This case study documents the degradation of a 1960s sculpture by William Turnbull. Transparent Tubes is fabricated from polymethyl methacrylate (pMMA), a material commonly used by artist during the last 50 years and usually regarded as a stable material. The degradation of pMMA artworks is rarely documented. A marked difference was observed in the condition of the acrylic tube sections of the sculpture in comparison to the acrylic sheet components. The tubes had degraded while the sheet material had not. This paper highlights some of the possible contributing factors for the degradation of the pMMA and for the discrepancy in the level of degradation between the two forms of the material. The options for the future display of the sculpture are also discussed, and the conclusion reached in collaboration with the artist. -- SCMRE