For much of the above explanation of the nature and behaviour of gelatine, the author must himself take responsibility, and in this section he has freely quoted from his own papers upon the subject (see References). He claims that his view of a gelatine gel as involving a network of compressed water, liable to modification by lyotrope influence upon the continuous phase and by ionic adsorptions of the disperse phase, is most in harmony with the recent advances in our knowledge of colloids; that much of the theory is a necessary corollary of those discoveries; and also that he has found this view to be a sound guide in practice, both in tanning and in gelatine manufacture.
Many other theories have been advanced, but most are generalizations over too limited a field, and from experiments with only a few substances, and show little or no correlation with the wider facts of colloid behaviour. That of Procter, for example, discards altogether the idea of a two-phased structure of the gel as an "unproved and rather gratuitous assumption," dismisses surface tension considerations as "more complicated and less verified," and adsorption as "wholly empirical," whilst it ignores lyotrope influence and the analogy with agar gels completely. Procter's theory applies mainly to the swelling of gelatine by acids, which swelling he considers to be due to the osmotic pressure of the anion of a highly ionizable salt formed by the chemical combination of the acid with gelatine. On this assumption, mathematical considerations show that the electric charge on the gelatine is given by the expression z = √(4ex + e2), where z = the amount of ion taken up, x the concentration of the surrounding solution, and e the excess concentration of diffusible ions in the jelly.
The property of gelatine and glue which is chiefly used in classifying them into grades of different commercial value, is the strength of the jelly obtained as compared with any arbitrary standard gelatine. An enormous number of other physical tests have been devised, but none are nearly so simple or so reliable. Gelatine is unfortunately very liable to hydrolysis even by water, and long before any amido-acids, etc., have appeared there is a change to a not greatly hydrolyzed product (sometimes called β gelatine) which has lost the power of setting to an elastic gel. It is thus the lyophile nature which has been altered, and the fall in elasticity corresponds to the fall in power of compressing water, which is proportional to the concentration of α gelatine. Now the elasticity of a gelatine gel varies as the square of the concentration. Hence if one so arranges the concentrations of standard and unknown samples that gels of equal elasticity are obtained, the concentration of α gelatine is the same in both gels, and the relative amounts of α gelatine in the original samples are inversely proportional to the weights used to give gels of equal elasticity. The "strength" of a gelatine or glue is therefore usually stated as the number of grams of a standard gelatine which will yield a gel with elasticity equal to that from 100 grams of the gelatine or glue being tested. Elasticity is matched by lightly pressing with the finger-tips.
It is also possible to grade samples of gelatine and glue by the estimation of "peptones," whose amount indicates the degree of hydrolysis. Nitrogen is estimated by Kjeldahl's method in the sample and in the precipitate obtained by saturating a solution with zinc sulphate. The difference is calculated as peptones by multiplying by 5.33. Trotman and Hackford say that the results are in the same sequence as those of the finger test. The method, however, is much more laborious than the "finger test."
Gelatine is also graded according to the results of bleaching and clarifying, but with quite arbitrary standards, largely determined by the fancy of the customer.
Chemical analyses, involving estimations of ash, lime, fat, acid, water, insoluble matter, and poisonous metals, e.g. arsenic, copper, zinc and lead, are of value for special cases according to the destiny of the goods. Special physical tests, such as "breaking strain" and "foam test," are also of some little value in special cases.
REFERENCES.
"The Chemistry of Colloids," W. W. Taylor. 1915.
"Handbook of Colloid Chemistry," W. Ostwald. 1919.
"Chemistry of Colloids," Zsigmondy and Spear. 1918.
"Introduction to the Chemistry and Physics of Colloids," E. Hatschek.
"Surface Tension and Surface Energy," Willows and Hatschek.
"Chemistry of Colloids," V. Pöschl.
"Grundzüge d. Dispersoid Chemie," von Weimarn.
"The Lyotrope Series and the Theory of Tanning," Bennett, J.S.L.T.C., 1917, p. 130.
"The Swelling of Gelatine," Bennett, J.S.L.T.C, 1918, p. 40.
"The Swelling of Gelatine," Procter, J.C.S. Trans., 1914, 105, 313; and Koll. Chem. Beihefts, 1911, 2, 234.
"The Swelling of Gelatinous Tissues," Procter, J.S.C.I., April 16, 1916.
"Summary of Procter's Views, and Bibliography," Collegium (London), p. 3, 1917.
"Lyotrope Influence and Adsorption in the theory of wet work," Bennett, J.S.T.C., 1920, p. 75.
For the "finger test," see—
"Glue and Glue Testing," Rideal, 2nd ed., p. 158.
"Leather Trades' Chemistry," Trotman, p. 241.
SECTION II.—RAW MATERIALS AND PRELIMINARY TREATMENT
The raw materials for the manufacture of gelatine and glue may be classified according to their origin. The preliminary treatment, which comprises chiefly purifying and cleansing operations, is varied according to type of manufacturing process for which it is a preparation.