The addition of alurniniuni salts to gelatin solutions may give rise to considerable, though to some extent impermanent, viscosity increases. These effects are very dependent on p1-I. They also depend on the type of gelatin and on the concentration of the reactants. Similar effects are obtained when certain other polyvalent metal salts interact with gelatin i n solution. The complicated viscosity effects can be largely accounted for by the hypothesis that electrostatic cross-linkages are formed between a hydrated oxide of the metal, preseri-ed in a metastable state by the gelatin, and two gelatin molecules.
IntroductionIt has been a regular practice for a great many years to add aluminium sulphate to the gelatin solutions used in the tub-sizing of paper. The effects produced have never been fully studied, though it has been recognized in the paper industry that aluminium sulphate may affect the bonding of the gelatin to the cellulose fibres, the resistance to water, and the ink-bearing properties of the paper. However, the effect of the aluminium sulphate on the viscosity of the gelatin solutions is also of considerable importance, since this property controls the pick-up of size from the size-bath. Difficulties have been caused by the apparent irregularity of these effects and this work has been undertaken to elucidate them.Most of the reported work on the effect of polyvalent metals on gelatin has been concerned with their tanning properties and investigations of the effects in solution have been rare. Lottermoser & Matthaesl studied the effect of potash alum on a photographic gelatin. The effect of pH, the most important variable in the reaction, was not examined and no satisfactory explanation was given of any of the results. A. & L. Lumiere,2 investigating the influence of aluminium salts on the setting time of gelatin, showed that the effect of the aluminium was independent of the anion. After washing gelatins that had been treated with aluminium sulphate, they found
