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Abstract

Indigo has a long and prestigious history, from the ancient past to our blue jeans today. Indigotin is a dark blue colorant molecule that is produced from many plants, including indigo (Indigofera), woad (Isatis), and knotweed (Polygonum) when the leaves are crushed, fermented, and then reacted with oxygen. All of the indigo-producing plants undergo the same chemistry, so they are difficult to differentiate from each other in archaeological textiles. There are many analytical approaches to the characterization of blue dyes in conservation science, though few are able to reliably distinguish between blues obtained from the various plant sources. Archaeologists and conservators are interested in sourcing the colors that ancient people used, as this provides insight into their selection of materials and the chemical technology necessary to produce dyes. We present here the initial stages of work to use direct analysis in real time (DART) mass spectrometry (as well as other ionization methods) to differentiate blue dyes made from indigo, woad, and knotweed on a variety of fiber substrates. Based on the results for the lab-prepared materials, we will apply the best method to archaeological fibers previously shown to contain indigotin.

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