AIC's 51st Annual Meeting

Glass beadwork is a form of art practised in many Native North American cultures. Unfortunately, some glass beads have an inherently unstable composition leading to the occurrence of “glass disease.” The exact trigger for the onset of glass disease is unclear, but once it has begun, some of the alkali components of the glass migrate to the surface, and the remaining glass becomes weakened. If the relative humidity surrounding diseased beads is not kept at a stable value in the 40s %RH, the process of deterioration continues until the affected beads flake, crack, and detach from the decorated surface. Since beads of a similar colour have similar composition, this process can lead to loss of whole fields of colour in a design.

Since not all beads are affected, and because it is difficult and expensive to stabilise the relative humidity in the required range for large collections, we would like a way to identify which collections are actively deteriorating, or which may be at elevated risk of deterioration in the future so that these can be prioritised for storage in stable relative humidity.

O’Hern & McHugh (2013) have developed such a test: a moist pH indicator paper strip is pressed to a bead for 3 seconds, and then the indicated pH is recorded. This indicated pH test cannot actually measure “the pH” at a surface, but it can give some indication of the quantity of alkali on the surface which is available to be measured. From 2015 to 2019 the Field Museum conducted a survey of several thousand beaded items from the Native North American Plains using this indicated pH test.

Recently we have been reviewing whether the indicated pH results have any clear relationship to the elemental composition of the beads. In this poster, we report the results of using laser ablation - inductively coupled plasma - mass spectrometry (LA-ICP-MS) to determine the surface and bulk composition of surveyed beads of known surface pH. Beads which had become detached from items (either through the bead cracking or through thread failure) were selected for testing across a range of indicated pH values from 6.5 to 10. Comparison will be undertaken to assess whether a correlation exists between the surface pH of the beads and their depletion of alkali as measured with LA-ICP-MS.

Reference:
O’Hern, Robin & Kelly McHugh. 2013. ‘Deterioration and conservation of unstable glass beads on Native American Objects.’  Bead Forum. 63:1-13.