Smoke stain on marble

Having restored a large 200 year old marble sculpture recently, some
deep orange staining remains. Though the piece is now in a beautiful
condition, and the gallery curator is very satisfied with the
restoration, I would like to remove the remaining seemingly
irremovable blemishes. Any advice out there?

    **** Moderator's comments: At my request, Eoin wrote back to
    provide more information:

    The stains remaining are not treatment based--that is, not as a
    result of the treatment recently applied. In as far as I can
    ascertain, they are the result of 200 years of candle and
    incense smoke in a church environment, allied to the various
    internal atmospheric pollutants that build up over such a period
    of time. I consider it unlikely that any previous treatment was
    applied to the piece, given it's dreadful condition and the film
    thickness of the pollutant present at the beginning of this
    project.

    The only tests carried out on the piece were those performed by
    a local laboratory to determine a suitable chemical application
    to remove the pollutant surface coating. This chemical was very
    successful in removing 95% of the contaminants, while leaving
    the stone beneath unharmed. However,certain areas of the white
    marble surface have an orange stain, which seems irremovable.

    I have tried the usual formulae used here e.g powder pumice, cut
    lemon and carbon tetrachloride,(all used separately) as well as
    diluted caustic based chemicals (which we quickly washed down
    with plenty of water) and ammonia based chemicals using strong
    lamps to draw out the stains--but to no avail.

Eoin O Riabhaigh

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                  Conservation DistList Instance 12:73
                  Distributed: Friday, March 12, 1999
                       Message Id: cdl-12-73-007
                                  ***