Reducing the risk of open display: optimising the preventive conservation of historic silks

Abstract

English Heritage properties contain a wealth of textiles on open display, however these are ephemeral objects. Amongst the natural fibres found in historic houses, silk is reported to be the most vulnerable to damage, especially from light. The critical deterioration factors for silk deterioration have been reassessed highlighting the important role of humidity, which has previously been overlooked. Monitoring behind a number of tapestries has recorded the formation of high humidity microclimates. This is a possible reason for the similar condition of brightly coloured samples taken from the reverse of a tapestry and the same thread which had faded on the displayed side.

Kinetics experiments studied the rate of silk deterioration and suggest the activation energy is approximately 50 kJ mol-1, although this may vary for other types of silk, such as weighted materials. However as elemental analysis demonstrated around 10% of the 100 samples, taken from over 1000 objects containing silk in the English Heritage collection, were from tin-weighted silks, plain silks were the study’s focus. Year long accelerated ageing experiments have demonstrated that although the inclusion of UV radiation during light ageing increased the rate of deterioration, light ageing caused small changes to silk. Thermal ageing with different humidity levels demonstrated increasing the relative humidity (RH), increased the rate of silk deterioration significantly. Further degradation of silk was observed when samples had been contaminated by the saturated salt solution used to create the RH environment during ageing. During ageing increased RH and increased temperatures led to greater yellowing of silk samples.

Experimental results have been used to make preventive conservation recommendations including lowering the RH below 50%, reducing the temperature and the continued exclusion of UV radiation. A theoretical silk deterioration curve for unweighted silk has been drawn, from which initial isoperms have been plotted. The analytical results have been compared with near-infrared (NIR) spectroscopy using multivariate analysis (MVA). This developed a predictive model for the tensile strength of silk using the NIR spectra. The potential of this non-invasive, non-destructive technique to monitor silk condition in situ has been tested at Brodsworth Hall and shown to rank the condition of samples successfully.

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