Hi all.  A method that is commonly used in restoration of wood
and porous stones to extract undesired absorbed substances is the
following.

Apply on the object a poultice made with cellulose pulp soaked with
the proper solvent for the given matter.  The solvent mobilizes the
matter, and then it is absorbed out by the poultice.  Usually, several
applications of new poultice allow to eliminate all of the extraneous
substance.

Cellulose pulp is sold here in Italy under the trade name "Technocel"
or "Arbocel" or simply "cellulose pulp", and is the best for this
application.  Cotton-wool could be used as well.

As to the solvent, a completely volatile solvent is the best, so that
no residue remains in the object after ventilation.  To limit the loss
of solvent in the course of the treatment, due to evaporation, poultice
could be covered with a sheet of insulating material.

For oil and grease, white spirit and ethanol could be good.  I have
found that trichloroethylene is a very good solvent for grease; I
usually use it to degrease new bone keytops.  It has the disadvantage
of evaporating very quickly, and is toxic.  With due precautions, it
could be used safely.

Best regards

Leonardo Perretti,  Italy