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Technical Communication. An improved tannin-based corrosion inhibitor-coating system for ferrous artefacts

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  • Ships of Discovery
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Technical Communication. An improved tannin-based corrosion inhibitor-coating system for ferrous artefacts

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The International Journal of Nautical Archaeology
(1996) 25.1:
38
45
Technical Communication
An improved tannin
-
-
coating
system for ferrous artefacts
Worth Carlin and Donald H. Keith
Ships of Discovery, Corpus Christi Museum, 1900 N. Chaparral St. Corpus Christi, TX 78401, USA
Introduction
Artefacts recovered from the sea in the late
1960s and early 1970s were re
-
examined in
1993 to determine their present condition,
to evaluate the effectiveness of the original
treatment, and to
re
-
treat those exhibiting
symptoms of active corrosion. Originally
conserved by mechanical cleaning, electro
-
lytic reduction, and coating in micro
-
crystalline wax, after 20 years the artefacts
were in good condition, but the micro
-
crystalline wax coating w
as thin and no
longer impermeable to air penetration. In
some places, slow corrosion was taking
place. More than 350 16th
-
century
wrought
-
iron items were re
-
treated with the
experimental tannic acid solution de
-
scribed
below followed by a tannin
-
derived
co
ating
which is impervious to air and moisture.
The Padre Island 1554 Plate Fleet
collection
A collection of 16th
-
century artefacts sal
-
vaged from two ships wrecked in the Gulf
of Mexico in 1554 is currently housed in
the Corpus Christi Museum of Science a
nd
History (Arnold, 1992). They were recov
-
ered at intervals between 1967 and 1973,
and conserved at the Texas Archaeological
Research Laboratory at the University of
Texas at Austin from 1969 to 1976 (Arnold
& Weddle, 1979; Olds, 1975). When recov
-
1057
2
414/96/010038+08 $18.00/0
ered, most of the wrought
-
iron artefacts
were heavily encrusted with calcareous
marine deposits. Following mechanical
removal of the encrustation, they were
cleaned electrolytically to remove corrosive
chlorides, then washed with
deionized
water, dehydrated in alcohol, air
-
dried and
immersed in molten microcrystalline wax
(Hamilton, 1976). To the authors' knowl
-
edge, this is the oldest, largest, collection of
wrought
-
iron artefacts conserved by
electrolytic reduction.
An inspectio
n in 1993 determined that
most of them were in good condition after
20 years, with less than 5% of the collection
showing any trace of 'rust'. The overall
excellent condition of the collection attests
to the effectiveness of this method of con
-
servation. I
nvestigators who have been
convinced that it is preferable to strip iron
to bare metal, removing all corrosion
products, should reconsider the advan
tages
of treatments which strive to preserve
and
pacify an artefact's 'rind' as well as its
metallic core.
During the passage of more than 20 years,
many of the objects had been exposed to a
variety of less
-
than
-
optimum environmental
conditions; part of the col
lection was used
in a travelling exhibit and other artefacts
were loaned to other museums for
tempora
ry exhibit and some changes had
taken place. The original
©
1996 The Nautical Archaeology Society
W. CARLIN & D. H. KEITH: CORROSION INHIBITOR
-
COATING SYSTEM
microcrystalline wax coating was very thin,
and susceptible to water vapour penetra
tion
and so
ning.
Each artefact was examined closely and it
was noted that a few of the smaller items,
such as bolts, had developed small cracks
perpendicular to their long or wrought
dimension. The cracks indicated that a
dimensional change
was occurring
consistent with continuing oxidation.
In order to quantify the physical differ
-
ence between surface and core, samples
were taken from a bolt (41WY3
-
158
3
.
0). A
dental pick was used to separate a flake
from the outer oxidized surface of a bolt
,
removing a fragment approximately 0
.
32
cm thick by 1
.
0 cm wide by 2
.
0 cm in
length. A sample of the underlying metal
was obtained by drilling into the bolt below
where the flake had been removed. These
samples were analysed by X
-
ray powder
diffraction (X
RPD) to determine their
composition. On a comparative basis the
surface sample (Fig. 1 a) contained less than
one
-
third of the elemental iron found in the
underlying material (Fig. 1 b), with
corresponding increases in the oxide con
-
tent of the surface mat
erial. In this study,
samples taken for XRPD analysis were also
analysed by X
-
ray fluorescence (XRF) in
the atomic numb
er range 9
-
92. No chloride
(Cl
_
) concentration greater than 0
.
001% was
detected among the trace elements found.
It was deduced that corro
sion was
continuing, as the cracks had developed
since the last inspection.
While the most expedient and direct
method of cleaning and stabilizing iron
artefacts from the sea is to remove
mechanically every trace of oxidized ma
-
terial, leaving only metall
ic iron behind, this
can be akin to 'throwing the baby out with
the bath water' if the outermost layer of
corrosion products preserves evidence of
manufacture and use, as well as unique
markings. The resulting dilemma for the
archaeological conservator is
to pacify the
metallic core while at the same time
preserving the stable 'rind' of corrosion
products, which although derived from the
core has become an entirely different
material. An additional complication is that
this rind prevents any coating applie
d to
the exterior from reaching the still
-
reactive
metallic core and leaves the artefact
susceptible to on
-
going corrosion.
Because the 1554 Plate Fleet artefacts still
have their outer, corrosion layers in
-
tact,
the decision was made to re
-
treat the
colle
ction with a corrosion inhibitor and to
provide a coating to minimize the penetra
-
tion of air and moisture. The decision to
treat the entire collection with tannic acid
was made because it would be relatively
simple to accomplish
-
provided an effec
-
tive t
annic acid/surfactant system and a
removable vapour barrier coating could be
developed.
Tannic acid
-
based corrosion inhibitors
Tannic acid and its derivatives have been
used as corrosion inhibitors for ferrous
materials for many years (Knowles &
White, 19
58; Matamala
et
al
.,
1990) al
-
though recently some investigators have
raised questions about its effectiveness
(Morcillo
et
al
.
,
1992; Ashton, 1993: 20).
The chemistry of tannic acid and its
derivatives is complex; the material itself
varies greatly from o
ne source to another.
Generally speaking, 'tannic acid' refers to
hydrolysable tannins which are essentially
galloyl esters of glucose. The hydrolysis of
tannic acid yields primarily gallic acid and
glucose. Tannic acid's corrosion inhibiting
mechanism has
not been determined due
to the complexity of the large tannic acid
molecule. Some investigators have sug
-
gested that anodic polarization of the metal
occurs, forming a passive film (Parkins &
Pearce, 1966: 649
-
650; Rosenberg, 1987:
15). Considering the co
mplexity of the
system, 'anodic polarization/film
-
forming'
is
a generality that can be accepted because
there are ample data to support the claim
that tannic acid is a corrosion inhibitor.
39
NAUTICAL ARCHAEOLOGY,
25.1
Figure 1. A
comparison between th
e composition of samples taken from the metallic core (b) and rind of
a wrought
-
iron forelock bolt (a) as determined by XRPD analysis. Peak heights reflect the proportion of
the element or compound present. The graph shows that the amount of iron in the me
tallic core is about
three times that of the rind, while the amount of magnetite/maghemite in the rind increases accordingly.
X
-
axis numbers are arbitrary. The repetition of peaks is a reflection of different energy levels.
Maghemite=1, magnetite= 2, iron=
3.
40
W. CARLIN & D. H. KEITH: CORROSION INHIBITOR
-
COATING SYSTEM
Sixteen to eighteen percent aqueous/
alcohol solutions of tannic acid have been
used to coat ferrous artefacts. However
tannic acid solutions have poor metal
wetting properties, particu
larly on arte
facts
such as those in the 1554 Plate Fleet
collection which have been coated with
microcrystalline wax. Theoretically, the
wax can be removed. But in practice, the
object must be immersed in a hot volatile
solvent (such as Stoddard's solvent
). For
small artefacts this could be accomplished
safely enough in a well
-
vented hood with
ample protection from sources of ignition.
For large artefacts the inhalation and
flammability hazards of microcrystalline
wax removal are too serious to be
contempl
ated.
Corrosion of the objects indicated that the
protective wax coating was no longer fully
effective. There was a possibility that
they
could be treated without removing the
microcrystalline wax coating, but this made
it imperative to design a tannic
-
aci
d
inhibitor with good penetration and wet
ting
properties. Although the surface was
hydrophobic and poorly wetted by either
water or the aqueous/alcohol solution of
tannic acid, when tested with an aqueous
solution containing a suitable surfactant,
the wax
-
treated artefact surface was suffi
-
ciently porous to allow penetration of the
tannic acid. Other tests demonstrated that a
fresh, thick coat of microcrystalline wax is
completely impermeable, even to the
surfactant solution.
A new tannic acid coating
Pri
or to determining if a surfactant could
increase the 'wetting' of aqueous tannic acid
it was necessary to determine what types of
surfactant were compatible with
concentrated tannic acid solutions. A se
-
cond mandate was that the surfactant
should contain n
o chloride ion due to the
inherent corrosive nature of that molecule.
For example, surfactants derived from
alkali metal sulphates or sulphonates are
compatible with tannic
-
acid solutions but
the presence of chlorides preclude their
consideration. The
properties of 35 com
-
mercial surfactants were reviewed and
seven chloride
-
free materials were selected
for testing. The surfactants tested were
products of PPG Industries, Specialty
Chemicals, Chemicals Group. Although
several manufacturers produce simila
r sur
-
factants these were selected because of the
investigators' familiarity with their product
line. The initial compatibility test was to
determine which chloride
-
free surfactants
were soluble in 18% aqueous tannic acid.
In
this simple test,
0
.
25 g of ea
ch surfactant
were mixed with 100 gm of aqueous (18%)
tannic acid solution and the degree of
solubility was observed.
As revealed in Table 1, only one of the
tested surfactants, Mazon 40
, was com
-
patible with the tannic acid solution. To
ensure that the surfactant incompatibility
demonstrated in Table 1 was not a func
tion
of tannin
-
acid concentration, the test
procedure was repeated with 8% tannic
-
acid concentrations. The results were t
he
same. To determine the effectiveness of the
Mazon 40
surfactant on the 'wetting'
properties of aqueous 18% tannic
-
acid
solutions, a series of simple tests was per
-
formed by adding known quantities of the
surfactant to the tannic
-
acid solution and
then
placing a 0
.
2
-
ml drop of the mixture on
a rusted flat iron plate. The spread or
coverage of the solution on the surface of
the plate is a reliable measure of the effec
-
tiveness of the surfactant's performance.
The tannic
-
acid solution contained 18%
tannic
acid (technical grade, water soluble),
11% denatured ethanol with the remainder
distilled water. To this solution Mazon 40
was added at various concentrations.
The data in Table 2 reveal that the
addition of the surfactant to the aqueous
tannic acid solution increases the surface
coverage and the penetration of the inhibi
-
tor into the oxide
-
metal interface. With
41
NAUT
ICAL ARCHAEOLOGY,
25.1
Table 1.
Compatibility of various chloride
-
free surfactants with 18% aqueous
solutions of tannic acid
Trade name*
Chemical type
Results
Macol NPG
POE (6) nonyl phenol
Insoluble
Macol OP
-
10
PS
POE octyl phenol ether
Reacts
Maco
l OP
-
30 (70)™
POE (30) octyl phenol
Reacts
Mazawet 30™
Nonionic surfactant
Reacts
Mazon LDA™
Lauramine oxide
Reacts
Mafo 13
Amphoteric surfactant
Reacts
Mazon 40
Caustic coupling agent
Soluble
*PPG Industries, Inc. Speciality Chemicals, Chemicals
Group, 3938 Porett
Drive, Gurnee, IL 60031.
Table 2.
Area of rusty iron plate covered by 0
.
2 ml
of tannic
-
acid solution in square centimetres (cm
2
)
Surfactant concentration (wt %)
0
0
.
5
1
.
0
2
.
0
Test No. 1
3
.
1
3
.
8
11
.
9
12
.
8
Test No. 2
2
.
8
3
.
6
12
.
3
14
.
0
Test No. 3
3
.
3
3
.
7
12
.
6
13
.
4
heavily
-
corroded artefacts where a large
percentage of the particles are iron oxides,
it is necessary to have the corrosion in
-
hibitor in contact with the metal/oxide
interface to prevent or impede furt
her cor
-
rosion. Therefore, it is essential to have an
inhibitor solution that thoroughly pen
etrates
the corrosion products to inhibit and
stabilize the corrosion process. A
comparison of the 'wetting' properties of
tannic acid, tannic acid with surfactant
added, and Fertan (a widely
-
used commer
-
cial tannin
-
based inhibitor/coating), are
presented in Table 3.
Artefact treatment
The collection contained objects of vari
ous
shapes and size ranging from small nails
and bolts to wrought
-
iron, breech
-
loading
can
nons 0
.
15
-
0.20 m in diameter and up
to 2
.
6 m in length. The initial
42
Table 3.
Area of rusty iron plate covered by 0
.
2 ml
of tannin test solution, in square centimetres (cm
2
)
18% tannic
acid
Fertan
18% tannic
acid w/1
.
5%
surfactant
Test N
o. 1
3
.
5
4
.
5
13
.
4
Test No. 2
2
.
8
4
.
1
14
.
2
Test No. 3
33
4
.
6
14
.
5
intention was to paint the tannic acid
solution on the artefacts, applying one or
more coats as necessary. It quickly became
apparent that due to the presence in the
collection of man
y tubular objects
(cannons, breech chambers) a better
method of application would be to im
merse
them in the solution. For treat
ment, they
were grouped by size in order to minimize
the quantity of solution necessary. Small
artefacts were treated simultane
ously in
small tanks, while large ones were treated
individually in larger tanks. The immersion
tanks were kept covered to minimize
evaporation and oxidation of the solution.
The fact that they absorbed a surprising
quantity of solution was indicated by an
incremental lowering of the solution level
each time an artefact was withdrawn.
W. CARLIN & D. H. KEITH: CORROSION INHIBITOR
-
COATING SYSTEM
A new vapour
-
barrier coating
The new vapour
-
barrier coating, known as
MOP
-
30, developed at the Corpus Christi
Mu
seum, is derived from a combination of
the surfactant MACOL OP
-
30 (PPG
Industries) and tannic acid. MACOL OP
-
30
is an ethoxylated octyl phenol containing
thirty ethylene oxide units. The composition
of MOP
-
30 has not been studied;
presumably it is an ester
or similar
condensation product. It is a good coating
for artefacts that will not be exposed to the
elements because its high ethylene oxide
.content provides a good oxygen barrier, a
good moisture barrier and excellent
compatibility with tannate films. E
asily
removed with alcohol, the coating will not
withstand prolonged immersion in water.
The effectiveness of any tannic acid
treatment is counteracted and eventually
negated by exposure to water, but the
addition of a vapour
-
barrier coating can
enhance th
e pro
tection offered by tannic
acid. Judging from the literature, those
investigators who have denigrated the
usefulness of tannic acid as a corrosion
inhibitor were really questioning its
effectiveness in wet environments.
Treatment procedure and formul
as
The treatment procedure is as follows:
(1) Synthesize the protective coating
(MOP
-
30) well in advance due to the
difficulty of preparing it in large
quantities.
Dissolve 180 g of tannic acid in 720
ml distilled water. To the resulting
solution, slowly a
dd 92 g of MACOL
OP
-
30 with minimum stirring. Let the
reaction product, a brown resinous
syrup, separate from the solution for at
least one hour. Slowly decant the
supernate and set aside for further
processing. Add a volume of deionized
water equal to the
resinous mass, stir
gently and let sit for one hour. Decant
and discard the aqueous supernate.
Repeat the previous step at least three
times. After discarding the final wash,
let the resin dry (by air or under a heat
lamp) until it becomes a hard, wax
-
li
ke
substance. This material, dissolved in
three parts (by weight) of denatured
ethanol, is the protective coating, MOP
-
30. The first decantation of the reaction
mix, which had been retained, is
performed after 48 hours. The recov
-
ered resin is added to fut
ure prepara
-
tions. A batch with 180 g tannic acid
should yield approximately 171 g of
MOP
-
30 product. The MOP
-
30 is
applied to the artefact which has been
previously covered with at least two
coats of tannic acid, making sure that
each coating is dry befor
e applying the
next.
(2) Mix the tannic acid
-
surfactant solu
tion.
Solution number 1: To 3080 ml of
deionized water add 340 ml of
(denatured) ethanol. Mix in 758 g of a
commercial grade of completely water/
alcohol soluble tannic acid. Note: tannic
acid is
very dusty, the weighing and
mixing should be performed in a well
-
ventilated area with the techni
cian
wearing a dust mask and eye protection.
Solution number 2: To 380 ml of
deionized water add 38 g of PPG
Industries Mazon 40, a thick, viscous
alkyl gluc
oside. The surfactant dis
-
solves slowly with moderate stirring.
Combine Solutions number 1 and
number 2. Agitate gently, as the mix
-
ture has a tendency to create foam.
(3) Clean the artefact with a stiff nylon
brush to remove dust or loose particles.
(4) I
mmerse the artefact in the tannic acid
solution for 25
-
35 minutes. Tubular
objects such as guns or breech cham
-
bers should be gently rocked to prevent
the formation of air pockets.
(5) Allow artefact to air dry for 24
-
48
hours.
43
NAUTICAL ARCHAEOLOGY,
25.1
Figure 2
.
Wrought
-
iron breech
-
loading guns on
display in the Corpus Christi Museum following
treatment.
(6)
Vigorously brush the dry, coated arte
-
fact with a nylon
-
bristled vegetable
brush to remove any loose oxidized
tannate residue.
(7)
Apply the protect
ive MOP
-
30 coating
with a brush or roller. Two coats is
normally sufficient. The coating dries
quickly, leaving a glossy black surface
on the artefact.
(8)
Artefacts destined for display should
be given an additional coat consisting
of a mixture of 40% powder
ed graphite
and the MOP
-
30 coating. Best results
are obtained when the coating is
applied with a small sponge roller,
which imparts a soft, dull finish (Fig.
2).
Conclusion
This coating system for the protection of
ferrous artefacts not exposed to the ele
-
ments consists of three components: (a) A
tannic acid solution with increased
penetration properties (Fig. 3). (b) A clear
coating derived from tannic acid that pro
-
vides good compatibility when applied over
a tannic
-
acid inhibitor coating. Ident
ified as
MOP
-
30, this coating has good vapour
-
barrier properties. (c) The addition of
graphite pigment into the MOP
-
30 coat
ing
produces a gray matt appearance. Applied
over the clear protective coating, this
mixture imparts an aesthetically
-
Figure 3.
Schematic c
omparison of the wetting characteristics of a drop of a normal 18% aqueous tannic
acid solution and a drop of tannic acid solution with surfactant three seconds after contact with the
surface of an artefact. The rind is composed of stable corrosion product
s.
44
W. CARLIN & D. H. KEITH: CORROSION INHIBITOR
-
COATING SYSTEM
pleasing finish. Like MOP
-
30, the coating
can be removed in an alcohol bath.
The three
-
coating system is based on a
time
-
proven inhibitor (tannin). It is revers
-
ible, and it penetrates pe
rmeable, thin
layers of microcrystalline wax. Although
the coating system is suitable for ferrous
artefacts having a porous structure, such as
cast or wrought iron, it is not recom
mended
for artefacts that will be exposed to the
elements.
Acknowledgments
The authors would like to thank Rick
Stryker, Director of the Corpus Christi
Museum and J. Barto Arnold III, Texas
State Underwater Archaeologist, for per
-
mission to research, develop and apply this
treatment to artefacts from the Padre Island
1554 Plat
e Fleet collection. Special thanks
go to Linda Zitting of the Corpus Christi
Museum history section, who is
undoubtedly more familiar with the 1554
collection than any other person. Messrs
Charles Holifield and Michael Feeney
obtained the XRPD analysis for
us gratis.
With regard to the artefacts them
-
selves, we
would like to thank Dr D. L. Hamilton and
all the people involved in the treatment of
this collection more than 25 years ago. The
excellent condition of the hundreds of
artefacts are a testa
ment to
their skill,
inventiveness, and determination.
References
Arnold III, J. Barto & Weddle, Robert S., 1969,
The Nautical Archaeology
of
Padre Island.
New York.
Arnold III, J. Barto, 1992, Shipwreck! the 1554
flota
exhibit.
IJNA,
21.4: 343
-
355.
Ashton, J.,
1993, Article Review: Tannic acid: does it work?
Australian Institute for the Conservation
of
Cultural Materials National Newsletter,
47.
Hamilton, D. L., 1976,
Conservation
of
Metal Objects From Underwater Sites: A Study in Methods.
Austin.
Knowles, E.,
& White, T., 1958, The protection of metals with tannins.
Journal
of
the Oil and Color
Chemists Association,
41:
10
-
23.
Logan, J. A., 1989, Tannic acid treatment.
CCI
Notes
9/5.
Ottawa.
Matamala, G., Smeltzer, W. & Benavente, R., 1990, Pine tannin rust con
verter for steel protection by
painting.
Surface Modification Technologies III.
Ottawa.
Morcillo, M.
et al.
1992, Corrosion of rusted steel in aqueous solutions of tannic acid.
The Journal
of
Science and Engineering
-
Corrosion,
48.12: 1032
-
1039.
Olds, Doris
, 1974,
Texas Legacy from the
Gulf.
Austin.
Parkins, R. N. & Pearce, A. S., 1966, Inhibition of corrosion of mild steel by tannins.
National
Association
of
Corrosion Engineering, Second International Congress on Metal Corrosion,
646
-
650.
Pelikán, J. B., 19
66, Conservation of iron with tannin.
Studies in Conservation,
11: 109
-
114. Rosenberg,
S. P., 1987, The inhibition of aqueous corrosion of iron by gallic acid.
Corrosion
Australasia,
11
-
15.
Ross, T. K. & Francis, R. A., 1978, The treatment of rusted steel
with mimosa tannin.
Corrosion Science,
18:
351
-
361.
Shreir, L. L., 1964, Tannins to control corrosion.
New Scientist,
403:
332
-
333.
45
... Artifacts from the 1554 Padre Island Plate Fleet Wrecks are not only on display, but are also the subject of continuous historical and conservation research efforts (Arnold et al. 1995;Carlin et al. 2001;Carlin and Keith 1996). Originally conserved in the early 2002 SAA Meeting Symposium Paper Denver, CO Drolet and Keith 5 1970's, the wrought iron artifacts from these two shipwreck sites are some of the first to be treated using electrolytic reduction. ...
... To counteract the problem, researchers in the Ships of Discovery conservation lab developed a new tannic acid-based three-layer coating that appears to have stopped on-going corrosion in its tracks. The formula for this coating system and how it was developed and applied was published by the Ship of Discovery investigators (Carlin and Keith 1996) and has been useful in similar conservation efforts internationally. ...
... The results of both tests have been published in the International Journal of Nautical Archaeology and Studies in Conservation. (Carlin and Keith 1996;Carlin et al. 2001) ...
... C'est le cas notamment de la cire Cosmolloïd qui a été très utilisée pour la protection de statues en bronze [13,76] et dont une étude a démontré la meilleur efficacité par rapport à d'autres cires microcristallines dans le cadre de la protection d'objets en cuivre naturellement patinés [63]. Cependant il a été démontré que les cires étaient relativement sujettes à la pénétration d'électrolyte rendant leur efficacité temporaire [69,71,77]. ...
Thesis
En milieu extérieur les objets en cuivre et alliages cuivreux subissent des altérations qui entrainent des modifications physiques et esthétiques de l’œuvre. Pour limiter ces dégradations il convient de réduire les interactions entre la couche de corrosion et son environnement d’exposition en appliquant un traitement de protection. Ce travail de thèse propose d’étudier deux types de traitements de protection : une cire microcristalline (cire Cosmolloïd) et des solutions de décanoate (NaC10 et HC10). Pour cette étude des échantillons « modèles » en cuivre naturellement corrodés ont été utilisés afin de développer une méthodologie analytique visant à étudier le mode d’action et la pénétration des deux types de traitement de protection appliqués sur des couches de corrosion. Grâce à cette méthodologie il a été observé que bien que l’action en surface varie en fonction de la nature du traitement, la pénétration quant à elle semble dépendre essentiellement de son mode d’application.Un travail de remise en corrosion des échantillons traités a également été mené en conditions d’immersion et en corrosion atmosphérique sous cyclage d’humidité relative, à l’aide de traceurs isotopiques (D2O et 18O), afin d’évaluer qualitativement et de comparer l’efficacité des différents traitements de protection. Des dégradations sous UVB et lixiviation des traitements ont également permis d’étudier la tenue des traitements dans des conditions d’exposition extérieure. La détection des traceurs isotopiques dans les couches de corrosion par ToF-SIMS et analyses NRA, a mis en évidence des efficacités équivalentes pour les deux traitements. Elles ont en revanche également révélé des différences de tenues face à différentes sollicitations. Tandis que la cire microcristalline se dégrade rapidement sous rayonnement UVB par rapport au traitement HC10, le phénomène de lixiviation semble en revanche dégrader plus rapidement un traitement demeuré en surface de la couche de corrosion.
... Es el caso de los artefactos de hierro fundido de grandes dimensiones. Al respecto se han realizado estudios de deterioro y trabajos de estabilización y conservación (tanto in situ como en el laboratorio) sobre objetos metálicos corroídos de diversos naufragios, como anclas y cañones (MacLeod 1995(MacLeod , 1996Carlin y Keith 1996;Gregory 1999;Australian National Maritime Museum 2000) o sobre otros más pequeños, como balas de cañón (Oddy 1987;Bethencourt et al. 2004), aplicando diferentes técnicas de acuerdo a las características de cada una de las piezas. En otros casos, en cambio, los objetos metálicos cubiertos por las concreciones continúan deteriorándose hasta que las piezas se corroen en un alto porcentaje o totalmente; en estas situaciones la superficie original de las mismas y las marcas o detalles que tuvieran, incluso sus formas, se pierden por completo (Hamilton 1998a). ...
Article
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In this report we address the deterioration processes of iron archaeological artifacts in marine and fluvial-marine environments. Said study will allow us to understand the formation processes of the material record and to consider the archaeological potential of the site in relation to its particular characteristics, as well as the methodology that should be used in each case during the underwater survey, the planning of the preventive conservation of pieces and their later treatment and study in the laboratory. During the course of the work we analyze a set of concretions formed around iron artifacts recovered from a Dutch shipwreck of 17th century in Puerto Deseado, Province of Santa Cruz. We present the results of the methodology applied in the site and during the treatment of the pieces, which were subject of a casting process with resins for its later study.
Article
1. Introduction Neil Brodie 2. Greek Vases for Sale: some statistical evidence Vinnie Norskov 3. Walking a Fine Line: promoting the past without selling it Paula Lazrus 4. The Concept of Cultural Protection in Times of Armed Conflict: from the Crusades to the new Millennium Patrick Boylan 5. Law and the Underwater Cultural Heritage: a question of balancing interests Sarah Dromgoole 6. Negotiating the Future of the Underwater Cultural Heritage Patrick O'Keefe 7. Perceptions of Marine Artefact Conservation and their Relationship to Destruction and Theft Amanda Sutherland 8. Metal Detecting in Britain: catastrophe or compromise? Peter Addyman and Neil Brodie. 9. Britannia Waives the Rules: the licensing of archaeological material for export from the United Kingdom Neil Brodie 10. Mexico's Archaeological Heritage: a convergence and confrontation of interests Enrique Nalda 11. What's going on around the corner? Illegal trade of art and antiquities in Argentina Danial Schavelzon 12. Looting Graves/Buying and Selling Artefacts: Facing reality in the United States Hester Davies 13. Reducing Incentives for Illicit Trade in Antiquities: the US implementation of the 1970 UNESCO Convention Susan Keech McIntosh 14. The Rape of Mali's Only Resource Tereba Togola 15. Dealing with Dealers and Tomb Raiders: the realities of the archaeology of the Ghor es-Safi in Jordan Konstantinos Politis 16. Plunder of Cultural and Art Treasures: the Indian experience S. K. Pachauri 17 Point, Counterpoint Kathryn Walker Tubb
Chapter
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Nothing lasts forever--and that is just as well. Finds from underwater sites are often amazingly well-preserved, but difficult to conserve. More often than not, things that are naturally well-preserved upon initial discovery suffer from poor conservation and neglect subsequently. Practicality dictates that we must be selective about what we choose to preserve.
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Seen from the vastness of space, Earth is a pale blue planet with high white clouds and water covering nearly three-quarters of its surface. Global civilizations emerged on the margins of its vast seas. Watercraft allowed humankind to explore the earth and played a major part in the rise and fall of great empires. Underwater archaeological sites reflect the diversity of human cultures and endeavors, as well as the earth’s environments.
Conference Paper
Artifacts removed from maritime conditions are usually found encrusted together and can enclose items of disparate materials in one mass. To prevent further decay of these artifacts the conservator must keep the objects wet throughout the processes of recovery, cataloguing, and treatment To maintain the integrity of each artifact in conservation after recovery from an archaeological site requires appropriate solutions for immediate and long-term treatment. Iron is the most common material found in encrustations, so the most common conservation solutions chosen are those that will render passive chemical reactions of iron, and yet keep other possible associated materials safe. Unlike iron excavated from terrestrial sites, iron excavated from a maritime environment poses unique conservation requirements. It is essential to stabilize iron immediately upon removal from the archaeological site; even objects assumed to be hollow encrustations should be treated as though they contain an artifact until further study using X-rays or by other examinations. Chloride levels present a singular threat to maritime iron artifacts. The most popular form of iron stabilization is wet storage, but due to the higher chloride levels more intensive measures must be taken than those used traditionally for terrestrial objects. Although it may seem contradictory, a strong oxidizing solution will inhibit iron deterioration since a resistant oxide film will form and turn the iron passive. Alkaline potassium dichromate has been used in the past years successfully in this context, but unfortunately this chemical has high chromate ion levels, which are highly toxic and hazardous to the conservator. A successful and less-toxic method of initial stabilization is use of highly-alkaline inhibiting solutions with a pH 10-13. Subsequently, long-term storage tends to be more effective using chromate solutions, but care must be taken in the use and disposal of the chemicals
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The frontier orbital theory and the inhibitor adsorption theory were applied to the results of the quantum calculations and corrosion rate measurements, respectively, in order to elucidate the chestnut tannin inhibitory action on low-carbon steel corrosion in 2 M HCl. Nine major constituents of chestnut tannin—vescalagin, castalagin, vescalin, castalin, gallic acid, ellagic acid, mono-, di- and trigalloylglucose—were modeled by molecular mechanics, molecular dynamics and semiempirical quantum NDDO method with PM3 parametrization. The geometrical structure, the energy of the highest occupied (HOMO) and lowest unoccupied molecular orbital (LUMO), the HOMO–LUMO energy gap, the distribution of the HOMO electron density and the magnitude and direction of the dipole moment were calculated for each molecule. Molecular reactivity that is related to its adsorbability by the HSAB principle, was studied by calculating the absolute electronegativity, absolute hardness and the electron donating ability. The quantum calculations results, coupled with those derived from the adsorption theory, gave a consistent picture of the investigated corrosion system.
Article
Quoique l'emploi du tanin pour le traitement de l'eau pour protéger les chaudières à vapeur de la rouille ait été suggéré voilà plus d'un demi siècle, sa première application dans la protection antirouille en dehors de cette branche ne remonte qu'aux années 1930. Il y eut un renouveau d'intérêt dans ce procédé quand des objets de fer archéologiques s'avérèrent avoir été particulièrement bien conservés en présence de tanins naturels. Les tanins sont de deux principaux types chimiques: hydrolysables et condensés. Les tanins de ces genres ont pour le fer des propriétés antirouilles, tout d'abord parce qu'ils s'opposent à l'oxydation et ensuite parce que leurs groupes phénoliques forment des complexes avec les métaux. La tanification du fer a l'avantage de ne pas nécessiter des acides en excédent et de ne pas abîmer les matériaux en présence du fer, comme les éléments de construction en bois. L'article décrit à fond des méthodes de tanification par application au pinceau de solutions de tanin, ainsi que le 'vieillissement' oxydant ultérieur. On recommande pour les objets légèrement rouillés, une solution de 200 g de tanin dans un litre d'eau distillée plus 150 ml d'alcool. Pour traiter des objets plus fortement rouillés, il faut ajouter à la première couche de l'acide phosphorique. /// Obwohl man bereits gegen Ende des neunzehnten Jahrhunderts vorschlug, zur Korrosionsverhütung in Dampfkesseln Wasser mit Tannin zu behandeln, wurde Tannin ausserhalb dieses Gebietes als Korrosionsschutz erst in den dreissiger Jahren zum ersten Mal verwendet. Das Interesse an dieser Methode erhielt neuen Auftrieb, als archäologisches Eisen in besonders gut erhaltenem Zustand in der Nähe natürlichen Tannins gefunden wurde. Tannine kommen in zwei chemischen Hauptgruppen vor: als hydrolisierbarer Typ und als kondensierter Typ. Tannine beider Typen besitzen Anti-Korrosionseigenschaften für Eisen. Erstens, weil es sich in beiden Fällen um ein Antioxydans handelt, und zweitens, weil in beiden Fällen die Phenolgruppen Komplex-verbindungen mit Metallen eingehen. Die Stabilisierung von Eisen mit Tanninen hat den Vorteil, dass keine zusätzlichen Säuren in grossen Mengen erforderlich sind und dass Materialien, die mit dem Eisen verbunden sind--etwa Holzgriffe--nicht beschädigt werden. Ausfürlich beschrieben werden die Behandlungs-methoden, die darin bestehen, dass eine Tannin-Lösung aufgestrichen wird und das behandelte Eisen anschlies-send in der Oxydation 'reift'. Für leicht verrostete Gegenstände wird eine Lösung von 200 g Tannin in einem Liter destilliertem Wasser plus 150 ml Alkohol empfohlen. Bei stärkerer Verrostung ist dem ersten Anstrich Phosphorsäure beizugeben. /// Benché sin dalla fine del secolo scorso si fosse avuta una proposta d'impiegare il tannino nel trattamento dell'acqua per impedire la corrosione nelle caldaie a vapore, il primo impiego nei trattamenti anticorrosivi fuori di questo campo non risale che agli anni 30. L'interesse in questo procedimento venne ravvivato quando si riscontrò che il ferro archeologico poteva venire conservato egregiamente in presenza di tannini naturali. I tannini si suddividono in due tipi chimici principali: gl'idrolizzabili e i condensati. I tannini di entrambi i tipi posseggono caratteristiche anticorrosive nei riguardi del ferro, perché sono antiossidanti e perché i loro gruppi fenolici formano complessi con i metalli. Il ferro stabilizzato con i tannini presenta il vantaggio di non richiedere acidi excessivi e di non attaccare i materiali fissati al ferro, quali gli accessori di legno. I metodi di trattamento mediante la spazzolatura di una soluzione di tannino e la successiva 'maturazione' ossidante vengono ampiamente descritti. Per gli oggetti leggermente arrugginiti, si raccomanda una soluzione di 200 grammi di tannino in 1 litro di acqua distillata e 150 ml. di alcool. Per gli arrugginimenti forti, alla prima mano va aggiunto dell'acido fosforico.
Article
The use of tannic acid to inhibit metal corrosion has been a controversial issue, particularly in relation to its application to rusted steel prior to painting. In this work, the protective efficiency of aqueous solutions of tannic acid applied on uncontaminated rusted steel is studied. This is done by investigating the changes undergone by the rust layer, the solubility of the tannate films formed, and the resulting inhibition efficiency. This required the use of a variety of experimental techniques including climatic chamber tests, electrochemical measurements, and x-ray photoelectron spectroscopy. The high solubility of the films and the low anti-corrosion efficiency of the treatment question the suitability of tannic acid solutions for protection of rusted steel prior to painting.
Article
The Corpus Christi Museum of Science and History opened a brand-new exhibit in the spring of 1990. Entitled Shipwreck!, the exhibit tells the story of the three Spanish ships lost on Padre Island in 1554. The Texas Antiquities Committee (TAC) is the state agency that conducted the archaeological and historical work on the 1554 flota and has reported on it in a series of publications (e.g., Arnold and Weddle, 1976). The artifacts remained in the public domain. Therefore, detailed studies of the collection and new contributions to knowledge of the past continue (e.g., Skowronek, 1987). Public television produced a documentary film, Graveyard of the Gulf, that the TAC continues to circulate on videotape to schools and other interested groups. The public enjoyed a major traveling exhibition in the middle and late 1970s. Several exhibit formats are described in another IJNA article focusing on the Austin Children’s Museum exhibit on the wrecks (Arnold and Alsup, 1992). The present article describes the definitive permanent exhibition on the 1554 wrecks. Heeding the proverb about pictures, here the text is brief, and the plans and photographs carry the load.
Article
Tannin treatments for rusted steel have been in use for some years now but their mode of action is poorly understood. Investigations including the use of scanning electron microscopy, electron probe analysis and i.r. spectrophotometry, have determined some fundamental aspects of the behaviour of tannin when protecting steel.It was found that the tannin reacted with the steel to form a crazed outer layer of ferric tannate. About half the rust was impregnated with the ferric tannate or unreacted tannin. Some of the rust was apparently converted to protective magnetite over a period of time. Excessive moisture caused breakdown of the film by removing the unreacted tannin and causing regrowth of the existing rust.
Tannic acid treatment. CCI Notes 9/5 Pine tannin rust converter for steel protection by painting. Surface Modification Technologies III
  • J A Logan
  • Ottawa
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Logan, J. A., 1989, Tannic acid treatment. CCI Notes 9/5. Ottawa. Matamala, G., Smeltzer, W. & Benavente, R., 1990, Pine tannin rust converter for steel protection by painting. Surface Modification Technologies III. Ottawa.
Conservation of Metal Objects From Underwater Sites: A Study in Methods
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Hamilton, D. L., 1976, Conservation of Metal Objects From Underwater Sites: A Study in Methods. Austin.
Article Review: Tannic acid: does it work? Australian Institute for the Conservation of Cultural Materials National Newsletter
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Ashton, J., 1993, Article Review: Tannic acid: does it work? Australian Institute for the Conservation of Cultural Materials National Newsletter, 47.
The protection of metals with tannins
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Knowles, E., & White, T., 1958, The protection of metals with tannins. Journal of the Oil and Color Chemists Association, 41: 10-23.
The Nautical Archaeology of Padre Island
  • Arnold III
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  • Robert S. Weddle