121 



and the alkali salts of these acids, 0-025 per cent. Although 2-5 per 

 cent, of ammonia caused precipitation in Mayer's experiments, a less 

 amount favours deflocculation, or breaking up of the lump. 



The "fluidity " of any clay sUp depends cliiefly on the proportion 

 of water added, but it is largely affected by the presence or absence 

 of very small proportions of electrolytes. According to Rohland*, 

 the addition of hydrochloric, nitric, sulphuric, acetic, or propionic 

 acid increases the plasticity of the clay slip, apparently by coagulating 

 the colloidal matter present. Solutions with an acid reaction such as 

 sal-ammoniac, aluminium chloride, ferric chloride, and potassium 

 bichromate behave similarly. Alkahes such as ammonia, caustic 

 soda, caustic potash, lime-water, baryta, and basic salts, make the 

 slip more fluid and reduce the plasticity of the material, but their 

 behaviour depends on their concentration. The action of alkalies 

 in reducing the viscosity, sometimes requires several days, and is 

 accompanied by coagulation. An excess of alkali may cause a 

 reversion of this action, the viscosity increasing again. The addition 

 of salts usually decreases the osmotic pressure and increases the 

 viscosity. 



Acheson has patented the use of a solution of tannin and alkali 

 to make a clay " fluid," and, followed by the addition of an acid — 

 presumably by precipitating the coUoid matter — to increase the 

 plasticity of the clay. 



The viscosity of clay suspensions, before and after the addition 

 of various substances, can best be understood by assuming that it 

 varies according to the condition, and proportion of the colloidal 

 matter present. Mellor, Green, and Baugh^^ have arranged the 

 substances Ukely to be present in, or added to, clays into five grouj)s 

 according to their action on the viscosity of the clay* : — ■ 



(1) Substances which first make the sUp more fluid, while 

 further additions stiffen the sUp. Examples : sodium and 

 potassium carbonates, fusion mixture, potassium sulphate, 

 potassium bisulphate, potassium hydroxide, potassium nitrate, 

 sodium sulphide, tannin and galhc acid. 



(2) Small amounts thicken the slip ; larger amounts make 

 the slip more fluid. Examples : copper sulphate, dilute 

 ammonia, and potassium alummium sulphate. 



(3) Substances which make the slip thinner : magnesium, 

 mercury and sodium sulphates, sodium sulphite, sodium acetate, 

 sodium chloride, sodium phosphate, ammonium gallate, hydro- 

 chloric acid, water-glass. It is just possible that some of these 

 substances may have to be transferred to the first (or second 

 group) if greater (or less) concentrations be tried than those 

 employed by Mellor, Green, and Baugh. 



(4) Substances which only stiffen the sUp : grape sugar, 

 humic acid, ammonium chloride, calcium chloride, calcium 



* It should be noted that the slips used were not made trom a single clay, 

 but from a body mixture consisting of 16g. ball clay, 19g. China clay, 13g. 

 Cornish stone, 20g. flint and 100 c.c. water. To this mixture, varying quantities 

 of acid, alkali and salt — ranging from 0-1 to Og. or 0-1 to 35 c.c. — were added. 

 This may account for the difference between these results and those obtained 

 by some Continental investigators. 



