Written by Lu Allington-Jones, Senior Conservator at the Natural History Museum, London.

Whilst trying (not very successfully) to find a “cure” for fat burn (Figure 1), I made an unwelcome discovery: sometimes the shrinkage temperature of deteriorated skin is actually lower than room temperature. This means that the skin will irreversibly shrink as soon as any water-based treatments are applied.

Figure 1. Fat burn can cause skin to rip and specimens fall apart

Shrinkage temperature (Ts) is commonly used in leather conservation to determine the level of deterioration, and the effectiveness of treatments. Ts is the temperature at which 2 corian fibres immersed in water show simultaneous and continuous shrinkage activity. It shows the level of deterioration because it indicates destabilisation of collagen fibres. Ts of fresh skin is 65^oC and in deteriorated leather this can be reduced to 30^oC (Florian, 2006). Ts is measured by immersing samples of leather (or skin) in water and gradually increasing temperature until shrinkage activity is observed under a microscope (Larsen et al. 1996; Vest & Larsen, 1999).

I was testing some common consolidants and leather treatments on samples of unregistered historic taxidermy (Figure 2) when I found that Ts was far lower in deteriorated skin than in deteriorated leather, because it is untanned.

Figure 2. Heating deionised water on a hotplate in the lab

When testing a sample pre-treated with Lascaux 4176 Medium for Consolidation, I initially thought that the Ts was incredibly high – I kept increasing the temperature on the hot-plate and still no shrinkage occurred – was this then the ideal treatment? Then I wondered if it had already shrunk…had shrunk as soon as I had applied the consolidant… I therefore chilled the water, glassware and consolidant in the fridge (5^oC) before applying to fresh skin samples.

When I repeated the test on new untreated samples, I discovered that the Ts of the historic pheasant skin was 36^oC. This is potentially reachable on a hot day with no air-conditioning. Far more worrying was the Ts of the fat burned Pheasant skin at 7^oC, far below normal room temperature.

Figure 3. Bar graph showing Ts of historic Redshank and historic and fat-burned pheasant Pheasant, compared with fresh skin.

So, the lesson from this tale is – unless you know the shrinkage temperature of your specimen – chill any aqueous treatment before application to historic skin.

For more information on our battle against fat burn, please see Allington-Jones, L. and Bailey, R. 2020. Treatments for lipid oxidation in taxidermy and impact on DNA recovery. Studies in Conservation

Florian, M-L., E. 2006. The mechanisms of deterioration in leather. In Kite and Thomson (eds.) Conservation of Leather and Related materials. Butterworth-Heinemann, Oxford. 36-57.

Larsen, R., Vest, M., Vestergaard Poulsen, D. and Bøgvad Kejser, U. 1996. Determination of hydrothermal stability by the micro hot table method. In: R. Larsen (ed.) Deterioration and Conservation of Vegetable Tanned Leather. Protection and Conservation of European Cultural Heritage, Research Report no.6. The Royal Danish Academy of Fine Arts, School of Conservation, Copenhagen. 145-159.

Vest, M. and Larsen, R. 1999. Studies of changes in the shrinkage activities of leathers and parchment by the micro hot table method (MHT). In: R. Larsen (ed.) Methods in the analysis of the deterioration of collagen based historical materials in relation to conservation and storage. Preprints. Advanced Study Course 1999 6-10 July School of Conservation, Royal Danish Academy of Fine Arts, Copenhagen. 143-150.