147 



when removed from a filter press would not contain less than 30 per 

 cent, of water. The clay acts like a porous diaphragm made of 

 capillary tubes and this shows the well-known phenomena of 

 endosmosis. 



In making articles of clay and allied materials by the casting 

 process, i.e., by pouring a suspension of the materials, into a mould 

 and, after a suitable time, pouring off the surplus fluid, it is important 

 to have as concentrated a suspension as is reasonably possible. When 

 water is used, only moderate concentrations can be used, but by 

 adding a small percentage of a suitable electrolyte such as sodium 

 carbonate or water-glass, or both, the amount of clay in suspension 

 can be doubled and the casting process greatly facilitated. This 

 use of an electrolyte, is based solely on the assumed colloidal nature 

 of the clay. Care must be taken in choosing the electrolyte, as some 

 substances such as sodium carbonate if used alone will cause a very 

 high surface tension, with the result that the slip " balls up " and 

 may cause the inclusion of air bubbles, whilst sodium silicate used 

 alone causes the clay to flow in " strings " like a thick syrup. A 

 suitable mixture of the two, however, is excellent, and gives a smooth 

 flowing stream without any tendency to the defects just mentioned. 

 Slips containing a suitable electrolyte require far less time in the 

 mould than those slips in which plain water is used. This is necessary 

 in consequence of the smaller proportion of water present and is of 

 great practical importance as it reduces the number of moulds required. 

 With a good stoneware slip containing soda, a mould may be used 

 five or six times in succession without drying and with slips of leaner 

 clays, the moulds may be used still more frequently. On the other 

 hand, the salts absorbed by the plaster tend to make the moulds less 

 durable when soda is present in the slip. 



A sUp to which soda or other electrolyte has been added feels 

 more soapy and plastic than one with plain water ; on passing it 

 through a sieve, it does not flow so readily and tends to form long 

 S3rrupy strings, and, on long standing, little or no separation occurs. 

 A soda-slip also flows more steadily and with less tendency to include 

 bubbles of air than one made without an electroljrte and the painting 

 of portions of a mould with slip, which is sometimes essential to 

 ensure the production of a good surface on the ware is entirely 

 unnecessary when a suita,ble electroljrte is used. 



In increasing or reducing the plasticity of a clay or earth, so as to 

 make it suitable for the manufacture of xarious articles, the methods 

 most extensively used are based on the assumed colloidal properties 

 of the clay. Plasticity is increased by> methods (p. 130) which 

 increase the amount of colloidal gel in the material and it is reduced by 

 the methods (p. 131) which will lessen the amount of irreversible colloid 

 gel, or by converting it into a hydrosol. Thus, the processes of ageing 

 and souring the clay are dependent on an increase of colloidal matter 

 by the prolonged hydrolysing action of the water on the claj^, followed 

 by a fermentation or acid-producing action which coagulates any 

 hydrosols previously formed. 



X 11454 L 



