PREPARATION OF THE PA3TE. 



potters technically call slip. For this purpose the two slips, that 

 of clay and that of flint, are successively run off into the blending 

 reservoir, against the inner side of which are " gauging rods," 

 by which the necessary proportion of each material is regulated. 

 The mixture is now passed into other reservoirs, through fine 

 sieves on "lawns," woven of silk, and containing 300 threads to 

 the square inch. A pint of slip of Dorsetshire or Devonshire clay 

 weighs 24 ounces, of proper consistence : of Cornish clay 26 ounces ; 

 and of flint 32 ounces. Finally, the slip is conveyed to a series 

 of large open kilns, heated underneath by means of flues, and 

 about 9 inches deep. The excessive moisture is thus evaporated, 

 and in about twenty-four hours the mixture becomes tolerably 

 firm in substance. It is then cut into large blocks and conveyed 

 to an adjoining building to undergo the process of "milling." 

 The mill is in the form of a hollow cone, inverted, with a square 

 aperture or tube at the lower part. In the centre is a vertical 

 shaft, set with broad knives. When this shaft is in action (worked 

 by steam power), the soft clay is thrown in, and forced downwards, 

 being alternately cut and pressed until it exudes from the aperture 

 at the bottom, in a perfectly plastic state, and ready for the hand 

 of the potter.* 



The paste thus prepared would serve for the purposes of manu- 

 facture, but it is found that it may be considerably improved by 

 leaving it for an interval, more or less protracted, several years for 

 example, stored in damp vaults or cellars. It suffers a sort of 

 rotting, becomes black, and evolves an offensive odour of the gas 

 called sulphuretted hydrogen. These effects are easily explained. 

 The paste, as prepared, always contains a proportion, however 

 minute, of organic matter, which the previous preparation has 

 failed to extricate from it. This matter by the influence of the 

 humid air, undergoes a spontaneous combustion, and acting upon 

 some traces of sulphates, which also remain as unextricated 

 impurities in the paste, transforms them into sulphurets, and 

 accordingly sulphuretted hydrogen is evolved. 



The utility of all these processes, by which the minutest 

 particles of organic matter are disengaged from the dough, will be 

 understood when it is considered that even the presence of a single 

 hair in the dough would be sufficient to spoil completely an article 

 of porcelain of great beauty and value ; for the organic matter thus 

 buried in the material being decomposed by the action of the 

 ovens, a gas would be developed which would produce air-bubbles 

 or even cracks in the article. 



To work the paste, when ready for the manufacture, it is once 



Catalogue of the Great Exhibition, p. 718. 



2 179 



