15660 str (106)

68 PLASTICISATION

culate, tend to decrease the shearine _st_E6m , mcrease the mobility. Their effect on pl_aS % much less elear.    ^A - ^lc% a I

Bancroft and Jenks claim that plasticisati effected by the combined action of flocculatine ** peptising agents; so that, for exampl_e, soda and sodium chloride will not alone plasti • greatly (i.e., inerease the plasticity), whereas'^ mixture of the two will do so. They test the theo * by assuming a correlation between the time w slaking of a bali of dried clay in water and the plasticity, but they fully realise that the test inadeąuate. Scott Blair repeats the experiments using the flow-plasticity test and also the rolling test, and shows that Bancroffs theory appears to work for such mixtures as sodium chloride and caustic soda, but that other substances, such as hydrochloric acid, for instance, and certain vegetable juices also, will plasticise considerably even when used alone. The use of vegetable juices has already been alluded to in mentioning the Egyptianisation process. The uncertain naturę of any test for plasticity makes all this work rather unsatisfactory.

Schofield and Scott Blair have studied the effect of electrolytes on the settling of clay slips in different stages of flocculation, measuring the height of the sediment and its yield-value. The latter is measured by the displacement retained by a sphere, hung at a specified height in the suspension, when the vessel containing the suspension is shifted laterally. The results are tolerably reproducible; but the curves obtained are exceedingly complex.

When really thick clay pastes are allowed to stand, they gradually exude water, and tend to concentrate themselves. This is a kind of syneresis, analogous to that shown by jellies; but in slips it

• trenerally considered as a sedimentation. There •⁵no hard and fast linę between the two phenomena. There miist be a point at which the structure ftrst reaches a condition in the naturę of " open-packing |; but the process of the gradual closing up of the micelles into a close structure seems to be a fairly regular one. Flour suspensions lose viscosity on standing, and so do doughs ; but the phenomenon in this case has nothing to do with gravity effects. Flour is a mixture of protein and starch, and it seems that the protein takes up morę water at first than it can afterwards hołd. As it gradually gives up this water, the starch gains water, but does not immobiłise it in the same way as does the protein, and the viscosity falls. It is for this reason that doughs soften during fermentation.

BIBLIOGRAPHY

Bancroft and Jenks. J. Phys. Chem., 1925, XXIX., 1215.

Barr. " Monograph on Viscometry,” 1931. Oxford University Press.

Bleininger. J. Ind. Eng. Chem., 1930, XII., 436.

Bowmaker. J. Soc. Glass Tech., 1930, XIV., 330T.

Brewer and Atkinson. Ind. Eng. Chem. (Anal.), 1936, VIII.,

443 (a g°od bibliography on coal work).

BdzAgh, von. I Colłoid Systems," 1937. Tech. Press Ltd. Demolon and Barbier. Comptes Rendues Acad. Soi., 1927, CLXXXV., 542.

Francis. Physics, 1933, IV., 403.

Mellor. Trans. Farad. Soc., 1921, XVII., 354.

Schofield and Scott Blair. Trans. Farad. Soc., 1931, CXXV.,

629.

Scott Blair. (Flow-plasticity.) J. Phys. Chem., I93¹* XXXV.,

374. (Falling sphere.) J. Phys. Chem., 1935. XXXIX., 2x3. Scott Blair and Yates, J. Agric. Sci., i93²» XXII., ^39-Taylor and Quinney. Phil. Trans. (A), 1931, DCLXXXIX.,

323 ; Proc. Roy. Soc. (A), 1933. CXLIII., 307.

Zocher. Phys. Zeits., 1927, XXVII., 790-