause harm to the artifact itself. The vibrations produced by an ultrasonic tank can cause irreversible mechanical damage to brittle archaeological objects. At the same time, the technique does nothing to address chloride concentration within an artifact. The “special solvent” mentioned in correspondence may have initiated additional processes, though this solvent is unspecified. If chloride-contaminated objects were treated exclusively through the ultrasonic technique it is unlikely that any stabilization occurred, and corrosion is bound to appear after treatment.
In the Rijksmuseum archives there is a single, three-word line pertaining to the conservation of objects through desalination. The note mentions, “desalination with demineralized water,” with no further specifications. The aim of desalination treatments is to use solvation and diffusion to remove chlorides from objects. The alkaline solution provides a large amount of hydroxide (OH−) ions, which outnumber chloride (Cl−) ions, freeing them from their charge balancing role, along with replacing Cl− ions, which are adsorbed onto corrosion products. Alkaline solutions are often used in desalination, as a high pH helps to passivate metal.24 Desalination is a widely researched treatment when it comes to wreck finds. However, this treatment is rarely conducted through the use of only demineralized water. When this method is documented, it is reported to be impractical due to its time-consuming and potentially damaging nature.
Desalination is written of in conservation literature as early as 1882, with an early recognition of the importance of chloride removal from archaeological metals. In 1905, Rathgen published The Preservation of Antiquities: A Handbook for Curators which features a section on aqueous solvation techniques, called “washing methods” at the time.25 By the time Rathgen and Rosenberg published on desalination in 1917, they were aware that alkaline desalination is an effective way to remove chlorides as opposed to pure aqueous solutions such as the one mentioned in Hollandia documentation.26
22 Caldararo, “Ultrasonic Devices,” 299.
23 R Spier, “Ultrasonic Cleaning of Artefacts: A Preliminary Consideration,” American Antiquities 16, no. 3 (1961): 410.
24 Watkinson and Lewis, “Desiccated Storage of Chloride Contaminated Iron: A Study of the Effects of Loss of Environmental Control,” 279-289.
25 Friederich Rathgen, The Preservation of Antiquities: A Handbook for Curators, 1st ed. (Cambridge, United Kingdom:
Cambridge University Press, 1905). 143.
26 Ian Macleod, “Conservation of Corroded Copper Alloys: A Comparison of New and Traditional Methods of Removing Chloride Ions.,” Studies in Conservation 32, no. 25 (1987): 25–40.
The recorded treatment for these objects, however, lists desalination as being done using demineralized water.27 There is little record of cupreous desalination of wreck finds using water instead of alkaline solutions, and those that have been published did not effectively remove chloride ions. Macleod found that the mechanisms that drive simple aqueous desalination are very time consuming, and treatment may last between two and four years, likely initiating new corrosion in the process.28 This indicates that, if desalination treatments were in fact conducted, they were likely insufficient to encourage the migration of chloride ions from the metal due to a lack of time.
2.2.4. Electrolytic Reduction
In the treatments recorded for the Hollandia shipwreck, electrolysis was reported to be done in England through the use of sodium hydroxide and sodium carbonate with a current density of 69/41 Kv. Electrolytic reduction is, in essence, a chemical reaction as a result of an electric current going through an electrochemical cell, in which the artifact is the cathode (Figure 188.8.131.52).29
The setup for an electrolytic treatment begins with a treatment vat. This vat is filled with electrolyte, often alkaline in nature.30 Anodic material is then attached to the positively charged terminal of the power supply and placed either around or on either side of the object being treated. The current from the external power supply causes negatively charged chloride ions to travel towards the anode, away from the object.31 A series of journal articles were released
27 Pers.comm – handwritten note
28 Macleod, “Conservation of Corroded Copper Alloys: A Comparison of New and Traditional Methods of Removing Chloride Ions.” 1.
29 Selwyn, Metals and Corrosion. 2004. 17.
30 Macleod, “Shipwrecks and applied electrochemistry,” 291-303.
31 Macleod, “Shipwrecks and applied electrochemistry,” 291-303.
Figure 184.108.40.206. Electrolytic Reduction setup. Image source: Hamilton, Methods for Conserving Archaeological Material from Underwater Sites, 1998.
between 1970 and 1990, studying the impact of pH, time, electrolyte, and temperature in alkaline cleaning methodologies. As early as 1956, some of the dangers of aggressive electrolytic treatments were being discussed.32
If this treatment was utilized, it is one of the only documented treatments that is effective in chloride removal. However, while electrolytic reduction can be effective it is difficult to control and can indiscriminately remove corrosion layers, exposing a reactive metallic core.33 Additionally, if certain electrolytes are used there can be a transformation of the natural patina and complete removal of associated information through the destruction of associated corrosion.
2.2.5. Chemical Treatments
The handwritten note which acts as the source for much of the treatment records for the wreck mentions two chemical treatments, namely the use of phosphoric acid and benzotriazole.
The phosphoric acid treatment simply lists “phosphoric acid,” without going into further details.
The benzotriazole (BTA) treatment is said to have been conducted in an alcohol/benzotriazole bath for the treatment of every corroded copper lead and bronze object, though no further details such as concentration or treatment duration are mentioned.
Phosphoric acid treatment has been conducted on iron in the past but is rarely mentioned in the literature as a treatment for cupreous objects. It was one component of a complex electrochemical treatment. This treatment, described by Evans (1951), consisted of dipping a zinc sheet in hydrochloric acid, phosphoric acid, and sodium carbonate before touching it to corrosion products, producing a measurable current.34
In contrast to phosphoric acid treatments, BTA has been well-researched, and continues to be widely used within the field. BTA was introduced to the field of conservation in 1968 by Masden and has since been discussed at length, notably by Sease in a 1987 publication.35 It has been noted that a solution of BTA in ethanol alone can act as a desalination treatment, though this treatment is not well understood and often fails if conducted without prior treatment.36
Phosphoric acid and benzotriazole function in very different ways. Phosphoric acid alone does nothing to stabilize chloride contamination of cupreous wreck finds. On the other hand, benzotriazole alone can form a complex with copper and copper chloridions. Though this does provide a degree of stabilization, it is often done in combination with another treatment that encourages the migration of chloride ions from artifacts as, when used alone, it is rarely successful in preventing active corrosion.
32 Harold Plenderleith, The Conservation of Antiquities and Works of Art (Oxford, England: Oxford University Press, 1956).
33 Watkinson and Lewis, “Desiccated Storage of Chloride Contaminated Iron: A Study of the Effects of Loss of Environmental Control,” 279-289.
34 U Evans, “The Corrosion Situation: Past, Present and Future,” Chemistry and Industry 70 (1951): 109.
35 Catherine Sease, “Benzotriazole: A Review for Conservators,” Studies in Conservation 23, no. 2 (1978): 76–85, https://doi.org/10.1179/sic.1978.011.
36 Macleod, “Conservation of Corroded Copper Alloys: A Comparison of New and Traditional Methods of Removing Chloride Ions.” 1.
2.2.6. Treatment of Wood Composite Objects
The conservation of cupreous/wood composite objects is complex, as what helps one material can harm another. In general, wood treatments are often acidic and therefore corrosive to metals, while the treatment of copper alloys is often alkaline in nature which can initiate wood degradation.37 Polyethylene glycol treatment for wooden and wood composite objects is included in this summary, as some of the objects in the Hollandia collection are copper/wood composites, and treatment may have influenced the formation of corrosion on the surfaces of objects. Polyethylene glycol is the only recorded treatment for wood and composite Hollandia finds, though it is only mentioned in one line of a popular magazine article in which Cowan is quoted, though the clipping in the archive did not include the name of the publication.
There are no records that indicate which objects were conserved through the use of polyethylene glycol, though it is the author’s experience that there was a characteristic odor of polyethylene glycol in drawers which housed sword grips. When waterlogged wood is excavated from underwater sites, it has the potential to warp unless it undergoes conservation treatment.38 Some of the sword grips appear to have become warped, while others have retained their original rounded shape, although it is not certain that this is indicative of treatment. In treatments of waterlogged wood, a conservator may use a method referred to as bulking or impregnation. This is carried out through the use of materials such as polyethylene glycol to replace the water in the wood and so act as a structural reinforcement.39
The nature of wood composite objects is corrosive to metallic objects even without additional treatment. As wood deteriorates, formic and acetic acids can create an acidic environment for associated metals. This can be exacerbated by a well-meaning conservation treatment that aims to stabilize wood components. Polyethylene glycol is mildly acidic when oxidized, with a pH of 4.5-7, so, if it was used, it potentially initiated more extensive corrosion of cupreous components of the composite objects of the Hollandia collection.
2.2.7. A lack of treatment
In addition to the previously described treatments, there are some objects covered in a shell of corrosion, sediment, and marine organisms, which is called a concretion. Concreted objects within the Hollandia collection that seem to have been only partially cleaned or left untouched altogether. The decision of how extensively to clean an object was discussed during the excavation, with Jedrzejewska’s 1976 publication arguing for leaving at least part of a cupreous find untreated in order to accurately record its condition upon excavation.40 While this is a valid treatment for many stable bronzes, it can also cause problems in both copper alloy and
37 Lyndsie Selwyn, D. Rennie-Bisaillion, and N. Binnie, “Metal Corrosion Rates in Aqueous Treatments for Waterlogged Wood- Metal Composites,” Studies in Conservation 38, no. 3 (1993): 180.
38 Donny Hamilton, Methods for Conserving Archaeological Material from Underwater Sites (College Station, United States of America: Center for Maritime Archaeology and Conservation Texas A&M University, 1998), 86.
40 Hanna Jedrejewska, “A Corroded Egyptian Bronze: Cleaning and Discoveries,” Studies in Conservation 21, no. 3 (1976):
copper alloy composites with components that are unstable without treatment. It is unknown whether or not the concreted or semi-concreted objects were treated or left to corrode unaided.
2.3. Understanding Past Treatments
Even seemingly benign treatments on archaeological copper alloys recovered from a marine environment can have adverse effects. Treatment materials used before the turn of the 20th Century trended towards more extreme in basic or acidic nature.41 The residues of these solutions in combination with relative humidity can create corrosion issues. For example, when submerged in a sodium carbonate or sodium sesquicarbonate solution, there is a transformation of the natural patina (for example the formation of tenorite (CuO) or chalconatronite (Na2Cu(CuO3)2.3H2O)) and the development of active corrosion.42
In the fifty years since the beginning of the excavation, adverse effects such as the development of active corrosion resulting from treatment have been widely studied by researchers such as Ian McLeod, David Scott, Lyndsie Selwyn, and Christian Degrigny among many others. Their research provides the basis of the scientific knowledge on cupreous corrosion mechanisms for this thesis, particularly in objects recovered from saline underwater environments. Current understanding of the long-term impact of conservation treatments on marine archaeological copper alloys is limited. Because the first official excavations and conservation treatments of archaeological copper objects were only carried out in the late 20th Century, conservators are only now able to assess the real-time impact of these conservation treatments.
The impact of treatments discussed in this thesis include wood treatments, desalination, electrolytic reduction, and corrosion inhibition. MacLeod at the Western Australian Museum has also done some significant work on the mechanisms that drive treatments utilized on the Hollandia collection, with significant publications on desalination and electrolytic reduction.
Dr. David Scott discussed at length in his 2002 publication the treatments done on bronzes in the past, with a chapter segment dedicated to treated marine finds.43 It should be noted that terminology for the corrosion phenomena observed on wreck finds is varied, and the terms excrescence, efflorescence, and bloom are used interchangeably. For the purposes of this study, excrescence will be used.
Studies that investigate the influence of treatments on cupreous archaeological material have been undertaken using test coupons. One particularly in-depth study was undertaken by Karen Leyssens at the University of Gent. She used test coupons to monitor the conservation of corroded cupreous artifacts using electrochemistry and synchrotron radiation based spectroelectrochemistry. While the work of these figures has laid the groundwork for this
41 Scott, Copper and Bronze in Art. 352.
42 Karen Leyssens, “Monitoring the Conservation Treatment of Corroded Cupreous Artefacts: The Use of Electrochemistry and Synchrotron Radiation Based Spectroelectrochemistry.” (Dissertation, Ghent, Belgium, University of Ghent, n.d.).
43 Scott, Copper and Bronze in Art, 369.
research, there has yet to be a study that translates the complex results of analysis into a visual tool that focuses on previously treated wreck finds.
SCUBA technology made underwater archaeological investigation more accessible to the public, resulting in an increase in wreck site disturbance. The excavation of the Hollandia shipwreck was exceptional in that it took place directly before the publishing of regulations for the excavation and conservation of archaeological material from submerged sites. The Hollandia Compendium and the Rijksmuseum archives include an extraordinary number of archaeological records related to the Hollandia collection. The majority of the established knowledge of treatments that were potentially applied to the Hollandia collection comes from publications on wreck findings discovered in Western Europe contemporaneous to the Hollandia. The standards for the conservation of other wreck finds at the time of the excavation is used in this chapter to support the scant treatment records on the Hollandia. The Hollandia Compendium, the most comprehensive book on the wreck, details the archaeological discoveries uncovered there.
It is not possible to interpret the treatments applied to the Hollandia collection using the information gleaned from scant notes kept in the Rijksmuseum archives. There is, however, some treatment information to be found in auction catalogues, news articles, letters, and handwritten notes. The treatments recorded for copper alloys in these documents include desalination, electrolytic reduction, chemical treatment, and corrosion inhibition. Wood composite objects may have also been through conservation treatment, recorded as being polyethylene glycol. Despite the plentiful documentation on the wreck, there are still aspects of their treatments that are obscured.
The treatments carried out in England versus the treatments carried out in the Netherlands create a dichotomy between extreme conservation treatments such as ultrasonic cleaning and the treatment of wreck finds exclusively through the use of BTA.
While the work of these figures has laid the groundwork for this research, there has yet to be a study that translates the complex results of analysis into a visual tool that focuses on previously treated wreck finds.
CHAPTER 3 THEORETICAL CONTEXT
Explicitly discussing these theoretical frameworks is essential to form an understanding of the decision-making process that has guided and continues to guide conservators of wreck finds. This understanding provides context for why the materials were handled differently than they would be today. This section outlines the underlying assumptions that constitute the theoretical frameworks that guided both initial handling of Hollandia objects as well as this research. The ideas behind certain frameworks were the basis upon which the identification tool that resulted from this investigation was built.
The conservation of wreck finds is a distinct subfield, related yet different from conservation as a whole. Challenges faced by conservators working within this subfield include on-site work done in the field and some heavily degraded materials, which changes the decision- making process.44 An exploration of the theoretical frameworks through which these objects have been studied is vital because there is a significant gap between material culture and conclusions that can be drawn about the past. These conclusions are informed not only by the material itself, but also by the theoretical lenses through which objects are seen. An investigation of both the material and the context from which it was recovered is imperative to understanding the condition of a collection, and to create the theoretical framework for this study.
3.1. Past Approaches
Many of the ideas that form the threshold of the field of underwater archaeology are rooted in conservation theory and archaeological theory, both terrestrial and underwater. When the Hollandia excavation was initiated in the 1970’s, it followed on the heels of a new framework within conservation known as ‘scientific conservation’ which was recognized between 1930 and 1950 with regards to architectural features.45 The ideas that underpinned scientific conservation relied on an integration of hard sciences into conservation practice but were somewhat loosely defined, missing a theoretical body of literature.46 By the mid-twentieth century, this framework began to spread to those who handled archaeological material.47 This is reflected in the treatment of wreck finds from the Hollandia shipwreck, as the hard sciences clearly had some involvement in the formation of a treatment protocol, but were not integrated so much so that extensive treatment protocols were recorded.
During the same period within archaeological theory, there was a focus on collecting nautical archaeological material. This was conducted without much emphasis on the materiality
44 Elizabeth Pye, “Archaeological Conservation: Scientific Practice or Social Process?,” in Conservation: Principles, Dilemmas and Uncomfortable Truths, 1st ed. (London, England: Routledge, 2009), 12.
45 Salvador Muños Viñas, Contemporary Theory of Conservation (Oxford, England: Elsevier, 2005), 69.
46 Muños Viñas, 78.
47 Muños Viñas, 74.
and long-term stability of artifacts. There is an argument, however, that there were many lively, yet unpublished theoretical discussions at the time.48 Theoretical approaches within nautical archaeology were modeled after existing theoretical approaches in the field of terrestrial archaeology. Within the field of terrestrial archaeology, a paradigm shift was first called for at the 1963 Society for American Archaeology meeting in Boulder, Colorado. This was a result of an ongoing debate between the cultural history and processual approaches to archaeological theory.49 This shift gave way to what was termed “new archaeology,” a theoretical framework that proposed a more scientific approach to interpreting and preserving archaeological material.50
Both the fields of underwater archaeology and that of archaeological ethics were in their nascency at the time of the Hollandia excavation. By the early 1980s, there was a growing call from maritime archaeologists to put an end to treasure hunting and excavations that sought out artefacts to sell as souvenirs for profit.51 It would not be until 1986 that the first session focusing on the ethics of nautical archaeology was held during the 14th Conference on Underwater Archaeology. Within the field of nautical archaeology, the critiques from figures such as George Bass were multifold: there was not enough scholarship produced from these excavations, and that there was little curiosity on the part of the excavators.52 Daniel Lenihan and Larry Murphy published on the issue of underwater archaeology lacking explicit research design and showing little legitimate scientific interest.53 Legislation sprung from these discussions, codifying the theoretical discussions of the time. It has been stated that the Protection of Wrecks act in the United Kingdom was precipitated in part by the treasure hunting conducted on wrecks including the Hollandia, Amsterdam, and other shipwrecks around the Scilly Isles. 54
3.2. Present Approach
Today, there are many publications on the theoretical interpretation and conservation of objects from a wide range of burial environments. In an analysis of thirty-six years’ worth of articles from the International Journal of Nautical Archaeology, Matthew Harper made an effort to explicate the theoretical grounding of these excavations in order to better understand the discipline. He concluded that the field of nautical archaeology may experience the same introspective debate over methods and theoretical framework that existed within terrestrial archaeology during the shift from processual archaeology to new archaeology.55 It could be
48 George Bass, Promise of Underwater Archaeology in Retrospect (College Station, United States of America: The Institute of Nautical Archaeology, 1983).
49 Harpster, “Shipwreck Identity,” 588.
50 Oxley, “Preservation of Underwater Archaeological Sites.” 16.
51 Bass, Promise of Underwater Archaeology, 205.
52 Bass, Promise of Underwater Archaeology, 205.
53 Lawrence Babits and Hans Van Tilburg, Maritime Archaeology: A Reader of Substantive and Theoretical Contributions, 1st ed., The Plenum Series in Underwater Archaeology, I (College Station, United States of America: Institute of Nautical Archaeology, 1998). v.
54 Gawronski, “Hollandia.”17.
55 Harpster, “Shipwreck Identity,” 617.
said, in contrast to that idea, that this introspective debate was initiated in the same decade as the Hollandia excavation. Furthermore, this debate was precipitated in part due to the treasure hunting that was conducted on the site, which initiated some changes within the field of underwater archaeology.
The role of the conservator has changed significantly since the time of these early excavations, as a result of many governmental bodies adopting legislation to prevent the damage done to objects such as those from the Hollandia collection. These preventive measures include a shift towards non-invasive archaeological investigations and greater involvement of conservators prior to the recovery of objects. In some areas, it is legally required to budget for adequate conservation treatment before divers even enter the water.56
Ideas from the theoretical frameworks of conservation and archaeological theory were drawn upon to design the research described in this thesis. There is an acknowledgement of the importance of the “scientific conservation” view that analysis is vital in better understanding, and therefore conserving, objects. The following research tests the hypothesis formulated to match the research question outlined in chapter 1: it is possible to make a connection between the physical manifestation of corrosion products on cupreous wreck finds with analytic markers of treatment materials and seabed environment. It also relies on the author’s experience, and with the conventional knowledge within the field that there is little money or time for these types of analyses to be carried out on most collections. Therefore, the visual concept map was developed, both to communicate the findings of this thesis concisely, and to make them more accessible to a range of object custodians who may observe similar corrosion phenomena within their collections.
For a clear understanding of the decision-making process that has influenced and continues to influence conservators of wreck findings, it is imperative to explicitly discuss the theoretical frameworks through which the work is studied. This comprehension puts the handling of the materials in context and explains why it differed from how it would be done today. There is a huge gap between material culture and the inferences drawn from observing them, thus it is crucial to investigate the theoretical frameworks through which these things have been investigated. These judgments are informed by both the content itself and the theoretical frameworks that we use.
The foundational concepts of the field of underwater archaeology are heavily influenced by terrestrial and underwater archaeological theory as well as conservation theory. The concepts that supported scientific conservation relied on the incorporation of hard sciences into conservation practice, but they lacked a theoretical body of literature and were very vaguely defined.
There was a concentration on gathering nautical archaeological evidence throughout the same time period within archaeological theory. The materiality and long-term stability of the
56 Oxley, “Preservation of Underwater Archaeological Sites,” 521.
objects were not given much consideration during this. However, there is a claim that there were a lot of interesting, unpublished theoretical talks at the time. The research project presented in this thesis was conceptualized using concepts from the theoretical frameworks of conservation and archaeological theory. The "scientific conservation" perspective, according to which analysis is essential for a better understanding of items and their conservation, is a significant factor in this study.
CHAPTER 4 CASE STUDY OBJECTS
There are thousands of metallic objects in the Rijksmuseum’s Hollandia collection, in varying states of corrosion. Cupreous corrosion can contribute to the story an archaeological object tells by holding clues about its alloy, burial environment, or treatment. At the same time, it can obscure surface details and be destructive to objects.57 Visual inspection through optical microscopy was the determining factor for the selection of objects for this study. The inspection included a look at corrosion which could be defined as “active,” which in this thesis refers to corrosion that is ongoing when stored in a controlled environment.
These issues generally fall into the following categories: white excrescences, blue excrescences, or pitting corrosion. Pitting corrosion is a driving factor in the development of bronze disease, while excrescences have the potential to induce corrosion as they can form an electrolyte which initiates galvanic corrosion.58 Cupreous objects in the Hollandia collection exhibit several conservation concerns including corrosion or excrescences that are both visually disturbing and potentially harmful to the objects. There are many objects within these categories of disrepair, but there were limited analyses available, so representative objects were chosen as case studies for this project.
Sword hilts59, munition sideplates60, grenadier furnishings61, and buckles62 were selected to represent the collection, as they all have three or more nearly identical objects that
57 Lyndsie Selwyn, Metals and Corrosion: A Handbook for the Conservation Professional. 2004. (Ottawa, Canada: Canadian Conservation Institute, 2004). 16.
58 Selwyn. 16.
59 The cashbook of the Amsterdam Chamber of the Dutch East India Company lists a number of deliveries of bladed weapons in 1742.59 The three sword grips selected for this study are from different bladed weapons, one a dagger (NG- 1980-27-H-693) and two swords (Table 4.1). The sword hilts are composed of a wooden core wrapped in copper alloy thread. The copper alloys in these objects were manufactured through the techniques of pulling, twisting, and flattening wire.
60 Gun’s sideplates selected for this study belonged to VOC guns of a caliber of 16 mm with a barrel length of 4 feet (c. 1.12 m).60 These furnishings are composed of a central escutcheon with a crowned coat of arms of Amsterdam in relied, two flat wings on either side, curving in opposite directions with a circular eye and engraved AVOC marks on each wing.60 According to the financial records of the Amsterdam Chamber of Commerce, the small guns utilized by the fleet of 1743 were ordered from a single company. This was Jan van Solingen's firm, which was a continuation of the former Amsterdam City
gunsmithing business. Jan van Solingen was a member of the famed gunsmithing family of the time. Van Solingen sold a total of 1050 fully built snaphance guns with a significant number of pieces, including gun furnishings, according to his accounts for January 31, February 28, March 31, and June 30, 1743.60
61 Two types of military insignia which were manufactured the same way were included in this study (Table 4.1). They were both created out of sheet metal which was pressed into their current designs.61 Larger plates were attached to soldiers wielding grenades, also known as grenadiers (NG-1980-27-H-1458). Smaller military insignia with the VOC monogram was probably intended as decoration for the lid of the grenade case(NG-1980-27-H-1640, NG-1980-27-H-1717).
62 Buckles are present in a few different styles. The buckles selected for this thesis were likely cast, and do not show signs of cold working.
exhibit significantly different states of potentially active corrosion (Table 4.1). These categories also cover a range of manufacturing methodology. The methods of casting, or pouring hot metal into molds, can produce very different metallic structures when compared to an object that was cold worked through hammering, rolling, stamping, or pulling into a wire. Some objects, like the buckles, were likely cast and show few signs of cold working, while others, such as the grenadier furnishings, were likely extensively cold-worked. The difference in metallic structures produced by these methods of manufacture could have an impact on the way these objects corrode.
In addition to these different manufacturing methodologies, some objects, like the sword grips, are composite objects, still in contact with their organic components, while others like the sideplates were likely once associated with wood but have since become dissociated. Buckles were present in bulk and were likely not associated with other materials as composite objects.
The formation of corrosion phenomena within the categories of white excrescences, blue excrescences, or pitting corrosion could be influenced by composite materials, alloying elements, seabed environment, or previous treatments, a distinction which this research was designed to investigate. In the following pages, there is an explanation of the objects selected for this study along with the characteristics of the exhibited corrosion, and drawings of their corrosion stratigraphies, which were created as a visual communication of the distribution of corrosion on artifacts.
Table 4.1. Case Study Objects.
treatment Artillery sideplates Furnishings Sword hilts Buckles
Table 4.1. Objects selected for study. Nikon D750. Images by author.
4.1. White excrescences
White excrescences were selected for this study because the growth of excrescences may be indicative of either an unknown corrosion phenomenon, chemical residue, or both. Both the corrosion phenomena and chemical residues can cause harm to objects. If the excrescences prove to be the residues of chemical treatment solutions, this indicates that some solution may still be present in objects selected for this study. Objects exhibiting white excrescences range from fully concreted objects to objects that have been completely cleaned of corrosion to the bare metallic, or “stripped” surface (Table 4.1.1.). These corrosion products manifest evenly on some objects, while on others they appear to be circular or generally elliptical in shape.
Table 4.1.1. Selected White Excrescences. Overview images: Nikon D750. Macro Images: Olympus TG-6.
Table 4.1.1. White Excrescences
# Overview Macro Image Corrosion Stratigraphy Texture/
Thicker in crevases, otherwise even
Blotchy blooms, unevenly distributed on
Isolated, generally eliptical