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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 Czech Republic. Outline. Excavated ancient objects and goals of conservation Corrosion layers C onventional conservation technique

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  1. APPLICATION OF LOW TEMPERATURE PLASMAS FOR THE TREATMENT OF ANCIENT ARCHAEOLOGICAL OBJECTSFrantišek KrčmaFaculty of ChemistryBrno University of TechnologyCzech Republic

  2. Outline • Excavated ancient objects and goals of conservation • Corrosion layers • Conventional conservation technique • Plasmachemical reduction of corrosion layers • Deposition of protective coatings

  3. 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 variouscorrosion 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

  4. 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 ???????????? medieval horse shoe

  5. Structureof corrosion layer A – incrustation layers B – corrosion layers C – 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.

  6. Corrosion layers of iron and its alloys • Internalcorrosionlayers – mainlyconsistofmagnetite Fe3O4 • Outerlayers – compositiondepends on thesurrouding, usuallycontainoxides, oxide-chlorides and oxide-hydroxides of iron medieval iron axe

  7. Corrosion layers of copper and bronze • Cu (I) complexes – colourless, except chalcocite (black), cuprite (red) • Cu (II) complexes – red and blue colour ????????????

  8. Corrosion layers of silver 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.

  9. Conventional conservation procedure • 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,…

  10. Plasma chemical reduction of corrosion layers - mid 80's 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).

  11. Contemporary plasmachemical process • Real application in Technical Museum in Brno • Museum of Central Bohemia in Roztoky • Swiss National Museum • Contemporary conservation technology using plasma: • Vacuum drying (80C, 15 hours) • Plasma cleaning in 1 or more cycles (T200C, 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. ofConservatorandRestorer Symposium, 65-69, Teplice 1997. Perlík, D., Proc. ofConservatorandRestorer Symposium, 89-95, České Budějovice 2001. Schmidt-Ott, K. and Boissonnas,Studies in Conservation 31, 29-37 (2002).

  12. 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

  13. 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  200C  changes in iron crystallography, lost of manufacturing information, lost of metal hardness • T  150C  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

  14. Our experimental set up

  15. Plasma process monitoring

  16. Plasma process monitoring

  17. Plasma process monitoring end of plasma treatment Imax/10 Treatment time Rašková, Z., Krčma, F., Klíma, M., Kousal, J., Czechoslovak Journal of Physics 52, Suppl. E (2002).

  18. Plasma process monitoring – multiphase treatment Rašková, Z., Krčma, F., Klíma, M., Kousal, J., Czechoslovak Journal of Physics 52, Suppl. E (2002).

  19. 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.

  20. Chemical composition of the surface layers SiO2 2,32 % Cu2(OH)2(CO)356,36 % Cu2O12,61 % Ag2S 7,12 % AgCl21,59 % After 16 hours of plasma treatment SiO2 0,57 % Cu(CO)34,93 % Cu2O3,74 % Ag2S 0,17 % AgCl0,59 % Ag 36,56 % Cu 53,44 % “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.

  21. Chemical composition of the surface layers medieval iron axe

  22. Chemical composition of the surface layers Before FeO(OH),Fe2O3, H2O, FeOCl After Fe3O4, Fe2O3, CaFe3O5 RBS diagnostics

  23. Chemical composition of the surface layers

  24. Chemical composition of the surface layers

  25. Application on various objects ??? - silver (9th century) Stud – silver (9th century)

  26. Application on various objects ??? - silver stirrup - silver

  27. Application on various objects ear-ring - silver ??? -silver

  28. New approaches • Optimization for the most abundant metallicobjects (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

  29. 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. continuos X pulsed

  30. 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 bronze HCl HNO3 H2SO4

  31. Monitoring at different conditions – iron + HCl

  32. Bronze before (left) and after (right) plasma treatment HCl HNO3 H2SO4

  33. Surface analyses before and after plasma treatment HCl iron bronze

  34. Surface analyses before and after plasma treatment brass 400 W, 25% 400 W, 75%

  35. 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.

  36. Temperature monitoring during the plasma treatment Temperature is measured by thermocouple inside the brass sample.

  37. Temperature monitoring during the plasma treatment brass

  38. Temperature monitoring during the plasma treatment brass

  39. Visual results – iron, HCl 100 W 300 W 25% 75%

  40. Visual results – copper, HCl, 200 W original 25% 50% 75%

  41. Visual results – iron, HCl with sand original 100 W, 100% 200 W, 100%

  42. Matching Box RF Generator 13.56 MHz Optical Emission Spectrometer Optical Fiber HMDSO MFC Rotary Oil Pump Turbomolec. Pump MFC Rotary Oil Pump O2 Deposition of the protective thin layers - HMDSO

  43. Deposition of the protective thin layers - HMDSO

  44. Deposition of the protective thin layers - HMDSO

  45. Oxygen Transmission Rate measurements

  46. Oxygen Transmission Rate measurements - HMDSO

  47. 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

  48. Comparison of parylene layers with standard application of Paraloid B44 varnish • 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 • Parylene • Used modification Parylene C • Thickness 10 microns • Test • According to ISO 9227 in salt chamber Ascot 450 • 300 hours • Temperature of 25°C

  49. Comparison of parylene layers with standard application of Paraloid B44 varnish - iron 300 hours 0 hours Paraloid Parylene

  50. Comparison of parylene layers with standard application of Paraloid B44 varnish - brass 300 hours 0 hours Paraloid Parylene

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