O volume 365 (números 1-2) da revista Thermochimica
Acta, publicado no ano 2000, tem artigos, exclusivamente, dedicados ao
património cultural e quatro deles relacionados com cabedais e pergaminhos. São, essencialmente, artigos sobre a deterioração
e o diagnóstico do estado de conservação deste tipo de materiais.
Os autores, os títulos dos artigos e os respectivos abstract
são:
- René Larsen, Experiments and observations in the study of
environmental impact on historical vegetable tanned leathers, pp.85-99
doi:10.1016/S0040-6031(00)00616-X
(restricted access)
Abstract:
The hydrothermal stability in the form of the shrinkage temperature (Ts) is a
fine measure of the deterioration of vegetable tanned leathers. Previously, it
has been demonstrated that the Ts can be predicted by multiple regression
modelling based on parameters for the chemical breakdown of the collagen and
tannin structures, the sulphate content and acidity of the leather as well as
the interaction of these four parameters. In the present paper, several
deterioration profiles are suggested for both the natural and artificial aged
leathers based on significant regression analyses of the grouped data. The possible
existence of several deterioration profiles is supported by more than 100 years
of observational and experimental studies of natural and artificial ageing, and
is probably mainly due to difference in tannin type and storage conditions. On
this basis the problems in using simple standard artificial ageing systems are
discussed, and improvement of more complex systems suggested. The latter based
on continuation of combined observational and experimental studies using the
multiple regression modelling of the Ts as an instrument for adjusting and
developing the ageing system.
- Claire Chahine, Changes in hydrothermal stability of leather and parchment with
deterioration: a DSC study, pp.101-110
Abstract:
Differential scanning calorimetry (DSC) allows to study the enthalpy changes
which are associated with the denaturation of collagen, of which shrinkage is
the macroscopic manifestation. Damage to skin, leather or parchment may be
expected to manifest as a decrease of either the temperature of denaturation
and/or the enthalpy changes. These modifications are associated with those of
other chemical and mechanical characteristics. DSC is a very fine tool to follow
in an easy way the reaction of leather or parchment to an ageing or a
conservation treatment. While for leather ΔH seems to have less than Td, for
parchment, it must be looked at very carefully since an unchanged Td value can
be accompanied by a fall of ΔH. The case of archeological waterlogged leather
is particular since the state of deterioration is apparently not correlated
with the hydrothermal stability.
-Neil S. Cohen, M. Odlyha and G. M.
Foster, Measurement of shrinkage behaviour in leather and parchment by
dynamic mechanical thermal analysis, pp. 111-117
doi:10.1016/S0040-6031(00)00618-3
(restricted access)
Abstract:
The mechanical behaviour of modern and historic parchment and leather samples
through their shrinkage temperature is described. The measurements are made
possible by the novel modification of a standard dynamic mechanical thermal
analyser (Rheometric Scientific DMTA Mk3) to enable experiments to be conducted
on samples immersed in water. A sample of parchment was clamped in tensile mode
under a small static applied force. The temperature of the water could be
controlled and thus it was possible to heat the sample and monitor the change
in its displacement as it passed through the shrinkage temperature. Information
could therefore be obtained on the percentage shrinkage and the temperature
range over which it occurred. Furthermore, the method also provided useful
information on the expansion or contraction of the samples on initial immersion
in water, which relates to ease of sample wettability, and on the changes
during drying process and rehydration.
-Tim J. Wess and Joseph P. Orgel, Changes
in collagen structure: drying, dehydrothermal treatment and relation to long
term deterioration, pp.119-128
doi:10.1016/S0040-6031(00)00619-5 (restricted access)
Abstract:
Collagen is the major component of most connective tissues in animals. The
structure of the helix and axial molecular packing within fibrils is well
documented. Less is known about the structural alterations that occur on drying
and the compounded changes that occur on dehydrothermal treatment. The
structural properties of collagen and its supramolecular architecture are of
importance in these states, since many of the industrial applications of
collagen-based materials involve dried collagen or dehydrothermally treated
collagen. The effects of drying and thermal treatment of collagen can be observed by
X-ray diffraction, the changes in the diffraction pattern relate to changes in the
axial packing of collagen molecules as dehydration occurs. The meridional
diffraction series becomes truncated indicating induced structural disorder,
the spreading of diffraction features indicate that the molecular orientation
is altered for some of the collagen chains or portions of collagen chains
within a fibril. Dehydrothermal treatment of parchment collagen for up to 24 h reduces the axial
periodicity of the collagen fibril from 64.5 to 60.0 nm. Analysis of the X-ray
diffraction data shows the possible alterations in molecular packing that may
explain the structural changes.
