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A Study of the Moir? Pattern of Tortoiseshell: Morphology of the Pattern, Techniques for Documentation, and Alterations of the Pattern and Shell by Accelerated Light Aging

  • Author(s): Day, Lesley Anne
  • Advisor(s): Pearlstein, Ellen
  • Scott, David A.
  • et al.
Abstract

The subject of this thesis is to increase understanding of an observed patterning that can be seen on both processed cultural materials made of tortoiseshell and unprocessed turtle shell scutes. The pattern manifests as random swirling lines that appear topographical or have the appearance of darkened lines embedded within the scute that are reminiscent of watered silk or a moir? pattern. Though the mechanism for the development of these patterns has not been reported in any technical literature, multidisciplinary research and testing performed for this thesis supports the theory that the patterning is undoubtedly formed by the periodic depositions of keratin that occur as the turtle grows.

The patterning visible on the raw material of the scutes is also often visible on objects and artifacts that are made of tortoiseshell, though it is usually reported as a change in appearance due to aging. In response to this observation, this thesis research seeks to determine whether the visibility of the moir? growth patterns detectable in tortoiseshell become intensified upon prolonged exposure to light, leading to a darkening of the pattern lines and a shift in color in the lighter parts of the shell. The current study includes light aging, and color and gloss measurement, as well as imaging before and after aging, to determine whether the enhanced patterning visible in some tortoiseshell artifacts is indicative of environmental exposure, or has another source. An important outcome of this research is a better understanding of photochemically induced alterations in tortoiseshell, and preventive lighting guidelines for tortoiseshell materials based on the findings of the light aging study.

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