Journal of Film Preservation - FIAF

27 result from different combinations of raw materials (hides, tendons,
bones of cattle, oxen or pig), the extent and type of raw-stock
treatment (pre-treatment), and the conditions of extraction. Pretreatment
procedures swell and soften hides or ossein (the
demineralisation product from bones) in preparation for denaturation
and extraction. An acid pre-treatment (usually brief) yields gelatine
type A, whereas alkali pre-treatment (a liming process) leads to gelatine
type B with a low iso-ionic pH. After the pre-treatment, extraction with
hot water (at a controlled temperature) denatures the collagen, and
increasing the hydrolysis and solubility of the gelatine. Gelatine from
successive extractions have different physical and chemical properties.
Hence, the first extract is obtained at the lowest temperature and
exhibits the highest Bloom Value (the gel strength), which is a measure
of the gelatine quality. The challenge in the manufacture of gelatine is
to produce materials that satisfy the numerous chemical and physical
criteria required for their applications. For photographic applications,
gelatine is usually obtained from cattle bones by the liming process
(Type B) and higher Bloom gelatines are usually preferred for
photographic use mainly for their coating, setting and drying
characteristics. In the collagen-gelatine conversion, no-covalent bonds
are disrupted and exhibits a partially reversibility, and in fact this is
used to explain the gelling properties of gelatine. Gelatine gel is an
amorphous material in which collagen fold aggregates appear on
cooling. These rod-like structures forms a gel-network characterised by
a gel resistance value, a Bloom Value, which depends on the type of
gelatine, as mentioned before. The amorphous structure of the gelatine
is shown in figure 12 and some of the typical properties of gelatine are
summarised in Table 2.
Figure 12: Gelatine structure
48 Journal of Film Preservation / 67 / 2004