http://www.kunsthaus.ch/en/the-collection/conservation/examples-from-the-world-of-art-conservation/max-ernst/removal-of-the-added-coats-ofcolor-and-varnish/?redirect_url=title%3DSpecialist

Kunsthaus Zürich

Removal of the added coats of
color and varnish

At first the solubility of the added coats of discolored paint and varnish was determined. Only then was it possible to develop a method for their removal:  a mixture of chemicals (Shellsol T and acetone, at a ratio of 1:1) allowed for their removal from the original.

Because the chemical compound has a relatively quick evaporation time and thus only a very short application time, a thickening agent was added (filmstill 1). In addition, a foil was applied which helped prevent evaporation and extended its time span of action.
The solvent is so designed as to only act upon the polyester resin binding agents of the added layers of paint, the oil based binding agents of the original painting remaining unaffected.

The non-original layers of varnish and paint can be removed simultaneously.  Once the layers of varying thicknesses are sufficiently softened, it is possible to remove them from the painting by pushing them off with a wooden stick (filmstill 2).

The added layers adhere better where the original painting has been scratched or where bits of paint have been lost.  Here the removal of the added layers is more complicated because the task at hand demands more care.  The areas with lost paint loss or other damages become visible when the overpaint layers are removed (filmstill 3).

The removal of the overpaint showed that there are several small areas with missing original paint. These are mainly found along the edges but to some extend as well in the middle ground. It is obvious that most of the losses derive from the removal of the wall in the late 50ies. There is one almost continuous area in the upper row where one can see such a break line of the original with losses.

The removal of the overpaint has furthermore revealed, to some extend, massive discoloration (see intermediate status). It can be seen in form of overall darkening or as brownish, grid- like pattern.

Film still 1:  Application of the solvent gel
Film still 1: Application of the solvent gel
Film still 2:  Mechanical removal of the non-original layers
Film still 2: Mechanical removal of the non-original layers
Film still 3:  Exposed original after the removal of the added layers
Film still 3: Exposed original after the removal of the added layers