Mammalian and Fish Gelatines at Fluctuating Relative Humidity

It is well known in conservation, that humidity influences the material properties in different ways. Karpowicz (1989) and Zumbühl (2003) have shown that mammalian collagen-based glue films continuously contract after humidity cycles. The induced stresses that can lead to cohesive peeling of paint. This paper explores the question whether fish gelatines, which are supposed to be more elastic, builds up less tension after experiencing high humidities.
The strain tests at constant stress load over 10 climate cycles by Zumbühl were reproduced for gelatine. The following adhesives were selected: A medium Bloom grade, 180 Bloom, type A (Roth), was used as a standard gelatine. Two fish gelatines were further investigated. One from warm water fish and one from cold water fish. Both were selected with a low degree of bloom. All aqueous adhesive solutions were prepared 12% per weight. Standardised films were produced with a film applicator (Erichson, width: 13 mm, wet film thickness: 300 μm) on melinex. Subsequently, the films were dried 14 d at 21±1°C and 50±3% relative humidity. The films were measured in a specially manufactured PMMA climate chamber. The mechanical tests were performed using a tensile testing machine (Zwick 1120, test expert) at a constant force of 0.1 N. After preconditioned at 53% rH (saturated salt solution, magnesium nitrate, Mg(NO3)2 6 H2O), 10 climate cycles were recorded. Each climate cycle lasts 24 h (humidification: 9 h at 75% using a saturated NaCl solution, Drying: 15 h at 53%) at a constant temperature of 21±1°C.

Basically, polymer films show a characteristic behaviour in changing climates. During humidification, the films expand due to the swelling of the material. During the drying phase, the films contract again due to the desorption of water. However, it can generally be observed that the film dimension no longer corresponds to the initial length after a measuring cycle, which is due to internal, structural changes. Here, two processes run against one another. If the film is plasticised by water absorption, the film is stretched. If, on the other hand, a reorganisation of the molecular structure occurs during the moisture cycle due to the improved molecular mobility, then a smaller film dimension results after drying.
Mammalian gelatine: After the first moisture cycle, a considerable dimensional change of -1.3% was observed. After that, the dimensional fluctuation levels off. Within one moisture cycle, elongation and shrinkage is c.1.1–1.5%. The total contraction after 10 cycles was 1.2%.
Warm-water-fish gelatine: After the first moisture cycle, a dimensional change was observed as well, however, only -0.6 %. The total shrinkage after 10 cycles was 0.9 %.

Cold-fish gelatine: This material shows a completely different behaviour than the other two gelatines. Even with a very low application of force of 0.1 N the gelatine showed within four humidifications a plastic deformation of up to 55%.
Finally the consequences for conservation will be outlined.