The Structural Conservation of Panel Paintings

Consolidation is a major intervention that is not to be undertaken

lightly. In particular cases of advanced deterioration, however, it may

become a necessary treatment. Once the necessity for consolidation is

determined, a number of decisions must be made regarding materials and

methodology. These decisions include choice of a consolidant, solvent

(and level of solution concentration), and suitable method of application.

Much will depend on the nature of the object to be treated, the type and

condition of the material, and the functional requirements of the object.

Usually structural function, as well as visual aspects, will be addressed.

The present discussion will be directed to a comprehensive examination

of various aspects of the consolidation of deteriorated wood, proceeding

from consideration of the general to the more specific problems that

might be encountered in the consolidation ofwooden panels that support

paintings. Hereafter, all references to wooden panels refer to painting sup-

ports. No attempt will be made to consider the consolidation of water-

logged wood, because that process presents problems and requires

approaches not applicable to panel paintings.

Consolidation of deteriorated wood entails the introduction of another

substance into its porous structure, a process requiring that the substance

be in fluid—liquid or gaseous—form. The ease with which a fluid can be

introduced is governed by the permeability ofthe wood. The transport

(movement) offluids through wood can be represented by Darcy’s law

(Siau 1984):

flux 5 permeability * gradient (1a)

where flux is the volume offlow per unit time and unit area perpendicular

to the flowdirection, and gradient is the change in pressure over the flow

path. Permeability depends both on the nature of the material and on the

viscosity of the fluid that flows through it. Hence we get:

Q/A 5 (K/h) (DP/L) (1b)

where: Q 5 rate offlow (volume per unit time); A 5 cross section perpen-

dicular to the flow path (area); K 5 specific material permeability (volume

per unit length); h 5 viscosity ofthe fluid (force per unit area times time);

DP 5 pressure differential across the flow path (force per unit area); and

L 5 length offlow path (length).

Inspection of Equation 1b reveals that fluid viscosity and pressure

differential are the only variables available for manipulation, because for a

given object, the cross-sectional area, the specific material permeability,

and the flow path are fixed. A high viscosity results in a low rate offlow,

while a high pressure differential produces a high rate offlow.

Alternatively, flow through wood can be modeled as capillary

flow, in which case Poiseuille’s law applies (Siau 1984). This is given by:

Q 5 (Npr^4 DP)/(8hL) (2)

where: N 5 number of capillaries (no.); r 5 capillary radius (length); and

all other variables are as previously defined. Here radius and number of

capillaries take the place ofthe cross-sectional area and specific material

Permeability ofWood

88 Schniewind