Unraveling the history and science behind ancient decorative metal threads

The researchers will present their results at the American Chemical Society (ACS) Fall 2019 National Meeting & Exposition.

"This project began when we were asked to investigate the metal threads of a 14^th century Italian textile using a proteomics-based approach," says Caroline Solazzo, Ph.D., who is one of the project's principal investigators. Her team published a study last year characterizing the protein-containing membranes and adhesives in the threads of this artifact, which were made from animal products such as cow hide and pig intestine. Now, the team is reporting on their investigation into the exact composition of metal fibers of this and other historical objects.

"Conservation science is a unique area of chemistry research," says Aleksandra Popowich, Ph.D., who is presenting the work at the meeting. "We are using microscopy techniques that allow us to build a 3D view of the threads, so we can see things like layering and micro-structure that give us insight into when and how the fibers were made." Both Solazzo and Popowich are researchers at the Smithsonian's Museum Conservation Institute, a center for specialized technical collections research and conservation of artistic, anthropological, biological and historical artifacts.

While decorative metal threads have been a subject of historical research interest for decades, studies to determine their manufacture and makeup have relied on cross-section analysis to view the internal metal structure. The current study, however, was driven by the desire to maintain the integrity of artifacts.

For this particular work, Popowich and her Smithsonian colleagues Thomas Lam, Ph.D., and Edward Vicenzi, Ph.D., obtained 30 samples from the Fashion Institute of Technology. The samples originated from Europe, Asia and the Middle East, and some of them were nearly 1,000 years old. The types of threads were diverse; some were strips of metal, others were strips of paper wrapped around fibers. Many of the pieces were religious textiles, such as vestments, demonstrating the cultural and historical importance of metal threadwork.

To get a closer look, the researchers developed a strategy that combined energy dispersive X-ray spectroscopy and correlated micro X-ray fluorescence. Together, the methods provided a high-resolution map of the threads' elemental composition and thickness. These techniques only required a few micrograms of material, leaving most of the threads intact for future conservation efforts.

The resulting surface images and cross-sections showed that most threads had a combination of gold, silver and sometimes copper or zinc, creating a layered structure that highlighted the intricacy of the craftsmanship. The researchers learned that some thread-making techniques vary by culture, but other methods did not change much over time. For example, data from two French threads, one from the 16^th century and one from the 18^th century, showed that the process of rolling super-thin metal wires and wrapping them around a core material was largely unchanged between those years. In addition, the measurements taken using this method align with historic sources and data from computer simulations.

With this pilot investigation completed, the researchers plan to further develop this strategy to the point where they do not need to destroy a piece of the sample at all. This advance could open up the list of artifacts for study to include those that are too culturally important to damage for the sake of research. Such a method could also expand their work to include other materials, such as gilt leather, tapestries or gilded furniture.