Excimer lasers

Recently I read a very brief article (which unfortunately I can not
find to reference) about the use of excimer lasers for the removal
of overpaint in conservation.  Two questions arise; 1.  how safe is
this method and 2. if it is safe, where might one find a qualified
practicioner.

Being a very new technology, I am certainly concerned about the
potential of causing irreversible damage.  Yet the potential of
removing down to a level of, I believe they stated, one or two
microns at a time certainly is interesting.

A painting I am working on would appear to be a perfect candidate
for such a procedure.  It is an early (c.1505) Florentine panel
painting that was overpainted during the Counter Catholic
Reformation with a loin cloth of a very heavy coat of dark green
paint with a high concentration of lead-white. The flesh beneath is
extremely thinly painted using the ground for the white tones.  The
paint is solvent resistant and the conservator was forced to resort
to removal by means of a scalpel.  Some of the overpaint was safely
removable by this means.  Unfortunately the thinness of the flesh
tones combined with the lower adhesion characteristics between the
flesh and ground (as compared to that of the flesh and overpaint
adhesion) caused the flesh to fracture off with the overpaint.
Overpaint removal was halted at all areas where fracturing was
likely, leaving perhaps fifty percent of the overpaint intact.

Certainly this is a relatively common event when dealing with
paintings of this age.  The use of an excimer laser would seem to
offer an extremely accurate means of removing overpaint such as
this, yet I wonder if more problems could be caused by its use.
Specifically:

    1.  Does the laser, particularly when removing thick overpaint
        which would of necessity require many 'shots',  generate a
        large enough amount of heat that the adhesion of the
        original paint layer to the ground would be further
        compromised?

    2.  Would the surface of the original paint layer become pitted
        through this procedure?

    3.  Is there discoloration of the paint through the heat
        generated, that would cause a disparity with the adjacent
        untreated areas of like color?

    4.  During the process does the area affected initially develop
        a cast (whitish I would guess)  that might make it difficult
        for the operator to determine when they are coming to a
        subtle glaze layer, that could then inadvertently be
        removed?

    5.  Are any museum conservation departments currently using this
        technique?

    6.  How qualified are current operators in any peculiar
        requirements that this application might present?

    7.  What is the cost relationship to other painstaking methods
        of removal?

Any references to publications on the use of the excimer laser in
this application would be appreciated.

Anthony "Tony" Ayers
Independent Scholar

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                  Conservation DistList Instance 10:75
               Distributed: Wednesday, February 26, 1997
                       Message Id: cdl-10-75-007
                                  ***