© Swiss National Museum, October 2006

Alkaline Sulfite Desalination-Tips and Tricks

Katharina Schmidt-OttNiklaus Oswald

Swiss National MuseumCentre for Conservation

© Swiss National Museum, October 2006

Post-Excavation Problems:

• iron is often contaminatedwith salts

• lower RH, higher O2concentration

• akaganéite can form in presence of Cl- ions

and with artefacts arriving lateto the lab:• loose flakes and cracks• loss of original surface

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Akaganéite β-FeOOH

(Stanjek, unpubl.), Cornell & Schwertmann, the Iron Oxides, Weinheim 1996, p. 18

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Iron Artefact without Desalination

Horseshoe in March2006

Horseshoe in Sept. 2006

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Iron Artefacts with Alkaline Sulfite Desalination

Desalinated 1996 and in stable condition 10 years later

Desalinated 1995 and in stable condition 11 years later

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Prior to Desalination

• X-radiography of all iron objects• Possible pre-treatment: plasma• mechanical cleaning (removal of loose earth and

sand at minimum)• stabilization of cracks or loose parts with Technovit

5071 bridges (methyl methacrylate)• analysis of organic material (SEM)• protection of organic remains or inlays with Paraloid

B44 (methyl methacrylate-copolymer) • weight of artefacts recorded

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Protection of Organic Material

• protection of organicmaterial with ParaloidB44 (~20% in acetone)

• after desalination: removal of Paraloid withacetone compresses

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Preparation of Artefacts

• sealing in PE-perforatedfabric (Flexifilm®)

• if fragile: support withcasting tape (e.g. DynacastExtra®)

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Preparation of Alkaline SulphiteSolution:

1. Only treat objects from one site together2. Object‘s weight ratio to solution amount useful: 200g/l3. Use stainless steel containers4. Na2SO3 & NaOH added to demineralised H2O 5. Take a sample of the solution (at least 5 ml)6. Immerse objects in solution, cover with PE- or PP- foil7. Heating to 50°C speeds the process up to 10x

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Heating Option A: Magnetic Stirrer

• stainless steel container with magnetic stirrer/heatingdevice

• useful if containers too large for water bath• isolation of steel container with PE-foam

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Heating Option B: Water Bath

• warming in water with heatingelement, no stirring

• use of stainless steel containers(cooking vessels)

• isolation of bath with PE-foam• lower heating and maintenance

costs• reliable for long term use

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The Process

• always sample fresh, unused solution first• sampling of solution & measurement of chloride level

with titration• changing of solution every 3-4 weeks as chloride level

rises• 3-4 changes of solution usually necessary• average desalination time 12-14 weeks• shorter desalination time if objects have inlays

© Swiss National Museum, October 2006

Adopted Chloride Test

• Aquamerck® 1.11106.0001• glass beakers, magnetic

stirrers• 200-1000 µl gauged syringe• 0.5-5.0 ml gauged syringe• hydrogen peroxide (35 %)• nitric acid (30 %)• purified aluminium oxide• tetranatrium diphosphate (5%)• sodium hydroxide (30%)

© Swiss National Museum, October 2006

NaOH alkaline sulphite solution

40 g/l 1 M pH 14

20 g/l 0.5 M pH 13,7

4 g/l 0.1 M pH 13

0.4 g/l 0,01M pH 12

126 g/l 1 M

63 g/l 0.5 M

standard Rinuy-Schweizer

6.3 g/l 0,05 M

tested and approved

0.63 g/l 0,005 M

tested and o.k.

tested Na 2

SO

3

0.063 g/l 0.0005 M

tested tested

Alkaline Sulphite Solution

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Changes in Solution

0,1 M (4 g/l)0,005 M (0,63 g/l)

possible if no air enters

0,1 M (4 g/l)0,05 M (6,3 g/l)

new standard

0,5 M (20 g/l)0,5 M (63 g/l)

old standard

NaOHNa2SO3

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Subsequent Treatments

• thorough rinsing with de-mineralised water (2-4 days)• effective washing can be monitored by measuring pH• removal of Flexifilm• drying of objects in vacuum oven at 70 °C & about

250 Pa• final cleaning of surface with air abrasive• drying in acetone (not if artefact has been stabilised

with Paraloid before)• protective coating with Paraloid B44

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Stability of Treated Artefacts

Desalinated in old standard solution(63 g Na2SO3 & 20g NaOH /l),

11 years after treatment: 10 years after treatment:

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Artefacts 4 Years after Treatment

Desalinated in dilutedsolution (0,63 g Na2SO3 & 0,4g NaOH/l)

Desalinated in dilutedsolution (6,3 g Na2SO3 & 4g NaOH/l)

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Summary

The removal of chlorides is essential for long-term stabilityof archaeological iron artefacts

Alkaline sulfite solutions can remove the water-insolublechlorides as well

A diluted solution 6,3 g/l Na2SO3 (0,05 M) + 4,0 g/l NaOH(0,1 M) shows the same positive results as the 0,5 M Na2SO3 + 0,5 M NaOH solution

The diluted solution is cheaper, and causes less pollutionto the environment

A plasma pre-treatment can speed up the desalination time

© Swiss National Museum, October 2006

For further details please seepublication:

Katharina Schmidt-Ott, Niklaus OswaldNeues zur Eisenentsalzung mit alkalischem SulfitVDR Beiträge 2, 2006Pp 126-134

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Acknowledgment

We would like to thank

the Swiss National Museum, Zürichthe Staatsarchiv Obwaldenthe Kantonsarchäologie Zürichand our collegues, especially Markus Leuthard andKim Travis

for supporting the presented project