Desalination of polychrome limestone altarpieces

I am sculpture conservator at National Museum of Machado de Castro
in Coimbra, Portugal. The museum is closed for refurbishment and
enlargement which includes a new conservation studio among other
facilities.

Eight polychrome limestone altarpieces mounted against the former
museum walls had to be dismantled in order to be treated. Their
dimensions are around 405 x 1295 x 54 cm (H x W x D). All of them
are composed by several fragments (ranging from 8 to 40) which will
be conserved separately. The treatment will include desalination,
cleaning, stone and paint consolidation.

The major cause of decay is salt contamination, resulting from the
previous location of these works in the museum In fact salts were
carried into the altarpieces by rising damp. Therefore the source of
damage was eliminated when the altarpieces were removed from the
walls.

The next step will be desalinisation. Elements to be desalinated are
quite numerous and often very big and poulticing will be very
expensive and time consuming. We consider that complete immersion of
these elements would be the most sensible method in this case. Of
course precaution will be taken bearing in mind that all the
altarpieces are polychrome.

As we would like desalinate all the fragments of each altarpiece at
the same time, and as all the fragments present considerable
dimensions, the only possibility of doing that was to build a big
tank. This will allow us to immerse all the fragments at once.
However, this tank does not have separate compartments so it does
not allow for the control of conductivity for each fragment
separately.

I would like to know if anyone has experienced large scale salt
removal, namely on  polychrome altarpieces, and the method used to
control the process and decide the precise time to achieve it. In
addition, I would be very grateful if someone could let me know
about his/her experience on protecting polychrome surfaces for
long-term immersion.


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                  Conservation DistList Instance 19:15
               Distributed: Thursday, September 15, 2005
                       Message Id: cdl-19-15-013
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