Patricia Leavengood, John Twilley, and Thomas Van Halm
The monumental sculpture by Isamu Noguchi entitled Black Sun was carved in Mure, Japan from a single 30-ton block of stone quarried from the Tijuca formation in Brazil. The finished piece, a polished torus shape nine feet in diameter and weighing 12 tons, was installed on an outdoor viewing plaza in front of the Seattle Art Museum in 1969. Small hairline cracks in a radial pattern were noticed on the west face of the sculpture during the 1980s. In 1992 the cracks were mapped and measured; by 1994 a definite increase in the width and length of some of the cracks was verified. A core sample and acetate peel samples were taken and analysed. The stone was characterized mineralogically as a black gabbro. The core sample was examined petrographically for evidence of weathering or mineral transformations that might be associated with the crack development. The results disclosed nothing of this sort and suggested that the cracking is a purely physical phenomenon related to stresses in the stone. A program of thermal monitoring was established and measurements were taken over a one year period. A computer model of the sculpture was constructed and the collected data was used to plot expected thermal tension stresses in the sculpture due to temperature differentials. The computer modelling indicated thermal stress patterns consistent with the cracking pattern on the sculpture.
To prevent water penetration into the cracks and the development of secondary deterioration phenomena a fill material of Acryloid B-72 bulked with microballoons was applied in January 1995 and has been monitored for loss and shrinkage since. To date the cracks appear to have stabilized, which might indicate a state of thermodynamic stasis has been reached. Sheltering of outdoor stone sculptures during severe winter weather when the exhibit is closed has been undertaken at the Noguchi Foundation in New York. However, permanent shading of Black Sun to minimize cyclical solar heating has been deemed to be aesthetically unacceptable. Considering the size and weight of the sculpture, as well as its site specificity, movement of the piece to another location is not feasible. Constant monitoring and maintenance of the fill material is the recommended treatment for the present.