application of low temperature plasmas for the treatment of ancient archaeological objects...
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APPLICATION OF LOW TEMPERATURE PLASMAS FOR THE TREATMENT OF ANCIENT ARCHAEOLOGICAL OBJECTS
František Krčma
Faculty of ChemistryBrno University of Technology
Czech Republic
Outline
• Excavated ancient objects and goals of conservation
• Corrosion layers
• Conventional conservation technique
• Plasmachemical reduction of corrosion layers
• Deposition of protective coatings
Present state of ancient metallic objects
Materials:• iron and its alloys• copper• silver, gold, etc.• alloys – bronze (Cu + Sn), brass (Cu + Zn),…
The objects are commonly affected by various corrosion kinds with different intensity. The given corrosion state of object depends on:• artifact material• artifact manufacturing technology• time of storing before excavation• composition of corrosive surrounding• storage between excavation and conservation• precedent conserving procedures
medieval horse shoe ????????????
Goals of conservation
• elimination of the corrosive agents• to remove different stimulators of corrosion
(mainly chlorine ions) from corrosion layers • to remove or reduce the corrosion layers
(Bronze, copper – patina layer???)• to protect object from further corrosion during its
storage
Structure of corrosion layer
A – incrustation layersB – corrosion layersC – metal core
Cut of a medieval “silver” coin
Klíma, M., Ptáčková, M., Soudný, M. and Rusnák, V., Metodical Paper –Proc. of Symposium of conservation and restoration of the national cultural heritage, 81-118, Luhačovice 1994.
-FeOOH geotite
β-FeOOH akaganeite
-FeOOH lepidokrocite
-Fe2O3 hematite
Fe2SiO4 fayalite
Fe3(PO4)2 8H2O vivianite
FeCO3 siderite
Fe(OH)SO4
2H2O
FeOCl
FeCl2
• Internal corrosion layers – mainly consist of magnetite Fe3O4
• Outer layers – composition depends on the surrouding, usually contain oxides, oxide-chlorides and oxide-hydroxides of iron
Corrosion layers of iron and its alloys
medieval iron axe
Corrosion layers of copper and bronze
????????????
• Cu (I) complexes – colourless, except chalcocite (black), cuprite (red)
• Cu (II) complexes – red and blue colour
Cu2O cuprite
Cu2CO3
(OH)2
malachite
CuCl2 2H2O eriochalcite
CuFeS2 chalcopyrite
CuO, Cu(OH)2
Cu2Cl(OH)3
CuS, Cu2S
Cu4(OH)6SO4 2H2O
Corrosion layers of silver
-Ag2S
Ag2O
AgCl
Ag3CuS2
AgCuS
Medieval “silver” coin
Klíma, M., Ptáčková, M., Soudný, M. and Rusnák, V., Proc. of Symposium of conservation and restoration of the national cultural heritage, Metodical Paper , 81-118, Luhačovice 1994.
• Mechanical cleaning sanding, ultrasonic needle, dental drill, etc.
• „Desalination“ dipping the metal artifact in: distilled water (1 – 4 months), sodium sulphite (cca 6 months)
• Drying• Fine mechanical cleaning• Final conservation
tanate, varnish, wax,…
Conventional conservation procedure
Veprek, S., Patscheider, J. and Elmer, J., Plasma Chemistry and Plasma Processing 5, 201-209 (1985).Veprek, S., Eckmann, Ch. and Elmer, J., Plasma Chemistry and Plasma Processing 8, 445-465 (1988).
Plasma chemical reduction of corrosion layers - mid 80's
Contemporary plasmachemical process
Real application in Technical Museum in BrnoMuseum of Central Bohemia in RoztokySwiss National Museum
Contemporary conservation technology using plasma:
• Vacuum drying (80C, 15 hours)
• Plasma cleaning in 1 or more cycles (T200C, 2-6 hours, H2 or H2/Ar mixture)
• Mechanical/chemical cleaning between the cycles
(sanding, ultrasonic bath, Chelaton3, citric acid, etc.)
• „Desalination“
• Fine mechanical cleaning and final conservation
Havlínová, A., Perlík, D., Proc. of Conservator and Restorer Symposium, 65-69, Teplice 1997.Perlík, D., Proc. of Conservator and Restorer Symposium, 89-95, České Budějovice 2001.Schmidt-Ott, K. and Boissonnas, Studies in Conservation 31, 29-37 (2002).
Advantages of plasmachemical treatment
• Dry removal of chlorine ions• Easier removal of the incrustation and corrosion
layers• Shorter desalination procedure• Possibility of full reduction of some corrosion kinds
up to the pure metal• Applicability for the hollow or very broken objects• Full excavation of the surface relief with many
details• Passivation and stabilization of object
Disadvantages of plasmachemical treatment
• Method is not applicable for the fully corroded samples (anisotropic stress at elevated temperature)
• Patina removal on copper and bronze object
(esthetic as well as historical problem)
• T 200C changes in iron crystallography, lost of manufacturing information, lost of metal hardness
• T 150C changes in copper alloys composition and crystallography, lost of manufacturing information, lost of metal hardness
• Financial expenses of experimental device
• Optimal conditions are unknown
• How to measure the real temperature of object
Plasma process monitoring
)(OHrel
312
305
rel tfII
Treatment time
end of plasma treatmentImax/10
Rašková, Z., Krčma, F., Klíma, M., Kousal, J., Czechoslovak Journal of Physics 52, Suppl. E (2002).
Plasma process monitoring – multiphase treatment
Rašková, Z., Krčma, F., Klíma, M., Kousal, J., Czechoslovak Journal of Physics 52, Suppl. E (2002).
Chemical composition of the surface layers
SEM-EDX
“silver” coin
Klíma, M., Ptáčková, M., Soudný, M. and Rusnák, V., Proc. of Symposium of conservation and restoration of the national cultural heritage, Metodical Paper , 81-118, Luhačovice 1994.
SiO2 2,32 %
Cu2(OH)2(CO)3 56,36 %
Cu2O 12,61 %
Ag2S 7,12 %
AgCl 21,59 %
SiO2 0,57 %
Cu(CO)3 4,93 %
Cu2O 3,74 %
Ag2S 0,17 %
AgCl 0,59 %Ag 36,56 %Cu 53,44 %
After 16 hours of plasma treatment
“silver” coin
Chemical composition of the surface layers
Klíma, M., Ptáčková, M., Soudný, M. and Rusnák, V., Proc. of Symposium of conservation and restoration of the national cultural heritage, Metodical Paper , 81-118, Luhačovice 1994.
Chemical composition of the surface layers
BeforeFeO(OH), Fe2O3, H2O, FeOCl
AfterFe3O4, Fe2O3, CaFe3O5
RBS diagnostics
• Optimization for the most abundant metallic objects (iron, copper, bronze, brass) using model samples with identical material and corrosion – it allows to compare different treatment conditions
• Temperature measurement directly inside the model sample
• Decrease of the mean energy using plasma in pulsed regime
New approaches
continuos X pulsed
Pulsed regime
duty cycle = 100 % • tON / (tON + tOFF)
Peff = Ptotal • tON / (tON + tOFF)
High energy in pulse but the mean energy is significantly lower and sample temperature is also lower.Moreover the process kinetics is different.
Preparation of model samples
• Surface with defined roughness - sanding
• Material characterization of metal (SEM-EDX)
• Preparation of corrosive layers
(HCl, HNO3 and H2SO4)
• Storage for 7 days in dessicator
• SEM-EDX analyzes of surface corrosion
bronzeHClHNO3 H2SO4
Temperature monitoring during the plasma treatment
brass
300 W – 50% 300 W – continuous
Plasma temperature is nearly independent on conditions but sample temperature is significantly different.
0 20 40 60 80 100
300
400
500
600
700
800
tem
pera
ture
[K
]treatment time [min]
temperature of sample rotational temperature
0 20 40 60 80 100
300
400
500
600
700
tem
pera
ture
[K
]
treatment time
temperature of sample rotational temperature
Temperature monitoring during the plasma treatment
Temperature is measured by thermocouple inside the brass sample.
0 20 40 60 80 100 120
300
325
350
375
400
425
sam
ple
tem
pera
ture
[K
]
treatment time [min]
100W, 25% pulse 100W, 50% pulse 100W, 75% pulse 100W, continual
0 20 40 60 80 100
300
350
400
450
500
sam
ple
tem
pera
ture
[K
]
treatment time [min]
300W, 25% pulse 300W, 50% pulse 300W, 75% pulse 300W, continual
Temperature monitoring during the plasma treatment
brass
100 W 200 W 300 W 400 W
100% 149 206 229 188
75% 122 189 185 239
50% 108 129 152 197
25% 60 83 102 121
Deposition of the protective thin layers - HMDSO
RF Generator13.56 MHz
Matching Box
O2
HMDSO
Optical FiberOptical Emission
Spectrometer
MFC
Rotary Oil Pump
Turbomolec. Pump
MFC
Rotary Oil Pump
Application of parylene (poly-para-xylylene) layers
Parylene coatings are • chemically inert, • conformal • transparent • with excellent barrier properties • relatively small adhesion
Preparation by classical CVD from dimer
Comparison of parylene layers with standard application
of Paraloid B44 varnish
Parylene• Used modification Parylene C• Thickness 10 microns
Paraloid B44• Samples dried at 100°C for 4
hours under vacuum• 2 layers of varnish (delay 6
hours), dried at ambient air• Solution of 4% for iron samples• 3% of benztriazole in ethanol
added for other materials
Test• According to ISO 9227 in salt chamber Ascot 450• 300 hours• Temperature of 25°C
Comparison of parylene layers with standard application
of Paraloid B44 varnish - iron
0 hours 300 hours
Paraloid
Parylene
Comparison of parylene layers with standard application
of Paraloid B44 varnish - brass
0 hours 300 hours
Paraloid
Parylene
Future research
• Application of gas mixtures at low pressure RF discharge
• Application of sample bias at low pressure RF discharge
• Combination of active discharge with post-discharge
• Construction of underwater plasma jet based on capillary discharge
• Deposition of diamond like carbon thin layers
• Deposition of gradient thin layers and multilayer systems
• Study of thin layers stability
• Colorimetry of protecting layers
• Study of protecting coatings removal
• Verification of all processes and their transfer to the technology
Research staff and students
Assoc. Prof. František KrčmaDr. Zdenka KozákováDr. Věra MazánkováDr. Radek PřikrylDr. Martin ZmrzlýDr. Lukáš RichteraKarel ŠtefkaIng. Drahomíra Janová - FMIDr. Hana Grossmannová -TMDr. Martin Hložek - TMIng. Alena Selucká - TMIng. Jitka Slámová - TMIng. Věra SázavskáIng. Michal ProcházkaIng. Lucie HlavatáIng. Lenka HlochováIng. Petra FojtíkováIng. Lucie ŘádkováIng. Radka BalaštíkováIng. Lucie Němcová
Ing. Přemysl MenčíkIng. Ondřej SedláčekBc. Adam KujawaBc. Lucie BlahováBc. Jakub Horák Finished studentsDr. Zuzana RaškováIng. Kamil BrandejsIng. Marek CihlářIng. Nikola ZemánekIng. Tereza ŠimšováIng. Osvald Kozák
Main collaborationTechnical Museum, BrnoFaculty of Mechanical Eng., BUT, BrnoComenius University, BratislavaInst. of Nuclear Physics, CAS, Řež
All this work is supported by Czech Ministry of Culture
National Identity Research Program
Plasma Chemical Processes and Technologies for Conservation of Archaeological Objects
1. 2. 2011 – 31. 12. 2015 € 1 000 000