APPLICATION OF LOW TEMPERATURE PLASMAS FOR THE TREATMENT OF ANCIENT ARCHAEOLOGICAL OBJECTS František Krčma Faculty of Chemistry Brno University of Technology

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

Our experimental set up

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


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




medieval iron axe


BeforeFeO(OH), Fe2O3, H2O, FeOCl

AfterFe3O4, Fe2O3, CaFe3O5

RBS diagnostics



Application on various objects

Stud – silver (9th century)??? - silver (9th century)


??? - silver stirrup - silver


??? -silver ear-ring - silver

• 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

Monitoring at different conditions – iron + HCl

Bronze before (left) and after (right) plasma treatment

HCl HNO3 H2SO4

Surface analyses before and after plasma treatment

HCliron bronze

Surface analyses before and after plasma treatment

brass

400 W, 25% 400 W, 75%

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 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


brass


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

Visual results – iron, HCl

25%

75%

100 W 300 W

Visual results – copper, HCl, 200 W

25% 75%50%original

Visual results – iron, HCl with sand

100 W, 100%original 200 W, 100%

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



Oxygen Transmission Rate measurements

Oxygen Transmission Rate measurements - HMDSO

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


of Paraloid B44 varnish - iron

0 hours 300 hours

Paraloid

Parylene


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

Thank you for your attention

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APPLICATION OF LOW TEMPERATURE PLASMAS FOR THE TREATMENT OF ANCIENT ARCHAEOLOGICAL OBJECTS František Krčma Faculty of Chemistry Brno University of Technology