permeability. It should be pointed out, however, that while the term Npr^2
defines an area, this is the area of the capillary openings and not the same
as the cross-sectional area in Equation 1b. Again, a high flow rate can be
achieved by either a low viscosity or a high pressure differential and, in this
sense, Poiseuille’s law is the same as Darcy’s law.
Equations 1a, 1b, and 2 are for steady state flow, where the fluid
enters on one surface and exits on an opposite surface. In consolidation
treatments, a more realistic model is given by unsteady state conditions,
where the fluid enters from opposing surfaces. For a parallel-sided body,
the fractional volumetric retention offluid—that is to say, the volume of
fluid retained in the body expressed as a fraction of its total volume—is
gi ven by (Siau 1984):FVL 5 2/L[(2KDPt)/(Vah)]1/2 (3)where: FVL 5 fractional volumetric retention (volume per volume); L 5 dis-
tance between the opposing surfaces (length); K^5 specific permeability of
wood (volume per length); t 5 elapsed time from beginning of treatment
(time); Va 5 porosity of the wood (pore volume per total volume); and
other values are as previously defined.
Examination of Equation 3 shows that here also, all variables
except viscosity and pressure differential are fixed for a given object and
that high retention requires high pressure differential or low viscosity.
Pressure impregnation is ordinarily not a realistic choice in conser-
vation work, but vacuum impregnation is effective and relatively easy to
do (Schaffer 1974; Barclay 1981; Payton 1984; Simpson, Spirydowicz, and
Dorge 1992). For any other application methods, one must simply substi-
tute an alternate driving force for pressure differential (i.e., gravitational
forces or the surface tension involved in wetting and capillary action).
Thus, the viscosity of the fluid chosen for consolidation is the key factor
insuccessful consolidation treatments (Schaffer 1971).
The permeability of sound wood is an extremely variable prop-
erty. Permeability may vary from one species to another by as much as a
factor of 1 million. Longitudinal permeability is greater than transverse
(radial or tangential) permeability, with ratios varying from 500 to 80,000
in softwoods and from 30,000 to over 100 million in hardwoods (Siau
1984). Biological deterioration can cause dramatic increases in permeabil-
ity, particularly if the organisms destroy the pit membranes (Ellwood and
Ecklund 1959). Thus, the ease of treatment tends to increase with the
degree of deterioration.A number of authors have discussed desirable characteristics of consoli-
dants (Grattan 1980; Unger 1988; Rosenqvist 1963; Werner 1977). Grattan
lists as many as eleven “ideal characteristics” (Grattan 1980). The major
concerns of conservators are included in the following list of requirements
of consolidants:- Long-term stability is necessary so that the consolidant does
not deteriorate at a faster rate than the object itself. - The treatment should not change the appearance of the object.
Undesirable changes include darkening, color changes,and
glossy surface films where no gloss was extant or intended.
Criteria for Selection
of Consolidants
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