The Structural Conservation of Panel Paintings

surface deposits of such resins from treated wooden objects after the poly-
merization reaction was complete (Unger, Reichelt, and Nissel 1981).
Thermoplastic consolidants introduced into wood in solution,
however, do offer at least some degree of reversibility. Hatchfield and
Koestler made a scanning electron microscopic study of ancient wood
treated with Acryloid B72 in toluene and found that the consolidant
could be largely extracted again but that some of the resin did remain
(Hatchfield and Koestler 1987). Nakhla treated samples of cedar with
Acryloid B72 in trichloroethylene, or polyvinyl butyral (Mowital B30H)
in ethanol, and then extracted the consolidants by soaking in the same
solvent used for treatment (Nakhla 1986). Although gravimetric measure-
ments indicated that some consolidant remained in both cases, the
Acryloid B72 treatment proved, on the whole, more reversible.
It must be emphasized, however, that solvents interact not only
with the consolidant but also with the wood. Normal wood contains—in
addition to its main constituents cellulose, hemicellulose, and lignin—an
extremely varied group of compounds known as extractives. They are so
named because they can be extracted with neutral organic solvents.
Deteriorated wood may additionally contain degradation products that are
also soluble. In the course of extracting consolidant, extractives and degra-
dation products may also be removed (Schniewind 1987, 1988). Some sol-
vents will also cause wood swelling, or they may extract some of the
adsorbed water in the cell wall, and some may in turn be adsorbed on the
internal wood surfaces.
Table 4 shows results ofa systematic study of reversibility of
wood consolidation with respect to extractive removal. Deteriorated
Douglas-fir specimens (6 325 3 50 mm) were treated with 15% solutions
(weight basis) ofButvar B98, AYAT, and Acryloid B72, each in two
different solvents. After drying, the specimens were extracted by one of
the following three methods: soxhlet extraction, soaking with agitation, or
soaking only. Soxhlet extraction is the most effective extraction method
av ailable and should therefore indicate the limits of what is possible;

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Table 4 Reversibility of consolidation treatments as indicated by residual resin content

Residual resin (%)

Polymer Solvent Extraction method Measured Corrected

Butvar B98 Methanol Soxhlet –0.3 0.8

Toluene-ethanol Soak and agitate 2.7 2.9

Toluene-ethanol Soak only 6.0 5.7

AYAT Acetone Soxhlet –1.0 –0.3

Toluene Soxhlet 1.2 1.9

Acryloid B72 Toluene Soxhlet 0.2 0.9

Acetone Soxhlet –0.1 0.6

Acetone Soak and agitate –0.3 0.4

Acetone Soak only 0.7 1.3