OXYGEN ABSORBERS AND DESICCANTS IN THE PROTECTION OF ARCHAEOLOGICAL
IRON: MAINTAINING SOME CONTROL
A Boccia Paterakis*, M Mariano
Japanese Institute of Anatolian Archaeology
Kaman
Kirsehir
Turkey
1
*Corresponding author: alicepaterakis@yahoo.com
Abstract
Two methods for the stabilisation of excavated iron artefacts are storage in anoxic and in desiccated microclimates.
A series of tests were run by the Conservation Department of the Japanese Institute of Anatolian Archaeology (JIAA)
in Kaman, Turkey, to compare the efficiency of the anoxic and desiccating properties of the RP-A scavengers to the
anoxic properties of the RP-K scavengers (Revolutionary Preservation System [RP System] manufactured by Mitsubishi
Gas Chemical Company, Inc.) and to the desiccating properties of silica gel in Escal bags. In the first three tests, an
archaeological iron artefact from the Kaman collection was included in the test bags. The primary goals of the first
three tests were to compare the desiccation rates of RP-A and silica gel and the time required for the oxygen depletion
to reach 0.1% by RP-K and RP-A. Each test compared one method of anoxic control with a target of <0.1% oxygen
against one method of desiccation with a target of 10% RH. RP-A was found to reduce the RH in the bag to a ‘safe’
level for iron (i.e. 10% RH) in three hours from the start of the test, whereas the silica gel required 14 hours. The fourth
test assessed the efficiency of anoxic protection of mild steel coupons (CR1020) afforded by the RP-K scavengers
in 60% RH for corrosion prevention by exposing the coupons to 60% RH with and without RP-K scavengers.
Environmental Scanning Electron Microscopy with Energy-Dispersive X-ray Spectroscopy (ESEM-EDS) was performed
on three coupons on completion of test four. The goal of these tests was to determine the suitability of silica gel, the
RP-K scavengers, and the RP-A scavengers to create environments with which to protect iron artefacts from corrosion.
Based on the results of the four corrosion tests reported, RP-A can be recommended over silica gel for the immediate
protection of archaeological iron from excavation. RP-K required 48 hours to reach <0.1% oxygen content based on
the oxygen eye: it is recommended that a means more accurate than the oxygen eye indicator be implemented for
monitoring oxygen depletion.
Keywords
Iron, steel, corrosion, ESEM-EDS, volatile corrosion inhibitor
(VCI), relative humidity, anoxic scavenger, Revolutionary
Preservation System (RPS), Japanese Institute of Anatolian
Archaeology (JIAA), Turkey.
Research aims
Preliminary tests were conducted to assess the anoxic and
desiccated properties of RP-A scavengers and silica gel
for the storage of archaeological iron by the conservation
department of the Japanese Institute of Anatolian
Archaeology (JIAA) in Kaman, Turkey (Paterakis & HickeyFriedman, 2011). Encouraging results included long-term
desiccation and anoxic properties of the RP-A scavengers in
EscalTM bags (Paterakis and Hickey-Friedman, 2011). RP-K
scavengers have been suggested for the storage of damp
iron or iron objects containing organic components (Greiff
and Bach, 2000; Mitsubishi, 2011). The efficiency of RP-K
scavengers as oxygen absorbers was also examined to
assess their utility in the storage of archaeological iron.
Introduction
Kaman-Kalehöyük in Central Anatolia, Turkey, was once
a settlement on the ancient Silk Road trade route, dating
from the Bronze Age (2300 BCE) to the era of the Ottoman
Empire (seventeenth century). Excavation began in 1986
and has been continued annually by the Japanese Institute
of Anatolian Archaeology (JIAA) of the Middle East Culture
Center in Japan (MECCJ). The Iron Age spanned the years
900 to 300 BCE at Kaman-Kalehöyük, and small iron finds
constitute the largest group of artefacts excavated annually.
One of the goals for the conservation of archaeological iron
is to prevent the development of the iron oxyhydroxides
(α-FeOOH, β-FeOOH, γ-FeOOH) with accompanying volume
expansion that leads to the physical disruption of the artefact
(Selwyn et al. 1999; Turgoose, 1982). Anoxic and desiccated
storage are two means of conservation to prevent iron
corrosion (Mathias et al. 2004). There is disagreement on a
‘safe’ level of RH for the storage of iron: two ‘safe’ levels that
have been proposed are 12% (Watkinson and Lewis, 2005)
and 18% (Thickett, 2005). The focus of the current study is
10% RH since iron artefacts can corrode even in very low
relative humidity in the presence of oxygen (Keene, 1994).
Soil analysis at Kaman indicated a maximum chloride content
of 335 ppm; sulphate 400 to 800 ppm; maximum nitrate
content of 500 ppm; and pH 6.5 to 8 (Paterakis 2011). The
average annual rainfall is 350 mm (Nesbitt 1993). The iron
artefacts in Kaman are known to contain chlorides based on
soil testing, monitoring of desalination treatments in earlier
years, and actively corroding iron in storage. Active corrosion
can form within days of excavation if the objects are not kept
in desiccated or anoxic conditions at Kaman.
A series of four tests were carried out to determine the
efficiency of the desiccating properties of silica gel, the
combined desiccating and anoxic properties of RP-A, and the
anoxic properties of RP-K for the protection of archaeological
iron artefacts and steel test coupons against corrosion (Table 1).
Iron artefacts of similar size and weight were selected for the
first three tests for the sake of reproducibility and to simulate
real storage conditions. Although changes in corrosion could
not be determined by visual observation, it was important to
185
OXYGEN ABSORBERS AND DESICCANTS IN THE PROTECTION OF ARCHAEOLOGICAL IRON:
MAINTAINING SOME CONTROL
Results of test One: RP-A compared to RP-K
include excavated iron artefacts for the influence that they
may have on the performance of the anoxic scavengers and
desiccants being tested.
Prior to testing the iron artefacts from excavation (which
preserved a metal core) were cleaned of soil leaving the
corrosion products intact. The iron did not undergo any form
of desalination treatment prior to testing, nor did it exhibit
active corrosion. The artefacts were stored with silica gel
until the tests began on July 12, 2011. One iron artefact
was inserted in each test bag. In the fourth test three mild
steel test coupons were used to monitor the development
of corrosion in each test bag. All tests were carried out in
heat-sealed EscalTM bags measuring 20 cm x 22 cm x 3 cm
with a volume 1320 ml (Mitsubishi 1996; McPhail et al. 2003;
Maekawa and Elert, 2003). A Rotronics HL-20 RH/Temp data
logger was placed in each bag to monitor relative humidity
and temperature. Oxygen eyes were placed in the anoxic
bags to visually monitor the oxygen content: pink indicates
< 0.1% oxygen; purple indicates > 0.5% oxygen (Elert and
Maekawa, 2000; Guggenheimer, 2006).
The test coupons (CR1020 (G10200)) were cold rolled mild
steel with a carbon content from 0.17 to 0.23%, 99.08-99.53%
iron, and a hardness of Hv 126. They measured 5 cm x 2 cm
x 0.3 cm and weighed 20 g each. Mild steel (i.e. low carbon
content steel) was selected because this alloy approximates
to the composition of the majority of iron objects analysed
in the collection of the Kaman-Kalehöyük excavation in
Turkey (Masubuchi, 2008). The hardness (Hv) of the Kaman
iron objects was found to range from 98.4 to 264 and most
objects were identified as hypo-eutectoid steel (Masubuchi,
2008). The mild steel test coupons were supplied by the
Metal Samples Company individually wrapped in VCI (Volatile
Corrosion Inhibitor) paper to protect them from corrosion.
The coupons with Grit 120 finish were selected to maximise
the surface area for studying the impact of the storage
environment. The coupons were not heated or annealed prior
to testing to take advantage of the increased corrosion rate of
stressed metal for the 12-week duration of the test.
The test coupons were degreased with acetone prior to
testing according to ASTM G50-76 (ASTM, 1976). The test
bags were cut from long rolls of EscalTM that the manufacturer
provides pre-sealed on both long sides. Two parallel heatseals of 3 mm width each were made along the two cut ends
to seal the bags with a FoodSaver V3240 Vacuum Sealer
(without utilising the vacuum function). Tests One and Two
were conducted in the conservation lab of the JIAA in Kaman,
Turkey. Test Three was carried out in Kaman in a JIAA
storage room without any form of climate control. Test Four
was carried out in the Getty Decorative Arts and Sculpture
Conservation Laboratory in Los Angeles.
Experimental
Test One: RP-A compared to RP-K
In the first test the RP-A scavengers were compared to the
RP-K scavengers for the duration of 5 days (Table 1). On
July 12, 2011 (Test 1A) 3 RP-A3 scavengers were placed in
one Escal bag with one iron blade fragment which had been
excavated on July 6, 2011. Three RP-K3 scavengers (Test
1B) were placed in another Escal bag with an iron horseshoe
fragment that had been excavated on June 30, 2011.
186
The oxygen content was reduced to < 0.1% after 7.5 hours but
before 18 hours had elapsed from the start of the test in each
bag (Table 1). Test 1A with the RP-A scavengers displayed
a stable RH (± 47.5%) during the first 4 hours of the test
after which the RH started dropping dramatically: 13% in the
following 20 minutes (Fig. 1). The rate then slowed considerably:
10% RH was reached after 77 hours (Fig. 2) and 8.6% RH was
reached with the RP-A scavengers by the end of the test.
Figure 1: Comparing desiccation rate in test 1 (RP-A vs.
RP-K), test 2 (RP-A vs. silica gel), test 3 (RP-A vs. silica gel).
Figure 2: Time required to reach 10% RH in test 1 (RP-A vs.
RP-K), test 2 (RP-A vs. silica gel), test 3 (RP-A vs. silica gel).
Test Two: RP-A compared to silica gel
In the second test the desiccating properties of silica gel were
compared with those of RP-A scavengers for the duration of
7 days. On July 16, 2011 (test 2A) 5 RP-A3 scavengers were
placed in one Escal bag with one unidentified iron artefact that
had been excavated on June 21, 2010. One RP-A3 scavenger
is manufactured to maintain an anoxic environment for a
volume up to 300 ml. In test 2B, 105 g silica gel was placed in
another Escal bag with an iron blade fragment that had been
excavated on June 18, 2010. The ratio of 400 g silica gel to
5000 ml air volume was used to determine the appropriate
weight of silica gel in the test (Lafontaine, 1984).
OXYGEN ABSORBERS AND DESICCANTS IN THE PROTECTION OF ARCHAEOLOGICAL IRON:
MAINTAINING SOME CONTROL
Results of test Two: RP-A compared to silica gel
The oxygen content was reduced to < 0.1% after 8 hours but
before 18 hours had elapsed from the start of the test in test
2A bag (Table 1). Test 2A bag with RP-A reached maximum
weight loss after 48 hours whereas test 2B bag with silica gel
reached maximum weight loss after 24 hours. Test 2B with
silica gel required 24 hours to reach 10% RH whereas test 2A
with RP-A required 48 hours to reach 10% RH (Fig. 2). Silica
gel reached a RH of 10% in half the time required by the RP-A
in this test. Accelerating the rate of desiccation by increasing
the quantity of silica gel was assessed in test Three.
Results of test Three: RP-A compared to (doubled)
silica gel
The RH in test 3A with RP-A reached 15% RH after one hour,
10% RH after three hours, and 5% RH after 18 hours (Table
1) (Fig. 1). Test 3B with silica gel required 7 hours to reach
15% and 14 hours to reach 10% RH (Fig. 2). Comparing the
results obtained with RP-A in test 3A and test 2A, we find that
the RP-A in test 3A required much less time than in test 2A
to reduce the RH: 3 hours to reach 10% in test 3A instead of
48 hours in test 2A (Fig. 2). This discrepancy cannot readily
be accounted for. Comparing the performance of silica gel in
test 3B with test 2B during the first 7 days of testing reveals
that increasing the weight of silica gel increased the rate of
moisture absorbency in these tests. The temperature and
RH in the storeroom showed extremes of -5° C/ 88 % RH in
February and 31° C/ 28% RH in July.
Test Four: RP-K at 60% RH
The anoxic properties of RP-K were tested by exposing
mild steel test coupons to 60% RH in EscalTM bags (Fig. 4).
Test 4A bag contained 3 mild steel test coupons, 5 RP-K3
scavengers, 2 oxygen eyes, 100 grams Rhapid Gel (silica
gel) preconditioned to 60% RH, and a Rotronics HL-20 RH/
Temp data logger. Test 4B bag contained 3 mild steel test
coupons, 100 grams Rhapid Gel (silica gel) preconditioned
to 60% RH, and a Rotronics HL-20 RH/Temp data logger.
Test 4C bag was the control containing 3 mild steel test
coupons. The test bags were kept in museum ambient
conditions (21-23°C) for the duration of 12 weeks. The
selection of RH 60% was based, in part, on the eqRH of
iron (II) chloride, that is, 56% (Selwyn et al, 1999). ESEM-EDS
was used to examine some of the coupons after testing.
Table 1: Description of corrosion tests.
Test Three: RP-A compared to (doubled) silica gel
This test parallels test Two with one altered variable: the
amount of silica gel was doubled (Fig. 3). On July 27, 2011
(test 3A) 5 RP-A3 scavengers were placed in one EscalTM
bag with one unidentified iron artefact that had been
excavated on June 28, 2010. 200 g silica gel (test 3B) were
placed in another Escal bag with an iron blade fragment that
had been excavated on June 29, 2010. The test ran for 12
months in the Kaman storeroom.
Figure 4: Mild Steel coupons after 12 weeks corrosion testing.
a) Test 4A with RP-K® scavengers 60% RH, b) Test 4B without
anoxic protection 60% RH, c) Test 4C Control (width of image
on left is +-6 cm, width of image on right is +-2 cm.
Figure 3: Test 3 comparing dry silica gel to RP-A scavengers,
area of photograph measures approximately 44 cm across.
187
OXYGEN ABSORBERS AND DESICCANTS IN THE PROTECTION OF ARCHAEOLOGICAL IRON:
MAINTAINING SOME CONTROL
Results of test Four: RP-K compared to silica gel
at 60% RH
Two days were required for the oxygen eyes in test 4A bag to
change from purple to pink indicating that the oxygen content
had reached less than 0.1% in the bag (Table 1). During the
first two days alteration could be seen with the naked eye to
develop on the test coupons in test 4B bag (with silica gel
preconditioned to 60% RH only, no RP-K scavengers). An area
rectangular in shape was seen to form on coupon 66 in test
4B bag as a result of edge effects since the coupons were not
annealed (Fig. 4). From the third day and until the completion of
the test after 12 weeks little visible change could be noted. The
surface of the test coupons from test 4B bag was noticeably
duller than the surface of the test coupons from the other
test bags when examined with the naked eye and the ESEM.
Weighing the test coupons before and after corrosion testing
revealed no change within three decimal places.
Comparison of RH in the three test bags shows that the RP-K
scavengers affected the RH. Test 4A bag with RP-K required
6.5 days to surpass 56% RH (eqRH of iron (II) chloride)
and more than 3 weeks to reach 60% RH. The RH initially
dropped in the bag from the first reading of 49.2% to 42.6%
over the first 1.5 hours and then slowly increased, reaching
60 % after 23 days (Fig. 5). Test 4B bag (without the oxygen
scavengers) showed a rapid increase in RH from 50% at
the start to more than 56% after 2 hours (Fig. 5). 59.5% was
reached in 24 hours and the target 60% RH was reached 33
hours from the start of the test.
Figure 6: ESEM
SE image of coupon 66 from test 4B in 60% RH after testing
showing light-colored particles of different sizes functioning as
nucleation sites surrounded by carbon-rich (dark) zones
Figure 7a: Secondary Electron image showing vertical dark
carbon-rich area with two light-colored particles (slag and
VCI?) interspersed (control coupon after testing).
Figure 5: Comparing RH in test 4 (60% RH with and
without RP-K) during first 3 days.
The fact that the oxygen scavengers held the level of RH
below 50% for the first 24 hours and below 60% RH for 23
days aided the corrosion protection of the test coupons.
Examination with the ESEM revealed that the degree of
corrosion and alteration was greater in test 4B (no anoxic
protection) than test 4A (RP-K).
One coupon (66) from test 4B (without anoxic protection) (Fig.
6), one coupon (61) from test 4A (with RP-K), and one control
coupon (Fig. 7a and 7b) were examined with Secondary
Electron (SE) and Back-Scattered Electron (BSE) imaging
and analysed with EDS (Fig. 4). ESEM-EDS incorporating a
188
Figure 7b: Back Scattered Electron image of Figure 7a revealing
the low atomic mass of carbon-rich area with two particles (very
dark among the carbon-rich area) (control coupon
after testing).
Philips XL30 ESED-FEG instrument fitted with an Oxford INCA
EDS analysis system was used. The operational parameters
were high vac mode, 20 kV, and spot size of three. The SE
detector was an Everhart-Thornley. Light-colored particles of
9-20 microns surrounded by dark areas were visible in SE
imaging on all three coupons (Fig.7a and 7b). EDS analysis
of the light-colored particles identified iron, silica, carbon,
magnesium, manganese, calcium, potassium, sodium,
chloride, and oxygen. Carbon, iron, and oxygen were
detected in all seven particles analysed. The dark areas were
identified as carbon-rich.
OXYGEN ABSORBERS AND DESICCANTS IN THE PROTECTION OF ARCHAEOLOGICAL IRON:
MAINTAINING SOME CONTROL
Discussion
The composition of corrosion products on steel in aqueous
medium depends on pH, temperature, oxygen content, surface
treatment of the steel, and presence of ions in solution (Music
et al. 1993). The pH of the layer of moisture on the surface
of the steel is influenced by contaminants such as chloride
that influence the dissolution of the oxyhydroxide surface. RH
exceeding 50% causes the formation of multilayers of water
with increased corrosion rates (Scott and Eggert, 2009). The
rust that forms on low carbon steel tends to adhere poorly to
the substrate and to be of a porous nature (Scott and Eggert,
2009). These facts compounded by the rough surface of the
test coupons (Grit 120 finish) contribute to increased access
of moisture and oxygen to the surface of the metal.
The test coupons were supplied wrapped individually in
Volatile Corrosion Inhibitor (VCI) paper by the manufacturer.
Although a recommended method, the degreasing
procedure with acetone prior to testing did not remove all
of the VCI protective coating (National Corrosion Service,
2013). VCIs are absorbed preferentially at various locations
depending on the cleanliness and surface roughness of
metal. Well-coated areas will tend to be cathodic as the
VCI inhibits anodic reaction, but the areas with lower VCI
concentration will have greater capacity to absorb moisture
and will be more prone to corrosion. Due to the rough
surface of the test coupons an uneven VCI coating can
be expected. A voltage gradient is established between
peaks and valleys on rough surfaces. The higher the voltage
gradient, the stronger is the adsorption of the VCI. Moisture
accumulates in the valleys where more corrosion occurs. The
shiniest areas of the test coupons demonstrate the areas with
the strongest adsorption of the VCI.
The light-colored particles are tentatively identified as bits
of slag inclusions, dislodged during the machining and
sanding of the test coupons, engulfed in organic matter
from the VCI or organic material used to clean the test
coupons after manufacture (Fig. 7a and 7b). These particles
function as nucleation sites that can establish a differential
adsorption environment leading to the formation of cathodic
and anodic sites on the metal causing localised corrosion in
the presence of moisture (MacLeod, 2013). Chloride would
undoubtedly alter the pH of the moisture layer on the surface
of the metal and contribute to the electrochemical corrosion
process. These light-colored particles are associated with
dark areas of the steel that are most prolific on coupon 66
from test 4B (without anoxic protection) (Fig. 6).
All three test coupons analysed by EDS were found to have
areas rich in carbon that appeared dark in the SE images
and darker still in the BSE images on account of their low
atomic mass (Fig. 7a and 7b). There were two sources of
carbon in the test coupons. Carbon was present as pearlite,
composed of ferrite and bands of iron carbide or cementite.
On account of the higher carbon content of the pearlitic
phases, they are less reactive and tend to form cathodic
sites and thereby promote corrosion around them. These
differences in the composition of the microstructure can
promote electrochemical corrosion even in low levels of
oxygen. The other source of carbon on these coupons was
the VCI referred to in the preceding paragraph. Test 4B
(60% RH and no anoxic protection) produced the largest and
most numerous carbon-rich areas (Fig 6). It is suggested
that as the metal corroded the residual (carbon-containing)
VCI on the surface migrated to active corrosion zones that
exhibited stronger chemical attraction (MacLeod, 2013). As
the VCI could not dissipate from the heat-sealed test bags
it contributed to this migration. This proposed migration
mechanism responded to the changes in reaction and to the
RH (60%) in test 4A and test 4B. This RH was sufficiently
high to affect the attachment of the VCI coating on the metal
surface resulting in changes to the surface pH. As changes
occur in the surface pH of the anodic and cathodic areas,
the chemistry of the adsorption of the VCI onto the metal
changes, as do the corrosion products (MacLeod, 2013).
Many VCIs such as cyclohexylamine have water insoluble and
water soluble components. The latter will be affected by the
high RH in the presence of oxygen. This proposed migration
mechanism may account for the increased formation of
carbon-rich areas in test 4A and test 4B in 60% RH.
Conclusions
The first two tests demonstrated that the ratio of scavenger
weight to airspace volume can impact the efficiency of
desiccation. Although doubling the amount of silica gel nearly
doubled the rate of RH reduction (10% RH in 14 hours), the
silica gel could not match the rate achieved by the RP-A
(10% RH in 3 hours) in test 3.
Although the RP-K scavengers afforded some protection
by retarding the increase in RH and by lowering the oxygen
content, alteration did take place in test 4A. The bulk of this
alteration most likely occurred in the first two days before the
oxygen content was lowered to < 0.1%. Similar findings have
been reported regarding the delay in oxygen reduction by
RP-K (Guggenheimer and Thickett, 2008).
Similar corrosion mechanisms apply in test 4A and test
4B: differential corrosion from dissimilar composition of
the microstructure (pearlite and ferrite), nucleation of
slag inclusions on the surface, and migration of the VCI
contributing to concentrated areas of carbon on the surface.
The carbon-rich areas with nucleation slag particles are most
abundant in test 4B in 60% RH (without anoxic protection).
The incomplete removal of the VCI during preliminary
cleaning with acetone may have exacerbated corrosion.
The time required for maximum oxygen depletion achieved
by the RP-A and RP-K scavengers varied from < 18 hours
to 48 hours. These figures are rough estimates based on the
color change of the oxygen eye that could not be monitored
on a 24-hour basis. More accurate methods for monitoring
oxygen depletion are preferable, such as the Mapcheck
oxygen meter (Thickett et al. 2008) and the Gas Sensor
Solutions GSS 450 oxygen analyser (Thickett et al, 2011).
Based on the desiccation performance of RP-A in test 3,
RP-A can be recommended over silica gel (even if doubling
the prescribed amount of silica gel) for the most rapid
desiccation of the storage environment. RP-A provides the
added advantage of anoxic protection that has been shown
to be of shorter duration than its longer-acting desiccation
properties (Paterakis and Hickey-Friedman, 2011).
Further studies should be carried out to monitor oxygen
depletion over the first 24 and 48 hours to determine when a
189
OXYGEN ABSORBERS AND DESICCANTS IN THE PROTECTION OF ARCHAEOLOGICAL IRON:
MAINTAINING SOME CONTROL
‘safe’ oxygen level is reached by RP-A and RP-K and to test
the impact of increasing the ratio of RP-K scavenger weight
to air volume. It is recommended that a means more accurate
than the oxygen eye indicators be implemented for monitoring
oxygen depletion.
Acknowledgments
We wish to express our thanks to Dr. Sachihiro Omura,
Director of the Japanese Institute of Anatolian Archaeology;
to Dave Carson and Joy Mazurek for the ESEM-EDS analysis
at the GCI; to Julie Wolfe, Jane Bassett, and Brian Considine
in the Getty Decorative Arts and Sculpture Conservation
Laboratory; and to Dr. Ian MacLeod for his helpful comments
regarding the test results.
Materials and suppliers
Heat-sealer, FoodSaver V3240 Vacuum Sealer,
http://www.foodsaver.com/index.aspx
http://www.target.com/
Mild Steel test coupons, Metal Samples Company,
P.O. Box 8, 152 Metal Samples Road, Munford, AL 36268
www.alspi.com
Rhapid Gel, preconditioned silica gel, available from Talas,
330 Morgan Ave., Brooklyn, NY 11211
http://talasonline.com/
Rotronics HL-20 RH/Temp data logger, Rotronic Instrument
Corp., 135 Engineers Road, Suite 150, Hauppauge, NY 11788
http://www.rotronic-usa.com/
RP System: Escal roll, RP-A, RP-K, oxygen indicators
available from Mitsubishi Gas Chemical America, Inc., 655
3rd Ave., New York, New York 10017
www.mgc-a.com
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