149 



result is obtained when a clay paste is stored in a cool place in a moist 

 atmosphere for a sufficientlj^ long time ; the requisite coagulation 

 occurs when sufficient acid is added to the material. This acid may 

 be produced internally by the putrefaction of the organic matter, 

 or it may be added artificially. 



The swollen gel produced on prolonged storage is very permeable 

 to water, and its structure may be compared to a series of solid grains 

 wholly surrounded by liquid films which are not sufficiently thick 

 to allow the particles to separate from each other or to flow appreciably. 

 Such a structure has a powerful capillary action and consequently, 

 it affects the distribution of water through the mass in a most thorough 

 and efficient manner. This uniform distribution of the water largely — 

 in conjunction with the coagulating and swelling of the colloidal 

 matter — accounts for the increased ease with which an old clay-paste 

 can be manipulated. The water in freshly pugged clay cannot be 

 so uniformly distributed as when such a j)aste has been allowed to 

 stand for several weeks, during which time the water is distributed 

 through the mass by capUlary attraction. 



A much shorter storage of the clay paste, frequently in open sheds 

 the material being covered with wet sacking, is known as souring. 

 Its effect is undoubtedlj^ to increase the active, as distinct from the 

 dormant, plasticity of the clay, though there is a great variation in 

 the extent to which this takes place. There is a widesj)read impression 

 that souring is the result of bacteria or ferment-organisms, and some 

 potters added sugar or honey to the clay to assist the fermentation 

 but, whilst this may account for some of the observed effects, the 

 hydrolysing action of the water present in the mass on the clay, sflica, 

 and iron hydroxide particles must not be overlooked. Rohland^ 

 suggested that the fresh clay paste is slightly alkaline owing to the 

 felspar, &c., j)resent in the clay being hydrolysed and converted 

 into the colloidal form. The acids j)roduced by the decomposition 

 of any organic matter also present neutralise the cations; and the 

 excess of hydrogen-ions produced coagulates the colloid matter and 

 correspondingly increases the plasticity of the clay. This explains 

 why the old vinegar " tip " of bygone potters develops the plasticity. 

 Previous to this, Seger^ had found that clays which remain alkaUne 

 do not increase in plasticity on storage, but do so if they are acidulated 

 with acetic acid. 



As heat is a disadvantage, souring must usually take place in a 

 cool, moist, shed or cellar, if it is to be really effective; though in 

 opposition to this, it may be noted that slips which are dried by heat 

 are often more plastic than those treated in a filter press. 



Some firms apply souring or storage to clays which are highly 

 plastic, not to develop more plasticity but to secure a better distribu- 

 tion of moisture through the mass, and, as they express it, " to bring 

 it into a better and tougher condition." 



The ^^•eathering of crystalline minerals generally yields gels or 

 mixtures of gels; thus, talc is formed from the weathering of 

 serpentine, and some forms of brown iron ore from yellow ochre, 

 but this is not invariably the case. It is, therefore, possible that 



