Dynamic Mechanical Analysis (DMA) and its role in assessment of state of preservation of collagen -containing cultural heritage objects

Our work has focused on using DMA to measure response of collagen containing materials (parchment and leather) to moisture. The aim has been to establish guidelines in terms of environmental conditions (RH and T) for their safe preservation. Parchment (accelerated aged and historical from several European archives) was first studied by DMA in the EU project MAP (Microanalysis of Parchment) and IDAP project (Improved Damage Assessment of Parchment) . This led to development of damage assessment categories. Measurements were made either using a temperature ramp (-100°C to 250°C) or by exposure to programmed increase in RH (20-80%) at 1%RH/min, and currently complemented with neutron radiography. From DMA-RH the rate of displacement was found to have some correlation with extent of intact D-banding in AFM (Atomic Force Microscopy) images of collagen in these parchment samples. The lower the displacement rate the more damaged the sample and the lower level of intact D-banding. DMA-RH was then used to evaluate effect of novel nano-based materials used in their conservation treatment (Nanoforart project). Results for treated samples showed minimum change in displacement rate and minimum change in modulus of treated samples after aging, in comparison to the untreated sample, where a significant increase in modulus was observed. DMA-RH was also used to test effects of their exposure to volatile organic acids (MEMORI project) and data contributed to the English Heritage decision support model. From thermal DMA data, one set of criteria used to assess damage was based on the behaviour of the low temperature relaxation peak. This peak, which has been observed in collagen and other hydrated proteins, has been widely studied , particularly by dielectric techniques. It is generally accepted that the process is related to water in the hydration shell of the protein and current literature has highlighted that changes in this peak are associated with extent of hydration processes. There are differences in moisture sorption between historical parchment and modern parchment shown by our DMA data due to differences in extent of collagen denaturation. The structural change will affect its ability to interact with water molecules and how they are accommodated in the structure, and will be affected by proportion of collagen and gelatine . This lies at the basis of our damage assessment and is supported by AFM which showed alterations in collagen D-banding as well as localised areas of gelatinization and loss of ordered structure.