8 Preservation of iron based archaeological artefacts exposed to

Transworld Research Network
37/661 (2), Fort P.O., Trivandrum-695 023, Kerala, India
Environmental Degradation of Infrastructure and Cultural Heritage in Coastal Tropical
Climate, 2009: 183-200 ISBN: 978-81-7895-426-4
Editors: J. González-Sánchez, F. Corvo and N. Acuña-González
8
Preservation of iron based
archaeological artefacts
exposed to tropical-humid
atmosphere
J. González-Sánchez1, D. Arano-Recio2, F. Bernes1 and H. Matos1
1
Centro de Investigación en Corrosión (CICORR), Universidad Autónoma de
Campeche, Av. Agustín Melgar s/n, Colonia Buenavista, San Francisco de
Campeche, Cam., CP 24030, México; 2Departamento de Restauración de
bienes culturales, Instituto Nacional de Antropología e Historia (INAH)
Delegación estatal Campeche, México
Abstract
Because of its historic context and relevance,
metallic artefacts considered part of the Cultural
Heritage of the city of Campeche are exposed in open
public places where prevails a tropical-humid
outdoor atmosphere. The exposure condition of iron
Correspondence/Reprint request: Dr. Jorge A. González-Sánchez, Centro de Investigación en Corrosión
Universidad Autónoma de Campeche, Av. Agustín Melgar s/n, Colonia Buenavista, San Francisco de
Campeche, Cam., CP 24039, México. E-mail: [email protected]
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J. González-Sánchez et al.
based artefacts induces a degradation process due to atmospheric corrosion
in the medium-high corrosive climate of San Francisco de Campeche city.
From an archaeological point of view, the preservation of iron based cultural
goods such as cannons, anchors, balls, guns, chains, swords, etc. is of
paramount importance in order to maintain the cultural identity of the
Colonial City of Campeche which was included by UNESCO in the list of
Cultural Heritage of Humanity in 1999. The study of the corrosion process
and corrosion products formed on metallic artefacts gives invaluable
information about their corrosion resistance and the best preservation
method. The present work provides results of the corrosion degradation and
preservation study of grey iron cannons and wrought iron anchors of the
XVIII century recovered from the sea of the Gulf of Mexico and exposed at
open public places in the city of San Francisco de Campeche, México.
Different analysis were applied also to wrought iron anchors and grey iron
cannons in order to determine the chemical composition of the corrosion
products formed during the period the artefact was immersed in the sea.
Electrochemical tests conducted on samples in natural seawater and in
rainfall water showed that wrought iron corrodes at a high rate in natural
seawater whereas in rainfall water the corrosion rate was three orders of
magnitude lower. Stable corrosion products films (patinas) were
electrochemically induced iron using 5% NaOH solution in order to provide
a natural corrosion resistance for anchors exposed at open public places in
the tropical-humid marine climate of Campeche.
1. Introduction
The study and preservation of tangible cultural heritage, specifically
metallic artefacts recovered from the seawater, involves the participation of
experts from different disciplines such as archaeologists, material´s scientists,
corrosionists, marine biologists and restoration professionals. Research
focussed on tangible cultural goods as testimony of economic and
technological development gives information about the modus-vivendi of
past cultures and people all over the world. In this sense, several branches of
anthropology have been developed in order to apply methodological
strategies to determine the relationship between cultural goods and their
historic context in terms of human evolution. The development and
application of materials science and novel analysis techniques to the study of
paleontological, historic and archaeological materials has open an enormous
opportunity to categorically assess the age, context, origin and even
manufacture processes of diverse artefacts used by human beings since the
beginning of times. Preservation of metallic artefacts of historic and
Preservation of iron based archaeological artefacts exposed to tropical-humid atmosphere
185
archaeological value is now a day an important activity in many places
around the world awarded as humanity cultural heritage. This is the case of
the historic city centre of San Francisco de Campeche city in which canons,
anchors, bells and other metallic pieces are exhibited at open places as
memory of the economic and cultural splendour of the city during its colonial
period. Preservation of tangible cultural heritage has become a primary need
for politicians and authorities of cities like San Francisco de Campeche in
which degradation of metallic pieces exposed to the atmosphere is highly
severe due to the humid-tropical marine climate. Metallic artefacts
manufactured of bronze are practically resistant to atmospheric corrosion
degradation compared with pieces made with iron base alloys in tropical
humid climates. Archaeological objects manufactured with organic materials
like wood and natural fibres suffer also from degradation induced by changes
in temperature, relative humidity and even solar radiation, in general due to
their interaction with the surrounding environment. Many metallic objects
with archaeological value are recovered from seawater in places near the
coast line where dozens of ships sank during the conquest and colony period
in the Americas. These pieces normally suffer a severe degradation process
when are removed from the sea and left to get in contact with the atmosphere.
In order to preserve them it is necessary to apply a methodology for
desalination and stabilization of the corrosion products formed and in some
cases the removal of them in order to induce the formation of new stable and
protective patinas. The application of electrochemical techniques to
determine the electrochemical behaviour of different metals and alloys in
contact with diverse electrolytes is the base for the understanding of the
corrosion process of metallic artefacts of historical/archaeological value. The
knowledge of the kinetics of the corrosion process, the thermodynamic
conditions for the formation of stable compounds on the surface of metals are
powerful tools for the professional devoted to the preservation of metallic
pieces. On the other hand, the application of metallographic analysis
permitted the identification of the kind of alloy with which metallic artefacts
were manufactured. This helps the archaeologists to approach the historic
context of the object in the case that no information is available. Changes in
the microstructure along the body of for example canons, allows determining
or suggest the manufacture process which gives an idea of the possible origin
place and period in which the canon was produced. For many years the
application of rust converters was a common practice for the preservation of
iron based objects. However no scientific base was used in order to determine
the effectiveness of the method. Potentiostatic formation of passive films
using sodium or potassium hydroxide solutions also has shown positive
results in terms of decrease of corrosion rate for iron base objects.
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J. González-Sánchez et al.
In the present work, the degradation condition of several canons
manufactured with grey iron was determined using electrochemical
techniques and measuring the thickness of the corrosion products formed on
the surface of canons exposed to the open atmosphere in the Museum of the
San Miguel Fort at the city of San Francisco de Campeche. Applying
metallographic analysis to the canons it was possible to identify or
corroborate the kind of iron based alloy with which they were manufactured.
San Francisco de Campeche city was an important port during the
colonial period. Its economy was based on the exploitation of natural
resources as bee wax, tropical woods, (palo de tinte) that was a colorant
obtained of a tree, and other products that were exported to Spain. The
significant commerce between Campeche coast and Spain made this region
attractive to pillagers and pirates. In order to protect Campeche port from the
pirat attacks, in 1686, the spanish government ordered the edification of
several militar constructions. The San Francisco de Campeche town was
protected with a limestone brick wall in an hexagonal form with four access
doors and eight bastions. Two forts estrategically located in the colines in
each extreme of the city with a perifecal view of the region. Their edification
conluded until 1769 when pirat attacks were over, never the less the militaries
construtions were used to protec the city during the mexican independence
and in the sublebation of native mayas during the Guerra de Castas [1,2].
As part of the conservation of cultural heritage, it is important the
knowledge of nature and behaviour of materials and their interaction with
their surrounding environments. The tropical climate is one the factor that
interfere the preservation of metallic pieces with historical value. Campeche
is a capital city located in the coast line in the Gulf of Mexico with a tropicalhumid climate, condition that makes this place aggressive to several materials
which suffer faster degradation. This situation has made us consider about the
care of cultural heritage of San Francisco de Campeche city.
In 2003 the Conservation Area of the National Institute of Anthropology
and History (INAH) in Campeche was created, then the possibility of manage
research projects focussed on conservation with a long term perspective. One
of the main problems to solve was the degradation of cultural goods made on
metal that are exposed in public places without periodical preservation
treatments. In the intervention of metallic objects that are consider cultural
heritage usually the conservator follows recipes and generally he/she applies
the same treatment to all pieces without making a previous investigation of
the real preservations needs of each piece. In Mexico the conservator is
trained to communicate with scientific community about the need of
generated knowledge for the preservation of cultural heritage. However, in
order to be able to make a specialized investigation, the conservator needs the
support of scientific knowledge and work with different disciplines.
Preservation of iron based archaeological artefacts exposed to tropical-humid atmosphere
187
The state of Campeche, Mexico counts with five museums under the
jurisdiction of Centro INAH Campeche, two of them exhibit cultural goods of
the colonial period most of them made on metal and the other three of
archaeological goods. In the old neighbourhoods, in the Malecón and in the
embankment of the two forts of San Francisco the Campeche city we can
observe the old cannons. Until now in the inventory of Underwater
Archaeology Section of Centro INAH Campeche there are 97 cannons exposed
in public places [3]. Besides, this institution has recorded more than 50 marine
sites with remains of shipwrecks that happened at different times like El
pesquero (eighteen century), shrimp boats and several pieces as cannons,
anchors, chains, canons balls, stoves and other historical objects. It is also
common that the Head direction of Museums of Centro INAH Campeche
receives donations of cultural goods that are found during excavation for
buildings construction, basements and undergrounded pipelines for gas, oil and
water conduction. Example of this fact has been the donation of two cannons
and an anchor for the fishers; another cannons donated by the National Bureau
of Water (CNA) which was found during their works [3]. With this background
it has increased the necessity to improve the preservation techniques of objects
and metallic pieces with archaeological and historical value, with the objective
not only to preserve but to optimize the material and human resources.
In 2005 a collaboration agreement was establish between the National
Institute of Anthropology and History through the Centro INAH Campeche
and the Autonomous University of Campeche in specific with the Centre for
Corrosion Research. From that moment several research projects have been
developed focussed on the conservation of cultural heritage such as the one
named “Determination of degradation degree and the applications of methods
of preservation of metallic pieces consider cultural heritage of the city of
Campeche”, which got financial support from a research founding
programme of CONACYT and the Government of the state of Campeche ID:
CAM-2005-C01-026 [4]. As part of this project it has been establish several
objectives as the creation of a catalogue of cultural goods manufactured on
metal, some of them still immerse in the Gulf of Mexico, the identification
and evaluation of the degree of preservation of the material that constitute
this goods and the establish and creation of viable treatments for their
preservation [4-6].
The principal objective is to apply chemical or electrochemical
treatments to form and stabilise corrosion products which can act as
protective patinas for metallic artefacts exposed to the outdoor tropical-humid
climate of the city of San Francisco de Campeche, Mexico.
As an example of metallic artefacts with historic and archaeological value is
the iron cannon shown in Figure 1 which presented severe corrosion degradation.
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Figure 1. Cannon No. 001 that was recovered by INAH at Campeche, the cannon was
found underground near the coastline.
Table I. Localisation and custody of grey iron cannons of the colony age at the city of
San Francisco de Campeche [7].
Quantity Total
Exposure place
Condition
Custody
10
10
Sector Naval Lerma
Regular
Mexican Marine
3
3
33va. Zona Militar
Good
Mexican Army
Dirección de Museos
Good
Baluarte de San Carlos
Regular
8
Reducto de San José
Regular
21
Reducto San Miguel
Good
2
Carretera a Campeche
Poor
2
Barrio de San Román
1
Baluarte de Santiago
Poor
Good
1
Barrio de Guadalupe
Good
1
Baluarte de San Pedro
Good
1
Aeropuerto
Regular
Mercado 7 de Agosto
Good
1
Guatemala y Ecuador
Regular
2
Baluarte de San Juan
Regular
2
Malecón
Poor
4
Baluarte de San Francisco
Regular
8
Archivo municipal
Regular
10
Centro Cultural “El Polvorín”
Poor
10
Puerta de Tierra
Regular
Hostal del Pirata
Regular
2
5
1
2
36
46
2
INAH
Campeche City
Council
Particular
Preservation of iron based archaeological artefacts exposed to tropical-humid atmosphere
189
In the city of San Francisco de Campeche there are officially registered
97 cannons of the colony period exposed in different places, gardens,
museums, open places and all around the city. Table I presents the official
record made for the National Institute of Anthropology and History (INAH)
of the grey iron cannons considered part of the tangible cultural heritage of
the city. The historic context of the cannons varies from XVI to XVIII
centuries and so their technical characteristics and manufacturing process.
2. Cannons of San Miguel´s Fort at Campeche city
The city of San Francisco de Campeche counts with two forts built in the
XVIII century, one of them, the Fort San Miguel has exposed to the open
environment 20 cannons from different historic context. All of them were
manufactured with gray iron as could be determined by metallographic
analysis carried out in situ due that the information about their precedence is
not complete. Historical information indicates that the majority of cannons
were manufactured with grey iron. Some cannons were recovered from the
sea of the Gulf of Mexico near the coast of Campeche; other ones were
recovered from the soil during excavations of archaeological investigations.
That is the reason the cannons present different degradation levels even they
are exposed to the same aggressive outdoor environment. Figure 2 presents a
schematic of the top part of the Fort and the preservation condition of the
cannons as well as the location in that part of the fort.
Figure 2. Cannons exposed to outdoor conditions at the Fort San Miguel, Campeche.
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J. González-Sánchez et al.
Due to the application of organic coatings (transparent lacquer) during
preservation activities carried out by non corrosion experts most of the
cannons present severe atmospheric corrosion. A corrosion products film is
developed and stabilized during the dry season, which provides some
corrosion protection. However during the rainfall season these corrosion
products are removed and the corrosion process continues with the
dissolution of iron that is evident by the presence of rust on the floor.
In order to determine the degradation level of the cannons we proposed
and conducted the removal of a 3 mm line of the corrosion products up to the
metal base along the complete body. Four lines of removed rust were made at
90 degrees to each other as shown in figure 3.
In some cases we found very thick rust films of about 10 mm in some
zones of the body of cannons. Figure 3 presents the results of cannon number
001. Other zones presented rust films as skinny as 0.5 mm. These results
indicated that the metallic artifact suffered corrosion degradation in
environments of different aggressiveness as can be the case of pieces that
were in the bottom of the sea for decades with periods during which it was
partially or totally in the sea ground.
Figure 3. Corrosion degradation profiles obtained from the iron cannon 001 obtained
through mechanical rust removal.
Preservation of iron based archaeological artefacts exposed to tropical-humid atmosphere
191
2.1. Metallographic analysis
The metallographic analysis made on the cannons showed that all of
them were manufactured through a casting process with grey iron as shown
in the micrographs presented in figure 4. The presence of graphite flakes is
10-343623 5X
10-343623 50X
001 5X
001 50X
10-343605 5X
10-343605 50X
10-343610 5X
10-343610 50X
Figure 4. Micrographs of some cannons exposed at the San Miguel Museum, samples
etched with 5% Nital; images taken from acetate replication.
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J. González-Sánchez et al.
the result of the addition of a substantial amount of silicon to a relatively low
carbon cast iron which induces also the cementite to ferrite transformation
[8-13]. This results in the properties of the resultant metal to be more similar
to that of grey cast iron, i.e. making it easier to machine and improved wear
resistance. The metallographic analysis was made near the muzzle and at the
breech in order to detect changes in the microstructure associated to the
manufacturing process, which may give information of the historic context of
cannons of unknown origin.
2.2. Chemical analysis of corrosion products
The corrosion products formed on the cannons were analysed in order to
determine the chemical composition and to associate their protective
characteristics as a function of the composition. In all samples (9 selected
cannons fot the preliminary study and preservation treatment) two crystalline
phases were found: Goethita FeO(OH) and Maghemite-C γ-Fe2O3 [14-17].
Quarz was present also in the corrosion products of the cannos exposed at the
Fuerte San Miguel. The corrosion products of cannon 001 contained
lepidocrosite γ - Fe3+O(OH), which was stored at the laboratory of the centre
for corrosion research under controlled relative humidity and temperature.
Table II presents the compounds that form the corrosion products of the
cannons under study.
Table II. Crystalline phases present on the corrosion products of 9 cannons under
study.
Cannon
No.
Hexagonal
SiO2
001
Orthorhombic
Goethite
FeO(OH)
Cubic
Maghemite-C
γ-Fe2O3
Tetragonal
Akaganeite
β-FeO(OH)
X
X
X
10-343612
X
X
X
X
10-343621
X
X
X
X
10-343617
X
X
X
X
10-343610
X
X
X
X
10-343616
X
X
X
X
10-343620
X
X
X
X
10-343604
X
X
X
10-343605
X
X
X
Orthorhombic
Lepidocrocite
γ -Fe3+O(OH)
X
Preservation of iron based archaeological artefacts exposed to tropical-humid atmosphere
193
2.3. Potentiodynamic polarisation
In order to determine the electrochemical behaviour of cannons and their
corrosion resistance, potentiodynamic polarization was carried out in situ on
the 9 selected cannons. The surface used for the metallographic analysis was
used again for the electrochemical test. The area was grinded, cleaned and
dried. Also electrochemical polarisation was conducted on areas with
corrosion products; finally potentiodynamic polarization was conducted on
surfaces treated (chemical stabilization of corrosion products). Due to the fact
that cannons are exposed at open places, the effect of the rainfall water is an
important variable as well as the condensed humidity during the night. For
this reason the potentiodynamic polarization was made using natural rainfall
water collected during the raining season in the city of San Francisco de
Campeche. Figure 5 presents the polarization curves obtained from
potentiodynamic polarisation. Here we just present the results from tests on
canons10-343620, 10-343604 y 10-343610 respectively.
The presence of rust gives corrosion resistance to the grey iron as can be
seen by the more positive corrosion potentials for samples with this condition
[18].
The complete body surface of cannon 001 was subjected to mechanical
cleaning, degreasing and nitric acid oxidation with a 5% HNO3 solution.
During the oxidation of the metallic surface with the acidic solution
mechanical grinding was conducted using a plastic brush. This helped to
remove residual impurities and in some cases graphite flakes from the surface
with which the surface became more uniform and the corrosion products
formed were continuous. Immediately after, once removed the acid excess, a
solution of 5% Tannic acid + methanol was applied on the surface.
Potentiodynamic polarisation was carried out on the surface subjected to the
above mentioned treatment. The results of the polarisation in rain water and
distilled water are presented in figure 6.
The effect of the treatment was obvious in terms of electrochemical
behaviour which indicated higher corrosion resistance compared with
samples without treatment.
Novegil-Anleo, et al [19] mentioned that rust converters act differently
depending upon the crystalline phase formed on the corrosion products. They
found that the most difficult phases to convert were hematite, maghemite and
magnetite respectively. The XPS analysis of the treated samples indicated the
presence of Carbon as principal component (54%) followed by Oxygen
(18%) and Iron (28%). The presence of these elements in the chemical
composition of the converted film formed on the surface of the iron cannon it
can be suggested the formation of iron tannins as mentioned by
Novegil-Anleo, et al [19]. The prevailing tropical humid climate in the city of
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Figure 5. Polarisation curves of cannons 10-343620, 10-343604 and 10-343610 in
natural rain water and distilled water on clean and rusted surfaces.
San Francisco de Campeche makes this place high aggressive for metals in
general. As mentioned in chapters 1 and 2, the average relative humidity is
>70% and the averge temperature is 25 °C, which is very corrosiove for iron
base pieces and artefacts exposed to the atmosphere.
Preservation of iron based archaeological artefacts exposed to tropical-humid atmosphere
195
Figure 6. Polarisation curves on cannon 001 in natural rain water after surface
treatment with tannic acid and exposed to the open atmosphere for different periods.
3. XIX century wrought iron anchor
The other type of metallic pieces that were considered for the present
study and preservation programme were archaeological wrought iron
artefacts recovered from the sea of the Gulf of Mexico such as anchors of
different period, design and size. Evaluation of the corrosion rate of wrought
iron samples after patina formation showed an increase in corrosion
resistance in rainfall water and no improvement in seawater. Preliminary
results of atmospheric corrosion tests conducted on wrought iron samples
with patina showed that these samples present better resistance to
atmospheric corrosion than samples without patina. Figure 7 presents a XIX
century anchor recovered from the sea of the Gulf of Mexico, which was at
least 100 years immersed in the sea.
Even this kind of artefacts such as cannons and anchors do not have real
use, they have historic value as be considered part of the cultural heritage.
Then it is of great interest to preserve then in good condition considering that
the majority of them will be exposed to the open atmosphere in Campeche.
The anchor showed in figure 4 was manufactured with wrought iron as could
be determined by means of metallographic analysis. Figure 8 presents the
micrographs obtained from a small piece removed from the anchor body and
from the stock. The microstructure is formed by equiaxed ferrite grains with
high quantity of non-metallic inclusions characteristic of this iron alloy
(figure 4c and 4d).
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Figure 7. XIX century wrought iron anchor in the as received condition after
recovered from the sea of the Gulf of Mexico.
a
c
Anchor shank
b
d
Stock
Figure 8. XIX century wrought iron anchor: a) The anchor and the broken stock, b)
cut cross section of the stock, c) micrograph from the anchor shank and d) micrograph
from the stock.
Preservation of iron based archaeological artefacts exposed to tropical-humid atmosphere
197
It is known that nearly all iron based alloys rust, but wrought iron does a
better job at handling it. As corrosion progresses, the non-metallic fibers tend
to disperse the rust into an even film, which gives the metal a natural
brownish appearance. This film repels the scattering spotty corrosive attack
that other metals endure [8,10,20]. This is the reason why even after more
than 100 years immersed in seawater the anchor and its stock were not
destroyed completely by corrosion in this aggressive environment. The task
was then to avoid accelerated corrosion degradation of the anchor after
removed from the sea to be exposed to the open atmosphere of the city of San
Francisco de Campeche. The electrochemical behavior of the wrought iron
anchor was devaluated by potentiodynamic polarization in natural seawater,
natural rain water and in a 5% NaOH solution. Samples free from corrosion
products were subjected to polarization in the NaOH solution in order to
establish the conditions for the formation of a passive film. Figure 9 shows
the polarization curve, which presents a potential range of about 600 mV in
which the wrought iron gets passive during anodic polarization.
Potentiostatic anodic polarisation was applied to samples of the anchor in
order to induce the formation of a passive film in the NaOH solution.
Samples were subjected to anodic polarization at a potential level of 50 mV
vs SCE in NaOH solution for 40 minutes after which the samples were
cleaned with distilled water and dried with air. The wrought iron samples
were subjected to potentiodynamic polarisation in rain water and natural
sweater in order to determine the corrosion resistance of the patina formed.
From the polarisation curve obtained and shown in figure 10, it was clear
that the formed patina presented poor resistance to corrosion attack in natural
seawater, whereas was more resistant to corrosion in rain water [21,22].
Considering that these pieces will be exposed to atmospheric corrosion damage
Figure 9. Polarisation curve of wrought iron in 5% NaOH solution.
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Figure 10. Polarisation curve of anodic treated wrought iron samples in rain water
and in natural seawater
and that condensed water and rain will be the most insidious electrolytes, the
protection offered by the patina formed in NaOH solution was an acceptable
preservation method for wrought iron artefacts.
Acknowledgements
The authors would like to thank The Universidad Autónoma de
Campeche for the facilities given for the implementation of this research
work; also to the Mix Founding CONACYT-Campeche State Government
for the financial support for the project CAMP 2005-C01-026. To the
National Institute of Anthropology and History (INAH) for the help in
managing the use of cannons and anchors for the studies carried out in the
mentioned project.
Conclusions
The application of methodological scientific principles to the study of
materials degradation and their preservation is a developing field of great
application and help for archaeologists and conservation specialists dedicated
to the preservation of tangible cultural heritage (metallic artefacts).
•
•
It is mandatory the use of scientific analysis for the study of metallic
artefacts of archaeological and historical value. Materials science, surface
analysis techniques and methods, electrochemical techniques are some of
the tools available for conservation specialists.
Even at the high aggressive atmosphere in the city of San Francisco de
Campeche, the methods used to stabilise iron based metallic pieces were
successful, non environment damaging, cheap and safe.
Preservation of iron based archaeological artefacts exposed to tropical-humid atmosphere
•
•
•
•
•
199
In Mexico this is one of the first research works devoted to the
preservation of iron based metallic artefacts that are exposed to the open
atmosphere. The Centre for Corrosion Reserach (CICORR) of the
Universidad Autónoma de Campeche is pioneer in this interesting field
of applied science and engineering.
The cannons exposed at the San Miguel Museum were manufactured
with grey iron with different chemical composition, which gave
information to archaeologists about the possible origin (manufacture
place) and historic context of some not known cannons.
The use of acidic treatment and stabilisation with tannic acid resulted in
an excellent method for atmospheric corrosion protection for grey iron
cannons exposed at the city of San Francisco de Campeche.
Preservation treatment of wrought iron anchors with anodic polarisation
in NaOH solution resulted an economic and effective method.
The above mentioned methods need to be repeated to the pieces at least
twice a year, principally after the raining season which could remove and
dissolve the patinas formed
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