Removing wax from wall painting

Danielle Sheard <danielle.sheard [at] scotland__gsi__gov__uk> writes

>I am attempting to establish the best methods of de-waxing a
>structural painting on lime plaster supports. Two wax applications
>are thought to have take place in the past. They are now endangering
>the paintings and must be removed or reduced to return some of the
>porosity to the structure. The waxes are thought to be beeswax and
>paraffin wax (not confirmed yet). The paint is thought to be a type
>of tempera (water based) possibly with the use of an animal glue or
>casein. I have read a bit about the use of Nitromors and solvent
>gels using xylene or toluene in the past. I would like to know what
>the current recommended treatment is and any suggestions for further
>reading or alternative treatments.

Your first question should be why is your wall painting
deteriorating (is it in fact deteriorating?). Beeswax and paraffin
wax are both have a high level of chemical and generally do not, in
themselves, cause wall paintings to deteriorate (although the low
glass transition temperature allows them to absorb dirt particles,
causing the painting to develop a dark or discoloured appearance).
What wax does sometimes do is to reduce the porosity of the wall
painting exacerbating the effects of environmental moisture problems.
However, depending on the nature of the paint layer the painting
might not have been that porous in the first place. So before
looking at treating the wax, identify and control the underlying
cause of the deterioration.

The next thing to consider is whether or not it is possible to
remove a wax coating and increase the porosity of the painting. Wax
generally is applied as a consolidant on paintings where the
cohesion of the paint layer (and sometimes the plaster substrate)
had deteriorated. Removal of the wax is practically impossible
without removing pigment. Wax can certainly be reduced, but this
will have little effect on the porosity of the painting. So in
truth, the reason that wax is removed in most case is for aesthetic
effect rather than increasing the stability of the painting. (The
situation with wax resin materials is rather more complex and in some
cases the coating is the cause of deterioration not merely a
secondary factor)

With regard to materials, the first test should always be the
simplest--is the wax is soluble in an aromatic solvent mixture
applied by swab. If you do need to use supports, the situation
becomes more complex. Polyacrylic acid gels (Carbopol) tend to be
unstable under the conditions encountered on wall paintings and the
risk of residues is high. Using poultices for aromatic solvent
moistures is a possibility, but rather difficult on a vertical wall.
Aromatic solvent mixtures supported in a wax emulsion (Pappina) is
relatively stable, but should be used with caution by an experienced
practitioner. Under no circumstances should aggressive commercial
paint strippers such as nitromors be used for the treatment of
historic objects.

Tobit Curteis
Tobit Curteis Associates
36 Abbey Road
Cambridge CB5 8HQ
+44 1223 501958
+44 1223 304190


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                  Conservation DistList Instance 16:38
                Distributed: Thursday, December 5, 2002
                       Message Id: cdl-16-38-002
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