Chloride ion removal from archaeological iron and #beta#-FeOOH.
The stabilisation of archaeological iron is considered as one of the major problems
facing the archaeological conservator today. Free chlorides, held as counter ions at the
metallic anode, act as electrolytes and promote corrosion of metallic iron
This study examines and compares the efficiency of a range of aqueous wash methods
currently used to remove chloride ions from archaeological iron and determines how
these various treatments work. It also investigates the effect of selected aqueous wash
treatments on the chloride rich metastable corrosion product 0-FeOOH. The amount of
Cl'removed from 0-FeOOH and any transformations to new compounds are recorded.
Iron objects were treated using aqueous Soxhlet extraction with nitrogen, aqueous
sodium hydroxide and aqueous alkaline sulphite solutions. Synthetic O-FeOOH and PFeOOH1Fe3Om4
ixtures were washed in aqueouss odium hydroxide, aqueousa lkaline
sulphite solution, aqueous sodium sulphite, hot and cold deionised water. The outcome
of the work showed that:
(1) For archaeological iron objects:
O. SM nitrogen de-aerated aqueous NaOH solutions are very efficient chloride
extractors, often approaching a 100% efficiency. Almost as efficient were 0.5M
aqueous... NaOH/Na2SO3 treatment solutions. Nitrogen de-aeration of treatment
solutions improves their cr extraction efficiency, by freeing Cl' from its counter ion
role. Hydroxide ions improve extraction efficiency of Cl- from iron objects in nitrogen
and Na2S03 de-aerated environments. The de-aerated aqueous Soxhlet extraction
method, as suggested by Scott and Seeley (1986), is a less efficient chloride extractor
than either nitrogen de-aerated 0.5M NaOH or 0.5M NaOH/Na2SO3. De-aerated
NaOH treatment solutions are to be preferred to NaOH/Na2SO3 solutions, as they do
not introduce residual electrolyte ions (SO 42).
(2) For O-FeOOH samples:
Pure O-FeOOH and P-FeOOH/Fe3O4 mixtures reacted differently during washing.
Pure 0-FeOOH completely transformed to cc-FeOOH in the alkaline environments
provided by NaOH and NaOH/Na2SO3. This released all the chloride from the PFeOOH.
In NaOH solutions within the range pH8.5 to 10.5 0-FeOOH/Fe3O4
transformed mostly to Fe304. Above pH1 1 there was limited conversion. Improved
release of Cl' from P-FeOOII/Fe3O4 mixture correlated with the transformation of PFeOOH