3 PRELIMINARY NOTES ON AGING PROPERTIES OF THE FILL MATERIAL

Degradation of polymers is complex and depends on numerous factors (McNeill 1992). To date we have observations only on color fastness and physical stability of the fill material;2 we also have some indication of whether or not the wax-resin shows cold flow relative to the individual components.

The PVAC component in the wax-resin has been shown to be a relatively stable material and was classified by Feller (1978) as a class A material.3 When Burke conducted the blue-wool test on the EAA A-C 540, he found that it actually bleached slightly. A recent article by Down et al. (1996) describes PVAC AYAC as having a “fair resistance to yellowing,” but also confirmed its emission of small amounts of acetic acid.

We do not know the glass transition temperature of the fill material, but the combination of polymers probably affect their respective Tgs. The PVAC's Tg goes up slightly, and the EAA's melting point goes down. It has been suggested that fillers such as pigments and marble powder may behave like tiny hurdles that the polymers have to move around, which slow them down (Schlumpf 1990). While one very large fill of the fills described below has changed its shape, all the others have remained apparently unchanged over the course of a decade.

To further investigate the Tg of the material, a batch of pure PVAC AYAC and a batch of the fill material were made in the Objects Conservation Laboratory at LACMA and placed in dated containers on January 10, 1994, where they are being monitored. The PVAC has already begun to move, but the wax-resin has not. Another test being conducted at LACMA with a starting date of April 15, 1995, involves placing a 50 gm weight on a block of wax-resin to see if it displaces the resin. No movement has occurred to date (July 12, 1996). Certainly the ongoing tests described above are rudimentary, and many more tests will have to be done to examine the possible degradation of the material.