Microsoft PowerPoint - Schmidt-Ott_AS_def_ohneTxt_für webAlkaline Sulfite Desalination- Tips and Tricks
Katharina Schmidt-Ott Niklaus Oswald
© Swiss National Museum, October 2006
Post-Excavation Problems:
• lower RH, higher O2 concentration
• akaganéite can form in presence of Cl- ions
and with artefacts arriving late to the lab: • loose flakes and cracks • loss of original surface
© Swiss National Museum, October 2006
Akaganéite β-FeOOH
(Stanjek, unpubl.), Cornell & Schwertmann, the Iron Oxides, Weinheim 1996, p. 18
© Swiss National Museum, October 2006
Iron Artefact without Desalination
Horseshoe in March 2006
Horseshoe in Sept. 2006
Iron Artefacts with Alkaline Sulfite Desalination
Desalinated 1996 and in stable condition 10 years later
Desalinated 1995 and in stable condition 11 years later
© Swiss National Museum, October 2006
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
© Swiss National Museum, October 2006
Protection of Organic Material
• protection of organic material with Paraloid B44 (~20% in acetone)
• after desalination: removal of Paraloid with acetone compresses
© Swiss National Museum, October 2006
Preparation of Artefacts
• if fragile: support with casting tape (e.g. Dynacast Extra®)
© Swiss National Museum, October 2006
Preparation of Alkaline Sulphite Solution:
1. Only treat objects from one site together 2. Object‘s weight ratio to solution amount useful: 200g/l 3. Use stainless steel containers 4. 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- foil 7. Heating to 50°C speeds the process up to 10x
© Swiss National Museum, October 2006
Heating Option A: Magnetic Stirrer
• stainless steel container with magnetic stirrer/heating device
• useful if containers too large for water bath • isolation of steel container with PE-foam
© Swiss National Museum, October 2006
Heating Option B: Water Bath
• warming in water with heating element, 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
© Swiss National Museum, October 2006
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
0.4 g/l 0,01M pH 12
126 g/l 1 M
63 g/l 0.5 M
Changes in Solution
possible if no air enters
0,1 M (4 g/l)0,05 M (6,3 g/l)
new standard
old standard
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
© Swiss National Museum, October 2006
Stability of Treated Artefacts
Desalinated in old standard solution (63 g Na2SO3 & 20g NaOH /l),
11 years after treatment: 10 years after treatment:
© Swiss National Museum, October 2006
Artefacts 4 Years after Treatment
Desalinated in diluted solution (0,63 g Na2SO3 & 0,4g NaOH/l)
Desalinated in diluted solution (6,3 g Na2SO3 & 4g NaOH/l)
© Swiss National Museum, October 2006
Summary
The removal of chlorides is essential for long-term stability of archaeological iron artefacts
Alkaline sulfite solutions can remove the water-insoluble chlorides 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 pollution to the environment
A plasma pre-treatment can speed up the desalination time
© Swiss National Museum, October 2006
For further details please see publication:
Katharina Schmidt-Ott, Niklaus Oswald Neues zur Eisenentsalzung mit alkalischem Sulfit VDR Beiträge 2, 2006 Pp 126-134
© Swiss National Museum, October 2006
Acknowledgment
the Swiss National Museum, Zürich the Staatsarchiv Obwalden the Kantonsarchäologie Zürich and our collegues, especially Markus Leuthard and Kim Travis
for supporting the presented project