Waterlogged ivory

I'm about to conserve a beautifully carved waterlogged ivory plaque,
probably carved from a walrus tusk, dating to about 1100 A. D.
(size: 0.8 x 7 x 15 cm).

The carvings show Christ on the cross with the Virgin Mary below to
his right and the beloved disciple John below to his left. From
above, the hand of God is stretching down towards the head of
Christ. There are both latin and runic inscriptions to the left and
right side respectively, of Christ's head.

The whole plaque curves a little from one long side to the other, so
that the front is slightly convex, indicating that it was once
carved out of something with an oval cross section. The front seems
to consist mainly of the denser primary dentine while the back,
apart from the edges along the long sides, is almost wholly made up
the more porous secondary dentine.

The object is considered to be a very important archaeological find,
since it is quite unique for this country.

The main problem now is how to dry the plaque from its waterlogged
state without any warping, cracking and flaking. I have had one bad
experience before with a small dolphin made of elephant ivory. This
object was dried in successive baths of water/ethanol, starting with
20% ethanol and increasing by 20% every second week, for a final
concentration of 95%. The dolphin was then placed on polyethylene
foam in a deep plastic container inside a plastic bag with only a
tiny opening to allow for a very slow final drying. And yet, on the
fourth day the dolphin cracked and flaked badly and had to be
consolidated immediately with Paraloid B-72 in acetone in order to
prevent it from totally disintegrating.

Does anyone have any idea of how to avoid a similar catastrophe with
the above described ivory plaque?

Eva Christensson
Conservator, Archaeological Material
Central Board of National Antiquities
Stockholm, Sweden

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                  Conservation DistList Instance 12:6
                 Distributed: Wednesday, June 24, 1998
                        Message Id: cdl-12-6-011
                                  ***