AIC's 46th Annual Meeting

There are many types of translucent papers, each with its own set of conservation issues stemming from various manufacturing processes. The characteristic that makes them stand apart from other papers—transparency—can itself be at risk when there is a need for applying mending or lining tissues. This project explores the physical aspects of paper transparency, and investigates the concept of optical clearing (transparentizing) of repair tissues, with the goal of achieving appropriate repairs on translucent papers without dramatically increasing the opacity of treated areas. The term “optical clearing” is borrowed from the fields of biology and medical research; it refers to the process of rendering biological tissues transparent through the application of clearing agents, which minimize the scattering of light and allow greater visibility for microscopy and imaging. This is similar to some historical processes of transparentizing paper, in which oils, waxes and rosins were added to fill light-scattering interstices, allowing more light to travel unimpeded through the paper web. This concept is applied to conservation repair tissues, with the goal of determining a coating to serve dual functions: optical clearing agent and reactivatable adhesive.

A wide range of adhesives and coatings familiar to paper conservation was tested for their transparentizing effects on a variety of repair tissues, including more traditional Japanese papers and the recently developed nanocellulose papers. Opacity measurements were taken using a spectrophotometer and the contrast-ratio method. Acrylic polymer dispersions proved to be the most consistently successful clearing agents. The most substantial transparentizing effects occurred in gampi-fiber Japanese tissues, with some cleared by over 90% of their original opacity. This can be attributed to the superior film-formation qualities of the acrylic dispersions and their amorphous polymeric structure. The heat-reactivation capability of acrylic adhesives also proves advantageous for the treatment of translucent tissues, which tend to react dramatically to moisture.

A range of repair methods was applied to modern translucent tissue samples. These were measured for opacity before and after treatment to compare to repairs made with cleared tissues. SEM cross-sectional imaging was used to visualize adhesive penetration. Attempts at removing each repair were also made to characterize ease-of-reversibility. The long-term stability of optically cleared repair tissues is considered alongside an aging test that measures the yellowing and turbidity of acrylic transparentizing coatings under different light exposures.

The application of the optically cleared tissues is discussed via the treatment of two large objects possessing damaged transparent overlays: Atlas Photographique de la Lune (Observatoire de Paris, 1896–1910) and Frank Lloyd Wright’s “Wasmuth Portfolio” (1910). The suitability of different clearing agents in varying contexts is also considered, such as in the treatment of coated transparent papers.