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Subject: Coral


From: Ruth Norton <rnorton>
Date: Tuesday, September 30, 2003
Flavia Perugini <fperugini [at] mountvernon__org> writes

>An old piece of coral in our collection needs immediate cleaning and
>consolidation.  The piece, still attached to a rock, has been
>mishandled and exposed to all sorts of environmental conditions for
>about 250 years. ...

How you clean or mend it will depend to some extent on what kind of
coral you have (hard, fan, red, black) and whether the coenenchyme
is still present.

The coenenchyme is the extremely porous, crusty layer (composed of
calcium carbonate and dried organic material) naturally covering the
core of fan, red and black coral. In dried coral it is usually very
weakly attached to the core and can be easily dislodged with
mechanical action. Vacuuming with a microtool, or very delicate
brush vacuuming could be done. The brittleness, weakness and
porosity of the coenenchyme argue against solvent cleaning. The
coenenchyme has often been removed to reveal the color of red and
black coral. The exposed red and black coral may be polished.

Hard coral and the core of red coral are almost 100% calcium
carbonate. The core of black coral and  fan coral is almost pure
protein (naturally tanned, so is quite hard and durable, tho still
thermoplastic). More immediately important for your situation may be
the coral's porosity.

Hard coral is very porous--I'd stick to mechanical cleaning, solvent
cleaning could drive dirt into the pores. If mending with Paraloid
B-72, I'd use a fairly thick solution (~50%) so it is not pulled
into the coral.

Fan coral is usually collected and displayed with the coenenchyme
intact, that is where the color is. The coenenchyme is very brittle
and the core very fine. If a piece of this is broken, I'd be
inclined to perhaps tack the pieces together with thick Paraloid
B-72, but rely on an external support to stabilize the mend.

The cores of red and black coral are dense. Mechanical, followed by
solvent cleaning if required (aqueous solvent is fine, damp swabs or
cloth), can usually be used to remove dust if the coenenchyme has
been removed and the surfaces are polished. If they are not
polished, the surfaces are naturally quite rough. Note: color of red
coral is sometimes augmented with dyes and coatings which may
respond differently to solvents than the coral itself would. Red and
black coral broken surfaces are usually hard and smooth. Paraloid
B-72 usually works fine. If the geometry of the break is such that
gravity is working against you rather than with you, and the piece
is heavy relative to the area of the break surface, Paraloid B-72
may stretch over time. In that case, it may be necessary to consider
a higher Tg acrylic, or even an epoxy if it is a huge piece. The
finer branches of black coral are thin and wire-like. The break
surface area is likely to be too small to make a secure mend with
adhesive alone. Reinforcement with external thread or bamboo pin
splints may be necessary.

Ruth E Norton
Chief Conservator
The Field Museum
1400 S Lake Shore Dr
Chicago  IL  60605-2496
Fax: 312-665-7193

                  Conservation DistList Instance 17:32
                  Distributed: Monday, October 6, 2003
                       Message Id: cdl-17-32-002
Received on Tuesday, 30 September, 2003

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