toluene, chalk, and sawdust were combined to make a filling material
(Brachert 1955b:30), or sawdust was mixed with cellulosic nitrate or
acetate. Mixtures of epoxy resins with filling pigments, such as Master
Model Paste, were also used. Not all these fill materials containing modern
adhesives are reversible.Protection of unpainted backs of panels
Authors of early restoration books complained that the old masters had
often failed to apply a protective barrier to the backs of one-sided painted
panels to protect against warping (Köster 1828:16). An application of lin-
seed oil with red pigments, typically red ochre, was recommended (Köster
1828:14–15). Old brownish, reddish, or yellowish paint layers on the origi-
nal backs of paintings can be found in the Alte Pinakothek in Munich and
in the Wallraf-Richartz-Museum in Cologne (Goldberg and Scheffler
1971:passim; Achternkamp 1991:18; Zehnder 1990:passim). Other authors
were convinced that oil paint layers on the back would help suffocate
woodworms (Hampel 1846:21). Panel backs were sometimes painted with
red lead (Wolters 1952:6). In Munich around 1900, hot linseed oil was used
to impregnate the wooden back, after which an oily pigmented paint layer
bulked with chalk or mixed with shellac was applied (Kinkelin 1912:5). At
the end of the nineteenth century, some new binding media such as cellu-
lose nitrate were recommended.
Linseed oil impregnation was sometimes done before cradling.
Köster recommended covering the entire reverse, including all cradle slats
(Köster 1828:14–15). Application of shellac is often reported (Zehnder
1990:422, Wallraf-Richartz-Museum, Cologne, inv. 179.59, 208). Sometimes
marks or labels are helpful for dating the application of the paint layers
(Zehnder 1990:335). Application of shellac after cradling was an important
part ofthe Vienna method of cradling (Keyselitz 1960:73–75). Since the
1950s wax-resin mixtures have been used for impregnation.
Wolters summarizes a wide range of binding media that can be
used to protect the reverses of panel paintings: beeswax; beeswax mixed
with natural resins; beeswax mixed with colophony and linseed oil; wax
combined with AW2 resin (cyclohexanone resin); pigmented oil paints; hot
unpigmented linseed oil with subsequent layers of shellac; shellac mixed
with Manila copal; cellulose nitrate; cellulose acetate; latex emulsions com-
bined with paraffin, sodium silicate, and water; PVA dissolved in toluene;
and an emulsion of animal glue and linseed oil pigmented with chalk or
gesso, sometimes followed by a pigmented oil paint layer (Wolters 1952:6).
Concerning the effectiveness of paint layers applied to protect the
backs of painted panels against humidity, Wolters presented the results of
important experiments (Wolters 1963). Mühlethaler tested the effective-
ness ofSaran coatings, recommended by Buck (Mühlethaler 1975).
Paper, foils, and metal sheets were also used to protect the backs
ofpanel paintings. Large paper sheets, probably applied in the nineteenth
century, have been documented on some panel backs in the Kunsthalle
Karlsruhe, sometimes as a type of counterveneer on sawn panels (Fig. 21)
(Koch 1981; Achternkamp 1991:23). The use of paper to flatten thin panels
before partial transfer is also reported (Wolters 1952:10). Apparently, foils
of synthetic materials such as polyethylene are not used as frequently out-
side the United States (Achternkamp 1991:23), but there are some examples
with cellophane foil (Wolters 1952:5). Tin and aluminum foils were firstH P P C A, G, S 227