FILLS FOR WHITE MARBLE: PROPERTIES OF SEVEN FILLERS AND TWO THERMOSETTING RESINS
ELEONORA E. NAGY
1.1 SELECTION OF PROPERTIES AND METHODS FOR TESTING
Fillers have been used primarily as extenders, and the full range of their benefits has not been extensively explored. This article investigates how properties of fillers such as shape, size, color, microstructure, density, and translucency affect the appearance (color, texture, translucency) and physical properties (density, hardness, elasticity, plasticity, mechanical reversibility, water sensitivity) of their composites. The evaluation of these properties is addressed for fills intended to compensate white translucent marble or similar stones (alabaster, limestones) in the outdoor environment. It is hoped that the conclusions of this study will provide some guidance for the use of fillers in sculpture and architectural conservation.
To determine important parameters and testing methods in relation to optimal fills for whitish translucent calcareous stones exposed to the outdoor environment, the following properties are considered: density, water sensitivity, color, refractive index, elasticity, and plasticity.
Three kinds of samples were used: prepared composite samples with various fillers; two different pure thermosetting resins; and natural marble samples. The latter two were used as controls. Testing methods listed below were carried out on a total of 187 samples.
The density of a fill should be close to or less than that of the original material. This density permits reversibility of the fill by mechanical means and has a potentially faster rate of deterioration than the original stone. A possible direct correlation between the density of the filler and its resulting composite was also investigated.
1.1.2 Water Sensitivity
The water absorption and drying rate of a fill should be close to that of the object, so that a wet fill does not introduce moisture and therefore potential decay in the original stone. A fill should also follow the changing hues of the wet-dry cycle of the object. Although a 48-hour water immersion test is extreme relative to normal ambient external conditions on stone surfaces, such a test will indicate comparative wetting and drying patterns of stones and fills. Considering time restrictions and the relatively small size and number of samples in this study, immersion may also draw attention to possible long-term weathering effects sooner than a capillary rise procedure.
1.1.3 Color and Texture
An acceptable color and texture match between the fill and the surrounding stone is an important requirement for camouflaged stone repairs. Evaluation of texture was accomplished empirically. Color matching was rated by both empirical and instrumental methods.
1.1.4 Refractive Index
Translucency is one of the most difficult requirements when matching white marble. On translucent stones, the factor that determines the appearance of a crack, gap, or filled lacuna is the difference in refractive index (RI) between the original stone and its fill. A material has only one RI if the physical properties of its particles are the same in every direction (isotropic). When the RI of an isotropic material matches that of the surroundings, the material is described as transparent. Anisotropic materials have more than one RI (birefringence) and are often indicated by average refractive indices. With the exception of the Microsphere and Eccosphere fillers, which are isotropic crystals, the fillers used in this experiment are not uniform crystals and have more than one refractive index.
1.1.5 Elastic and Plastic Behavior
A fill should allow expansion and contraction of the repaired stone without damaging the original material, thereby making the fill mechanically reversible. Should both materials be exposed to the same range of strain (deformation) and stress (compressive force), once the force is released, the fill may show some signs of permanent plastic deformation sooner (enter its plastic deformation range at lesser stress) than does the stone, which would still return fully to its original state (elastic deformation). To assess elastic and plastic behavior, each composite and control was tested for compressive strength and percent strain at break. Composites were sought that exhibited lower compressive strength and similar or greater strain at break (“longer” performance in the plastic region) as compared to the original stone (i.e., natural marble).
1.2 SELECTION OF MATERIALS AND THEIR COMBINATION
Thermosetting resins have considerably greater strength and hardness than other resins used in conservation, making their use for structural repairs of stone very common (Larson 1978; Domaslowski 1988; Barclay and Mathias 1989; Nagy 1991; Naudé 1991; Domaslowski 1995; Technology Organization 1995;). This was the primary factor for testing thermosetting resins here. Two stone adhesives, Sikadur 35 Hi-Mod LV epoxy and Akemi Marmorkitt 1000 Transparent/Clear Flowing polyester resins, were chosen for testing. Both were developed for exterior use and are widely used in architectural and sculpture conservation.
Seven fillers—calcium carbonate, Cab-o-sil, mica, marble dust, Globe-o-sil, Microspheres, and Eccospheres—were chosen because they are white or translucent and are inert. Both resins were combined with each of the seven fillers, and each resin was mixed with a filler in 3, 4, or 5 different concentrations. These composite concentrations are listed in tables 1–4, where each concentration represents the mean of 3–5 samples. To the total number of 173 composite samples, add 8 natural marble specimens (4 fresh and 4 decayed) and 6 pure resin samples (3 of each resin) that were used as controls, making the total number of samples used in this study 187.
TABLE 1. EFFECT OF FILLERS ON THE DENSITY OF RESINS
TABLE 2. PERCENTAGE OF WATER ABSORPTION AND DRYING RATE
TABLE 3. AVERAGE CIELAB MEASUREMENTS
TABLE 4. EFFECT OF FILLERS ON THE COMPRESSIVE STRENGTH OF THE PURE RESIN
Pigments, typically used as coloring agents, may be considered as fillers when used in large volumes. However, the investigation of pigments as fillers is not included in this article.