2 MICROBALLOON-FILLED EPOXY RESIN

THE CHIEF ARGUMENTS AGAINST THE USE of epoxy resins in the filling of wooden objects are their non-reversible curing and their comparative hardness. In ideal circumstances “…the conservator shall use materials which can be removed most easily and completely.”5 Obviously, direct application of a pure epoxy resin to a wooden object results in a dramatic and irreversible change in the characteristics of the wood. However, inert fillers can be added to these resins so that the resistance to compression of the resultant material is below that of the surrounding wood and its surface adhesion to a substrate very slight. In this study Araldite AZ 3456 (with catalyst HY 3456)6 was loaded with phenolic microballoons (Union Carbide Corp. No. BJO-0930)7 to form a putty with appropriate characteristics. This epoxy resin, which has furfuryl alcohol as a solvent, was chosen for its comparatively low viscosity (1,000mPas) which permitted a very high loading of microballoons. It thus forms a very light filler which can be carved and inpainted with ease. In addition, this material presents the wood substrate with an extremely small surface area of epoxy resin, thus virtually eliminating bleeding of the resin into the wood before curing (assuming the wood has not been consolidated prior to filling). The rapid curing time of approximately three hours also reduces the likelihood of resin transfer to the wood.

The choice of phenolic microballoons requires some explanation. The authors have used both glass and phenolic microballoons for experimental fill materials and the only reason that the latter is preferred is that they appear to be more easily inpainted when working with degraded wood. This is, of course, quite subjective and depends also upon the inpainting medium chosen. The possible degradation of the phenolic resin is not a problem as the balloons are totally encapsulated in epoxy resin, and the fill itself is isolated from the substrate.