Roman wall paintings

Karin Abelskamp <k.abelskamp [at] archeologie__nl> writes

>... would anyone have suggestions for me how to deal
>with our wall paintings? Ideas for treatment, storage or useful
>literature references, own experiences? Does anyone have experience
>with Primal or know about its effects? Or with Rhodopas?

Primal is a very good adhesive for wall paintings, but it is also a
very disputed one. Some say it may cause biological attacks, being
an organic material. Also you will notice it tends to form an
impermeable skin. However I have used it often as an adhesive on
wall paintings and at least on the short run have never had any
problems. If your paintings have a lack of ADHESION between the most
superficial layers of mortar, or flaking  paint, primal could be a
good solution for it.

If your paintings have a *cohesion* defect, that is the paint tends
to "dust" when you touch it, you should avoid primal (which will
form a thin blue-transparent impermeable skin on your surface). You
can use Paraloid B72 as a fixative. This is another acrylic resin
which is dissolved in organic solvents. (I should avoid using
dichlorethylene as a solvent on account of its high toxicity. You
can use instead a less toxic mixture of either acetone or
methyl-ethyl-ketone with some slowly-evaporating solvent such as
isotane or white spirit). A solution of paraloid (not more than
2-3%) evenly sprayed on the surface should be quite sufficient to
resolve the problem. You should try on a small area first because
there might be a darkening or glossy effect.

Another system to fix superficially dusty paint could be hydroxide
of barium which fixes the surface by transforming into calcium
carbonate. This might cause superficial whitening; however it is a
better method than paraloid being more compatible with the
paintings' materials (The procedure of using Ba(OH)2 is a little
complicated. I could post it if you're interested).

Personally I have always used paraloid as a fixative. You can't
avoid a certain change of color due to the darkening effect; the
concentration of the solution should be carefully determined after
making tests to see the smallest possible concentration which
renders the paint sufficiently stable.

I have used paraloid as an adhesive as well in one case where primal
couldn't be used because the paint was water-soluble. Increasing the
concentration to about 10% it could make flaking paint adhere to the
surface.

As for the white glue you mentioned, I suppose you mean Vynavil,
which is a vynilic resin, used as an adhesive in the past. It can
become acid with time and doesn't have any advantages. I would
definitely prefer primal.

The concept of reversibility when referring to adhesives and fixatives
is only a partial one; it is important to remember before
intervening that once you apply a substance, organic or inorganic,
on a porous material such as a wall painting, there is no way you
can completely extract it afterwards.

The choice of adhesives and fixatives depends also on factors such
as humidity and temperature. For instance in open or humid places you
have a higher risk of biological attacks and should consider
avoiding organic materials as much as possible.

Some literature you might find useful:

    D. Horton-James, S. Waltson, S. Zounis, "Evaluation of the
    stability, appearance and performance of resins for the adhesion
    of flaking paint on ethnographic objects," Studies in
    Conservation, 36 (1991), pp. 203-221

    J.L. Down, M.A. Macdonald, J. Tetreault, R.S. Williams,
    "Adhesive testing at the Canadian Conservation Institute--an
    evaluation of selected poly (vinyl acetate) and acrylic
    adhesives," Studies in Conservation 41 (1996), pp. 19-44.

Orit Soffer
Paintings Conservator
Via Posta Vecchia 78
00047 Marino RM Italy
+39 06 9367483


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                  Conservation DistList Instance 16:24
               Distributed: Thursday, September 26, 2002
                       Message Id: cdl-16-24-002
                                  ***