OBSOLETE FILL MATERIALS FOUND ON CERAMICS
3 IDENTIFICATION OF MATERIALS AND TECHNIQUES
Identifying repairs on ceramics has always been challenging, for both the collector and the conservator. Early recommendations for testing restorations included tapping with the edge of a coin; one source suggested that “the test of smell is also a useful one” (quoted in Litchfield 1951, 20). Unfortunately, as with most early conservation treatments, the lack of written records has left the objects conservator with the task of identifying the materials that were used, either by empirical testing or now, with the aid of sophisticated analytical techniques.
Visual observation under natural light and/or ultraviolet light (long-wave and short-wave), as well as microscopic examination, has long been recognized as the first steps in identification (Rorimer 1931; Noble 1965). These methods have limitations, however, and need to be supplemented with chemical spot tests (Feigl 1946), solvent solubility, and, if available, organic and inorganic analysis.
A good repair may sound the same, have a similar color, and be virtually invisible—exactly as the restorer planned. Many of the adhesive and putty mixtures were colored to match the ceramic and show little relationship to the color or texture of the original raw materials. Moreover, in most instances a final application of paint or surface coating completely hid the repair. Examination under ultraviolet light (long-wave or short-wave) may detect only the presence of a repair, or the fact that the surface has been repainted, sometimes with little or no fill underneath. Most inorganic pigments do not fluoresce (de la Rie 1982), however, and a resin bulked with pigment will have much less fluorescence, if any, than might be expected. Identification of repairs by smell is certainly a limited technique.
Many fill mixtures were “home-made trade secrets,” whose recipes were guarded carefully even if they were sold by workshops “to the amateur, hobbyist, collector, and housewife” (Klein 1962, xi). Although the Klein workshop was famous for its self-proclaimed “Klein method” (Klein 1962), the adhesives and fill materials that it used and sold to the public have yet to be accurately identified.
An adhesive, by its very nature, is a fill material. It bonds fragments together and fills in small losses along the break edge. Once clear or light-colored adhesives were begun to be used, it became possible to tint or color the adhesive so that the fine losses along a break edge were filled in as the fragments were assembled. The limiting factor was the resulting thickness of the adhesive, which would interfere with tight joins. Nevertheless, this practice was recommended for the repair of porcelain using epoxy adhesives (Larney 1975).
Many of the materials and techniques used on ceramics were certainly borrowed from other disciplines, including sculpture, painting, and architectural finishing. As noted above, however, the field of ceramics restoration displays historical trends as well as individual resourcefulness. The two main functions of fills on ceramics are to compensate small and large losses. Small losses and cracks can be filled with little need for structural support, assuming that fragments have been properly assembled with a suitable adhesive (not always the case, as experience has shown). Large losses require fill materials that can fill the area and contribute strength and structural support to the whole.