| Abstract: | SUMMARIES. The Museum of London has many archaeological timbers on display, most having been treated by freeze drying after immersion in polyethylene glycol (PEG 200 and 4000). In a recent temporary exhibition, a Roman wooden lintel that had been freeze-dried was displayed in a small case that was lit from below. Within weeks of installing the object, a white deposit was found on the internal glass surfaces of the case and on some of the objects. Samples of the deposit were taken and analysed by Raman microscopy and electron microprobe analysis, showing that the deposit was a form of PEG combined with silicon and calcium. This was unexpected as PEG is normally not volatile at ambient temperatures and may be due to the degradation of PEG to components of lower molecular mass. The source of the metal cations may be the wall painting fragment present in the display case. The cause of the problem was mainly attributed to the elevated temperature (4 °C above ambient gallery temperature) and relative humidity in the display case. The ventilation of the base of the case, which housed the fluorescent lighting, was insufficient even though trials with a mock-up case had indicated otherwise. This method of lighting has become popular in displays at the Museum of London; better ventilation of lighting compartments is now planned, particularly when organic materials are to be displayed. |
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