Christina Bisulca presented a fascinating paper that not only described the source of and uses for pine resins and insect lacs but also detailed various methods of testing for these substances in the lab. The Arizona State Museum is home to a collection of 35,000 objects representing every major cultural group in the southwest from Paleo-Indian to the historic period. As part of a conservation and rehousing project made possible by a Save America’s Treasures grant, conservators hoped to analyze and identify adhesive resins present within the collection.
Certain plants and insects are found only within particular regions of Arizona due to the huge range of elevation and environmental biomes within the state. Since three of the most prolific cultures, the Hohokam, Mogollan, and Ancestral Puebloan occupied very different biomes, it was assumed their use of natural adhesives would vary according to location. 
The Hohokam, for example used creosote lac, a polyester resin derived from an insect on the creosote bush found in desert lowlands. On the more northerly Colorado Plateau, where the Ancestral Puebloans lived, piñon pine was a plentiful source of diterpenoid resin. In general, Bisulca and her team assumed insect lac was used in the south and pine resin in the north.
Initial analysis was done with Fourier Transform Infrared Spectroscopy (FTIR) but researchers had trouble gathering reference plant material. They also realized these high tech, expensive tests are not readily available to many archaeologists, so they used microchemical tests in addition. Since no reliable microchemical test existed for shellac the team developed one based on the pH dependency of color in anthraquinone dyes, which are extracted from lac insects in Asia. In alkaline conditions the dyes turn from orange to purple. The test worked 80% of the time even in their archaeological collection. Where it did not work the resin had likely been charred or highly heated. The results of the tests challenged their assumptions about pine resin being used primarily by the north. In fact the team found more insect lac in the Ancestral Puebloan collection than pine resin. The lac would only have been available to these regions through trade. The researchers also investigated the use of each material. Bisulca used arrows, common to all three cultures, to highlight the different uses for each adhesive. Insect lac is stronger and less brittle so was used by each culture, regardless of location, for hafting the arrowheads to the shaft.
Overall, Dr. Bisulca’s talk was extremely informative and used excellent images to portray her ideas. The images she used of the lacs and resins as they are found in nature and on ethnographic objects helped to clarify the differences between the source and use of each. I have to admit I had not considered the significance of these resins in terms of their use in tracing trade routes and cultural practices. I am curious to know what similar substances would have been used by Eastern and Plains area cultures and how well such resins would be preserved in damp archaeological environments. This will certainly change the way I approach native material at my own site.
All images are from a poster on insect lac by Christina Biscula available here.
Information about the authors is available from The Arizona State Museum’s website.
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AIC Blog Archives: Conservators Converse
Former Blog of the American Institute for Conservation