Series editors' preface - Wood Tools

524 Conservation of Furniture
have an acidic or basic character or the effect
on solubility of acidic or basic conditions.
Thus aqueous cleaning systems and those that
utilize acid/base interactions cannot be evaluated
using solubility parameters alone. The
Teas’ chart does not take into account the
effect of aromaticity on a substrate. For
example, when solvent testing, it may be
observed that xylene, an aromatic hydrocarbon
solvent, has the desired effect in removing
dirt or an unwanted coating. The
conservator may wish to substitute a less
potentially harmful solvent blend, such as one
part mineral spirits and six parts ethanol.
Although this mixture matches the Teas
solubility parameters of xylene, it is possible
that it will not have the desired effect on the
dirt or coating. In such cases it is likely that
the dirt is characterized by the presence of
aromatic groups or structures. It may be necessary
to include a proportion of an aromatic
solvent in the final solvent blend, or an
aromatic solvent per se may be required to
take into account the aromatic nature of the
unwanted material. Michalski (1990) and
Phenix (1998) have identified limitations of
Teas’ solubility parameters and proposed
modifications and alternatives that may present
a more complete description of solubility.
Other systems of describing solubility
parameters also exist. The Edwards hydrogen
bonding index is one of several solubility
parameters used in industry and may be
encountered in literature about solvents
provided by the manufacturer. Hydrogen
bonding in solvents is largely restricted to
oxygenated solvents. Ketones and esters that
contain oxygen atoms that are not immediately
connected to hydrogen atoms act as acceptors
and, by convention, are assigned positive
hydrogen bonding indices. Alcohols, which
usually act as donors but can occasionally act
as acceptors, are assigned negative hydrogen
bonding indices. Hydrocarbon solvents are
non-hydrogen bonding, although aromatics
have limited acceptor characteristics.
Solvent removal of varnish
There are several important factors that should
be considered when choosing a solvent for the
removal of surface dirt or a varnish. These
include the solubility parameters and compatibility
of the solvent and dirt or varnish,
Table 11.3 Contribution of dispersion forces (f d)
to blends of cyclohexane, toluene and acetone
Cyclohexane Toluene Acetone Approximate
% volume % volume % volume value of f d
(Teas)
100 0 96 least polar
75 25 92
50 50 88
25 75 84
100 80
87.5 12.5 76
75 25 72
62.5 37.5 68
50 50 64
25 75 56
100 47 most polar
Source: Feller, 1976a
Table 11.4 Contribution of dispersion forces (f d)
to blends of mineral spirits, xylene and acetone
Mineral Xylene Acetone Approximate
spirits % volume % volume value of f d
% volume (Teas)
100 0 90 least polar
75 25 88
50 50 87
25 75 85
100 83
87.5 12.5 78
75 25 74
62.5 37.5 70
50 50 65
25 75 56
100 47 most polar
solubility parameters of the underlying decorative
surface, the rate of dissolution of dirt or
varnish in comparison to the rate of penetration
into the substrate (see Figure 11.9),
potential retention of solvent in the substrate,
controllability of solvent application, solvent
evaporation rate and the health and safety
implications for the conservator.
Feller (1976a) proposed using three solvents
of increasing polarity that were combined in
graduated steps to identify the least polar
solvent blend that would remove natural resin
varnishes (Table 11.3). The mathematical
method described in Box 11.1 can be used in
conjunction with Appendix 2.1 in Horie (1992)
to formulate a similar table using solvents such