Page 406 - Copper and Bronze in Art: Corrosion, Colorants, Getty Museum Conservation, By David Scott

the surface. In some harsh outdoor climates, as in Canada, for example, Incralac has performed
poorly and is not generally recommended for use.
Reversibility of both wax and Incralac coatings is another serious problem. As noted
previously, it can be very difficult to remove wax from porous or heavily corroded surfaces,
and removing aged Incralac may entail the use of large quantities of toxic solvents. In terms of
removal, Ormocer coatings may be no better.
The problems of coatings for outdoor statuary are so pervasive that, even in industry, there
are no organic coating systems systems recommended for use on unprimed metal surfaces.
Coupling agents, designed to enhance the bonding of polymer systems to a metallic sub­
strate, have become industrially important, but there are few accounts of their use in the
treatment of copper or bronze sculpture. One coupling agent that has been used in a series of
trials on copper objects by the Getty Conservation Institute and the Swedish Corrosion Insti­
tute is neopentyl (diallyl) oxy tri (dioctyl) pyro-phosphate titanate (Lica 38), which has the fol­
lowing structure:

= 12.7
CH 2 CH-CH 2 0-CH 2
I

CH 3 CH 2 C-CH 2 -0-Ti{0-P(0)(OH)-0-P(0)[OC 8 H 17 ] 2 } 3
This compound has two reactive ends. One end, the neopentyl (diallyl) oxy group, can react
chemically with protons of the surface hydroxyl groups by an alcoholysis (sovolysis) mecha­
nism. This results in the formation of a Cu-O-Ti linkage and the elimination of an alcohol, as
shown in the following general reaction where M is the inorganic substrate and RO is the neo­
pentyl (diallyl) oxo group:

MOH + RO-Ti{0-P(0)(OH)-0-P(0)[OC 8 H 17 ] 2 } 3 = 12.8
+ ROH
M-OTi{0-P(0)(OH)-0-P(0)[OC 8 H 17 ] 2 } 3
On the second reactive end, the linkage between the metal and the titanto-phosphate groups
provides a good substrate for polymer adhesion to the prepared metallic substrate, although the
nature of the polymer used is critical to the success of the system. The coupling agent has to be
chosen to match not only the metallic substrate but also the chemical groups of the polymer sys­
tem with which it can bond most effectively.
A potential advantage of coupling agents for conservation treatments is that they can be
i
used at very low concentrations, typically of the order of 0.25-0.5% (w/v) in ethanol. Even f the
surface of a bronze is first prepared with a titanate coupling agent, however, the ability of the
polymer to protect it from corrosion depends on an array of factors of which surface adhesion
is only one. For example, f the polymer coat is too thin, oxygen and moisture may still be able
i
to diffuse to the metal surface and cause general corrosion.

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