Degrigny, C. Cleaning CH Artefacts Overview. 2010

8/7/2019 Degrigny, C. Cleaning CH Artefacts Overview. 2010

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Leiden, 11-15 January 2010

Cleaning of CH artefactsAn overview

Christian Degrigny


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End of 19th c.

F. Rathgen

- Berlin: cleaning ofencrusted coin treasuresby electrochemicalreduction

A bit of history

1950s

Plenderleith (UK)

- use of constantcurrent I to clean &stabilise iron & copperartefacts- otherwise cleaning byelectrochemicalreduction (Zn/NaOH)

XXX

XXX

-

+VI


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1970s

Hamilton (USA)

Techniques applied to marine

metal artefacts

1980s

Pearson andMacLeod (Australia)

Use of constant V (voltage)for the treatment of marinemetal artefacts


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Since 1990s

-better control of the reactions takingplace on the metal surface (systematic useof reference electrodes when establishing

electrochemical parameters);

- more research on conservation

protocols and determination of treatmentparameters;

- development of new approaches and

design of new tools.

Corrosion scientists

Conservators


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- mechanical (superficial damage:

tarnished silver, marine cast iron, leadartefacts)

- heat treatment (modification of themicrostructure due to overheating)

Often applied in combinationwith other treatments

An alternative to othertreatments?

ArcAntique


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Ecorr (corrosion potential)

ArtefactM

M Mn+ + ne-: oxidation

2H2O + O2d + 4e- 4OH-: reduction

All reactionstaking place at

the metal

surfacewithout external

current

+ -

Aqueoussolution

Choice of the electrolyte:behaviour of metals in solution

EVTEK


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V

Artefact

Referenceelectrode

Ecorr/SHE= V + Eref/SHE

Ag/AgCl: 0.2V/SHE

Calomel: 0.242V/SHE

Ref

Ecorr measurement

V

Artefact

Local measurement


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Monitoring of Ecorr with time in acertain solution

Ecorr

time

Passivation

Corrosion

No reaction


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Influence of the pH of the solution

Several results fromPourbaix diagrams:

-Prediction of possiblereactions within the field ofstability of the aqueoussolution (normally no cleaning);-Choice of the appropriateelectrolyte (pH) to favour thepassivation effect;

Pourbaix diagrams arecalculated from

thermodynamics

Fe

Field of stability of

the aqueoussolution


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7 14E/SHE

0

1

-1

7 14E/SHE

0

1

-1

7 14E/SHE

0

1

-1

7 14E/SHE

0

1

-1Au

pH

Cu

pH

Fe

pH

Al

pH

I

C

I

CCP

I

CC P

I

C P

From gold to aluminium


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Modification of electrochemical parameterswith external current

Potentiostat

Power supply

Battery

Electrolytic cleaning

ArcAntique

MAH, Genve


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Definition of experimental conditions:voltammetric curves

Scanning of potentials in the cathodicand anodic regions

Determination of treatments parameters Follow up of treatments (I=f(t))

X

XX

X

X

X

XX

X

X

CE

Cathode

RE

WEPC

Potentiostat

Pourbaix diagrams cannot be used


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Cathodic scanning: reduction reactions

Ecorr

Mn+ + ne- M

Ecp

2H2O + O2 + 4e- 4OH-

Oxygen dissolved

I (A)

E (V)

EH2

2H2O + 2e- 2OH- + H2


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Anodic scanning: oxidation reactions

I (A)

E (V)Eap EO2

MMn+ + ne-

2H2O O2 + 4H+ + 4e-

Ecorr


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Coupons to be used for the definitionof parameters ?

Real artefacts

?

ArcAntique


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Use of artificial coupons

Silver tarnish

Ecorr

I (A)

E (V)

Steel oxidised

ArcAntique

ArcAntique


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Practically use of a generator

X

X

XX

X

X

X

XX

X

+

_A

Application of specific potentials

(given in the literature) Measurement of thecorresponding I

but

Monitoring is difficult (with basicequipment)

V

Cathode

anodes

HECR Arc


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Art-Mtal

CS1: cleaning of tarnished silver

Saxophone in silveredbrass, Muse de laMusique, Paris

Daguerreotype

pH

E/SHE

0

-1

1 Ag+Ag2O3

Ag2O

7

AgAg2S

Ag

10

Sodium sesquicarbonate

?


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Ecorr

2H2O + O2 + 4e- 4OH

Ag

Ag2S

2H2O + 2e- 2OH- + H2

I (A)

E (V)

Ag

H2

Ag

Ag+ + e- Ag

Red. Ag

Determination of the parameters

Potential range


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Monitoring of tarnish removal

I (A)

t (min)

Cathodic reductionat EpmaxEpmax

1

2

34


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Art-Mtal

Saxophone insilvered brass, Musede la Musique, Paris

Art-Mtal

After mechanicalcleaning

In practice

(in 1% (w/v) sodium sesquicarbonate, pH=10)

Silver vase,private collection,CH

HECR Arc


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Problem of silver alloys Ag-Cu

Ag2SAg

Cu2OCu2S

Ag-Cu

Red. AgRed. Cu

I (A)

E (V)Ecorr

Heterogeneous surface after treatment


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MAH, Genve

MAH, Genve

Silver

Gilt silver

Cleaning of tarnished gilt silver


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2 steps 1. Reduction

(Ag on Ag and Au) 2. OxidationEcorr

Ag2SAg

Ag Ag+I (A)

E (V)

MAH, Genve

In buffered NaNO3 solution


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Ag

Ag

reduced

Ag

Au

Ag2S

Reduction of Ag2S and AgCl inbuffered NaNO3, vb=1mV/s

AgCl

1st step: cathodic polarisation


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Before

ArcAntique

During

ArcAntique


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Results on several plates

Validation of heterogeneitiesdetected from visual observation


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AgAg+

I (A)

E (V)

Epmax

2nd step: anodic polarisation


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when the metal cannot be immersed

Tarnished gilt silverstags (17th c.)

Definition of the parameters ofthe treatment with a potentiostat

Use of the electrolytic pen with apower supply to clean thetarnishing Collaboration with GCI

M

+ -

V

J. Paul Getty Museum

GCI





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CS2: Cleaning of marine objects

EVTEK

ArcAntique

ArcAntique

Provoking thebubbling of H2

KOH

Iron-based objects


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00,2

0,4

0,6

0,8

1

1,2

1,4

1,6

1,8

2

0 200 400 600 800 1000 1200 1400 1600

time (h)

0

0,5

1

1,5

2

2,5Voltage (V)

voltage

I

I

Intensity (A)

Monitoring of the voltage and current

At -1V/SHE in KOH solution


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+

DC powersupply

Stainless steel tank

Stainless steelmesh (anode)

Electrolyte (KOH1%)

Plastic board

Insulated copper wires

Mild steel rodcovered by aPVC tube

Plastic support

Device


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In practice

EDFArcAntique

ArcAntique-

Remainingmaterial

marine crust

cracks

Li iti th b bbli f H t th i i


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Limiting the bubbling of H2 to the minimum

ArcAntique


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lead objects

-1.3VPb2+ Pb

E (V/HSE)

I (mA)

Needed?

In KOH solutions

EDF-Valectra

EDF-Valectra


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copper-based objects

1

23

4

I

E

Natural

patinadestroyed

In sodium sesquicarbonate

EVTEK

ArcAntique

ArcAntique

Limitations: composite artefacts


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Limitations: composite artefacts

Rifles at their arrival

Documentation

Investigation:Waterlogged?

ArcAntique

ArcAntique

ArcAntique

A collection of rifles (17th c.),Mediterranean Sea

When mechanical cleaning is the only


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g yway

Pneumatic ultrasonic

ArcAntiqueArcAntique

ArcAntique

Storage in Hostacor + H O solution


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Pre-storage witha corrosion inhibitor

Storage in Hostacor + H2O solution

ArcAntique

t (hours)

Ecorr (V/HSE)

Hostacor 1%

Hostacor 1.5%

Hostacor 2%

300100

-0.6

-0.7

-0.8

Other applications of the electrolytic


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Porthole with wooden frame19th c. Finnish wreck

beforeEVTEK

EVTEK

pen

EDF-Valectra

To clean


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Electrolyte: mixture of triammonium

citrate. Voltage: 4,6 V and Current: 45 mA

after

EVTEK

EVTEK

Documents

Degrigny, C. Cleaning CH Artefacts Overview. 2010