Borates

A recent literature search produced two interesting articles
describing the effect of borate solutions on the stability of the
iron sulfides, pyrite and pyrrhotite.

    Wang, X.H.

    "Interfacial electrochemistry of pyrite oxidation and flotation.
    1: Effect of borate on pyrite surface oxidation",
    Journal of Colloid and Interface Science, Vol 178 No 2, p
    628-637, 25 Mar 1996

    Nicholson, R.V., and J.M. Scharer.
    "Laboratory Studies of Pyrrhotite Oxidation Kinetics".
    Environmental Chemistry of Sulfide Oxidation.
    American Chemical Society (ACS) Series. 550, p14-30, 1994.

Borax (sodium borate)/boracic acid (3:7) buffers were used in
waterlogged wood conservation as biocides, but it appears from these
articles that this has the potential to generate sulfuric acid from
these sulfides while the wood is being impregnated with polyethylene
glycol (PEG).

In the case of the Swedish warship, the Vasa, vast quantities of
borax/boracic acid buffer were added to the PEG solution with which
the ship was being sprayed in order to try and maintain the pH. The
ship is now dry, but is suffering from sulfuric acid production.
X-ray adsorption spectra of core samples from the Vasa show little
or no iron sulfides present, unlike other timbers from marine burial
sites. Care should therefore be taken in the use of borates in the
treatment of waterlogged wood contaminated with iron sulfides. In
particular, the pH and sulfate content of the treatment solutions
should be monitored to ensure that the iron sulfides have been
completely removed. Borate solutions have the potential to resolve
or reduce the sulfur threat to waterlogged archaeological wood, and
tests are underway to investigate this.

Dr Theo Skinner
Conservator, Marine and Wetland Archaeology
National Museums of Scotland
Granton Research Centre
242 West Granton Road
Edinburgh EH5 1JA
+44 131 247 4484
Fax: +44 552 9361


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                  Conservation DistList Instance 19:44
                 Distributed: Thursday, March 16, 2006
                       Message Id: cdl-19-44-001
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