on a slightly humid support such as a stone or brick floor, sometimes with a
load on the warped panel to straighten it (Wolters 1952:8).Flattening of panels with polar solvents
Polar solvents such as ethanol mixed with water or pure ethanol may have
been used to moisten panels for the purpose offlattening. It may have
been observed that the flattening of a warped wooden panel could be
effected by ethanol when mercuric chloride was used in an alcoholic solu-
tion for pest control in wood. The use of organic solvents to straighten
panels is rarely documented. Spraying ethanol on the reverse of a warped
panel to moisten it has been reported (Wolters 1952:9). In two cases,
when the so-called shellac method was performed without efficacy,
swelling of the panel reverse was initiated with Cellosolve. Such treat-
ments were carried out in 1957 and 1959 in the Schweizerisches Institut
für Kunstwissenschaft laboratories in Zurich (SIK 1957, 1959).Cutting the backs of panels
During the nineteenth century and in the first half of the twentieth cen-
tury, the reverses of panel paintings were treated on the surface to assist in
humidification. Typically, such treatments drastically altered the original
surface ofthe reverse. The most aggressive method consisted of planing
the whole surface of the reverse to obtain pure and fresh wooden material
for moistening. From the 1950s this opening of the wooden structure was
done with a scraping tool to reduce loss of the original substance (SIK
1957, 1959). Another “classic” method was to make cuttings,notches, and
slits with a knife along the grain of the panel to promote penetration of
water into the wood structure (Lucanus 1832:115; Welsch 1834:63–64).
Such cuttings werealso carried out in the same way as the Italian sverzatura
by sawing along the grain. Around 1950 a modified paring chisel was used
to make slits in panel reverses (Wolters 1952:9; Brachert 1955b:14).Shellac, or Munich, method
Atechnique to flatten warped panels was developed at the Doerner
Institute in Munich by Christian Wolters (1952:10). Initially, water-insoluble
binding media were postulated for use, particularly those that contain
water in their liquid phase, such as watery dispersions of synthetic poly-
mers, urea resins, cellulosic esters, and high-molecular alcohols (Wolters
1952:10). Repeated applications of such binding media on the reverse of
warped panels were intended to flatten and reinforce the support simulta-
neously. Solutions of shellac in ethanol and Cellosolve were applied as a
type of solvent compress on the wooden surface. The polar solvent vapors
penetrate the wooden structure and cause swelling, while the shellac film
serves as a solvent-retention barrier. This so-called shellac method, or
Munich method, was described by Christian Wolters at the 1961 con-
ference in Rome of the International Institute for the Conservation of
Artistic and Historic Works (IIC) (Wolters 1963:163–64).
Today the Munich method is understood to have rather negative
effects, as the shellac film has a very strong gloss that covers the entire
reverse (Fig. 12). Typically, conservators no longer apply materials directly
to the support. However, at that time shellac was not the only coating
applied to panel reverses—wax layers were also used. That technique
may have the advantage of not requiring the removal of original material
(Straub 1965).214 Schiessl