Atmospheric microclimates : damaging & protecting indoor cultural heritage materials

• Main Author: Crawford, James Bruce
• Published: University of Warwick 2016
• Subjects: QC Physics
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.707428

This thesis in applied physics analyzes and solves problems regarding destructive and protective atmospheric microclimates in order to slow down the deterioration of indoor cultural heritage materials. Fresh approaches to two longstanding areas of concern are made in laboratory studies with a view to future field testing and evaluation by heritage conservation practitioners and museum display case manufacturers. Methods used for observing and analysing materials are macrophotography, optical microscopy, metallography, image analysis, X-ray diffraction, scanning electron microscopy and energy dispersive spectroscopy. Structural engineering methods comprise geometric surveys and compression and deflection tests. Airtightness measurement techniques were tracer gas decay and pressure decay. Investigations into the corrosion of lead by oak-emitted volatile organic compounds (VOCs) revealed the presence of a rarely reported crystalline phase which was confirmed to be associated with modern oak. The morphology and composition of the observed products of corrosion were almost identical to those found in field environments, but differed in specific ways to prior laboratory studies which used synthetic or oak-emitted VOCs. Five novel surface protection treatments using materials derived from ethanolic solutions of high molecular weight carboxylic acids (CH3(CH2)n-2COOH; n = 10, 12, 14, 16 or 18) were tested atmospherically for five years. Hexadecanoic (n = 16) and octadecanoic acid (n = 18) treatments inhibited corrosion of lead exposed to oak VOCs by up to ≈60%. These effective treatment materials showed lamellar morphologies similar to those made from aqueous solutions of lower molecular weight carboxylic acids (n = 10 or 11). In contrast, two materials with non-lamellar morphologies made from ethanolic decanoic acid (n = 10) and tetradecanoic acid (n = 14) were found to increase corrosion. Inspections of industry-made medium-sized (≈1 m3 ) museum display cases with doors located systematic leaks in sorbent compartments, around exhibit compartment doors and in ceilings: providing vertical leakage circuits driven by stack pressures (˜0.1 Pa). A walk-in test chamber with a custom-designed tracer gas and environmental monitoring system was made to evaluate three devices for passively controlling airtightness on a pair of display cases. Airtightness of the cases was increased by 7 to 13 times. Despite remaining leaks in the ceilings, the cases reached or went beyond the microbarometric limit; due to new pressure-proofed sorbent compartments and novel convex gaskets for the compliant unframed exhibit compartment doors; solving leakage caused by restorative forces imposed by gaskets. The greatest airtightness (0.013±0.004 air exchanges daily) was achieved by a case retrofitted with a pair of high compliance bellows (Δ±2 Pa) with high conductance pipework, while being subjected to temperature cycling (Δ+2°C daily) and natural barometric pressure fluctuations. A formula derived from Ohm’s Law to evaluate the minimum required airway resistance of gaps in display cases was proposed as a new way to calculate, and then measure, the capability of a case to have its airtightness increased by installing bellows.