Removing of iron stains from wet ivory

Wet archaeological bone and ivory can be stained with iron corrosion
products.  If the iron is left within the bone or ivory the iron
corrosion products can expand and physically fracture the object.
Chemical removal of the iron can harm the substrate through loss of
the object's collagen and/or mineral components.

An excellent paper published by Godfrey, Kasi, Schneider and
Williams  (2001) in the 8th ICOM working group on Wet Organic
Archaeological Materials (WOAM) conference titled 'Iron Removal from
Waterlogged Ivory and Bone'  discusses the impact of the  iron
removal treatments upon the substrates used in this study.

The introduction section mentioned treatments that use non-selective
reagents capable of reacting with the chemical components that make
up bone and ivory identified as oxalic acid, ammonium citrate,
thioglycollic acid, citric acid and hydrogen peroxide. These
treatments use a short duration of treatment time but can negatively
impact the substrate.

Their paper's experiment examined modern bone and archaeological
bone, modern ivory and archaeological ivory samples  treated with
four different iron removal solutions and deionized water: 2% w/v
diammonium citrate, 5% w/v sodium dithionite, 5% w/v sodium
dithionite followed by 2% w/v diammonium citrate, 5% disodium salt
of EDTA and finally deionized water.

All of their tested reagents were found to damage all the bone and
ivory samples but sometimes the modern bone or ivory was impacted by
a reagent, and sometimes the archaeological samples were impacted
the most.  It is a fascinating, complex paper that is very
informative and warns against the adoption of iron removal as a
standard course of action.  They suggest more research into
selective chelating agents such as crown ethers, cryptands and
polyphenols that use archaeological test specimens.

My current interest in wet ivory stained with iron corrosion
products is due to an archaeological object given to me for
conservation treatment.  It is a very damp walrus ivory harpoon head
(slightly ornately carved) with an intact iron spade-shaped tip.  It
is likely 70-100 years old and was excavated from Greenland, Kingdom
of Denmark.  While in the ground the ivory has become stained with
iron corrosion products to the degree that the color is quite
visible but not mechanically cleanable.   It was found in wet soils
and was kept damp within moss and refrigerated in the dark before
being brought to me.  The iron tip is black and there is a small
amount of orange iron oxides present on the tip.  Chloride ion
testing of other ivory and wood objects found nearby and soaked in
water found little chlorides present in the wash water, less than 7
ppm.

Walrus ivory will likely split upon going to the dry state.  Iron
corrosion products left within the ivory may further expand and
damage the ornate carvings.  Removal of the iron corrosion products
might be warranted if a technique can be found that does not damage
the mineral or collagen that remains.  Right now I cannot identify
an iron removal treatment that I trust will not harm the substrate.
I am leaning hard towards doing no iron removal treatment for the
ivory and focusing upon stabilization of the iron tip and slow
drying the object.  I am prepared to use Acrysol WS-24 in a dilute
solution to consolidate the surface of the ivory as needed.

Can anyone recommend further papers that explored the use of crown
ethers, cryptands and polyphenols to remove iron from archaeological
ivory?  Do you have any good treatment ideas?   What is best
practice for this harpoon head?

I welcome your thoughts and thank you in advance for any advice you
might share.

Molly O'Guinness Carlson
Archaeological Conservator


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                  Conservation DistList Instance 23:36
                  Distributed: Monday, March 22, 2010
                       Message Id: cdl-23-36-027
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