Műtárgyvédelem, 2009 (Magyar Nemzeti Múzeum)
Kovács Petronella: Zsugorodási hőmérséklet - a bőrök lebomlási fokának értékmérője
Kovács Petronella • Zsugorodási hőmérséklet - a bőrök lebomlási fokának értékmérője Shrinkage temperature - measure of the decomposition rate of leathers Petronella Kovács The physical and mechanic properties of leathers have rarely been determined with the measurement of the shrinkage temperature (Ts) in Hungarian conservation practice. The author had the opportunity within a DLA research to assess the conditions of 13 travelling chests prepared in Transylvania in the second half of the 1700’s, which were covered with leather and decorated with metal ornaments. She could also accomplish a series of representative examinations and the measurement of the shrinkage temperatures of 43 samples taken from the interior leather coatings. Most of the covers of the chests were prepared from vegetable-tanned calfskin or young cowskin, one of them was made from sheepskin, another case was covered with pelt, while yet another had an alum-tanned sealskin coating. Under a stereomicroscope, the surfaces of the samples appeared to have micro-fissures, the grain could easily be separated and the fibres could also easily be extracted from the samples, which indicated a very weak cohesion of the fibres. The leathers were permeated with iron corrosion, they blackened and hardened, the fibres aggregated and became brittle. The corrosion products of iron and copper catalyse the oxidation of collagen and the tanning and greasing materials, and within wet conditions they expand and so mechanically deteriorate the leather. During the sizing test of the samples, the fibres did not move or expand at a normal temperature, which proved that the examined leather samples did not yet reach the critical sizing limit. The pH measurement of the fibres taken from the samples brought astonishingly good results (pH 4-4.5- 5-5.5). The vegetable-tanned leather samples of three chests that suffered from leaking water in the museum store-room gave more acidic pH values: 3-4.5. The results of the described analyses did not give satisfactory explanation to the poor mechanic condition observed under the microscope, which attest to decomposition processes in the leather samples even by good pH values. Thus the shrinking temperatures of the samples were measured.* The fibres were examined under a microscope with a hot-stage. The measurements started at 20 °C with due respect to the parameters given in the technical literature. The temperature was raised with 2 °C per minute. The shrinking started, however, very slowly and happened so quickly that only its accomplishment could be observed and not the process. The rate of heating was modified to 8 °C /minute * Although the slightly acidic pH value of leathers generally becomes more acidic as they age, Larson and his colleagues observed that the sulphuric acid absorbed from the pollution can turn into ammonium sulphate during the oxidation and hydrolytic decomposition of collagen, in result of which the pH value of the leather increases, it becomes neutral or alkaline and so acidification cannot be measured after a certain period of time. 93
