AIC's 52nd Annual Meeting

Reflectance Transformation Imaging (RTI) has been well established as a useful tool to study cultural heritage objects. Published studies vary widely vis-a-vis media focus, ranging from paper-based works, to archaeological stone, polished metal and more. Glass, due to its specular surface and transparency, presents particular challenges to this technology. Cooper Hewitt Smithsonian Design Museum (CHSDM) has embarked on an extended research project focused on recording cold worked glass surfaces using micro-RTI, in order to determine the most successful way to image this media through this technique as well as to investigate the surface morphologies and tool marks created through physical and chemical working methods. RTI capture strategies have varied from digital microscopy to a high-quality macroscopic camera lens. As research progressed, we received support to facilitate the creation of a bespoke four-way illuminated RTI dome featuring UV, IR, blue and white light LEDs.

This paper focuses on unexpected outcomes, learnings, and lessons gleaned along the journey into this research topic. Glass studied included 18th Century Dutch wheel cut vessels and diamond stippled glass, and 20th Century acid-etched French glass. Wheel-cutting of glass surfaces traditionally relies on an abrasive oil slurry applied to copper wheels, which rotate as they cut into the surfaces. The technique flourished by the mid 17th Century and later in the Netherlands. Similarly, diamond tipped engraving, often executed by female artists, found favor throughout the same period while diamond stippling reached a pinnacle in the 18th Century. The cut and stippled forms on these cups, goblets and vessels create sculptural and even ghostly images that contrast with the light reflected from untouched surfaces.

In contrast, acid etching relies on hydrofluoric acid to cut into the glass surface rather than a mechanical tool or physical force. The technique comes into commercial use by the latter half of the 19th century. French glass firms such as Daum Freres took advantage of this technology to create frosted surface effects, while artists such as Maurice Marinot exploited the acid to create deep recesses and intricate surface textures. Samples prepared specifically for this study to complement the museum collections included both acid-etched flash glass as well as diamond engraved surfaces. Our talk will present strategies for successful RTI capture on reflective, water-white transparent glass using the methods discussed above, as well as deeply etched glass objects. We also investigated imaging cast proxies for challenging or awkward shaped pieces. Finally, we will discuss what we have learned about surface changes to the glass following varying working methods.