Stains on marble

Eric Miller <eric.g.miller [at] btinternet__com> writes

>We are currently cleaning a marble fireplace that has yellow/brown
>tar stains that do not respond to our attempts at removal. Laser
>cleaning works but it turns the marble yellow. (A 1999 DistList
>entry affirms Synperonic N, but this is no longer available.) Any
>suggestions?

Synperonic N is a non ionic detergent and this or other detergents
would not be my first line of attack for tar or any other bituminous
stain. The first issue is residual detergent and one can read the
1,000 most recent articles about this quite readily.

The first question I would ask is on solubility. Tar is readily
soluble in alkanes and aromatics, the challenge being for the
solvent to enter the marble and bring out the stain. This can be
achieved by applying an evaporative poultice which could be a sheet
of tissue paper or a thick cellulose based pulp. I would avoid the
use of clays for residue staining reasons but not be afraid to use a
suitable cellulose ether if necessary to retard evaporation. (See
1,000 articles on residues clearance to make this decision)

Having said that I would be looking at solubility somewhere in the
area lying between white spirit and toluene and applying this in a
poultice that will allow the necessary depth of action. Keep the
poultice covered to attain the required depth and then remove it to
reverse the dissolved stain.

Thank you for your comments on the impact of laser cleaning on
marble. Gradually some critical assessment of this wonder tool is
emerging at last.

Andrew Thorn, former conservation student at the City and Guilds of
    London Art School
ARTCARE
614-1926-9326
613-9326-9326
Co-ordinator ICOM-CC Murals, Stone and Rock Art Working Group
IIC 2010 Istanbul
ICOM 2011 Lisboa


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                  Conservation DistList Instance 22:44
                 Distributed: Sunday, February 8, 2009
                       Message Id: cdl-22-44-001
                                  ***