Clifton College Scientific Society. 35 



a quantity of silicious earth that a solid cup has been formed 

 around it, rising to a considerable height. This solution of silica 

 is probably owing to the solvent power of soda, which is present 

 in the two latter waters. The best flints are of a dark grey colour, 

 approaching to black, and having a considerable degree of trans- 

 parency. Those which exhibit brown or yellow spots on their 

 interior surfaces should be rejected on account of the ferruginous 

 particles which they contain, and whence would arise blemishes in 

 the ware. 



The clays principally used in the potteries of Staffordshire are 

 brought from Dorsetshire and Devonshire. These earths are both 

 of excellent working quality, and being free from any impreg- 

 nation of iron, are valuable for the extreme whiteness which they 

 exhibit when burnt. The Dorsetshire clay is brought from Purbeck, 

 being of two kinds, distinguished as brown clay and blue clay. 

 That from Devonshire comes from the southern part of the 

 county, and is of two distinct qualities, known as black and 

 cracking clay. 



The clay from Dorsetshire is considered preferable to that from 

 Devonshire for the potter's use, so that it commands a price in 

 the Potteries equal to one-eighth more than the latter. The good 

 qualities of brown clay are, that it burns an excellent white, and is 

 not liable to crack during the process of burning. On the other 

 hand, it is subject to considerable imperfection of crazing, which 

 causes some manufacturers to discard the use of it altogether from 

 their works. Crazing is a technical term used to denote the 

 Clacking of the glaze, which is supposed to arise from the imperfect 

 manner in whicli this is capable of uniting itself with the clay com- 

 posing the body of the vessel. 



The blue clay combines the greatest number of good qualities, 

 and is the most generally esteemed of all the four I have mentioned. 

 It burns exceedingly white, making a very solid quality of ware, and 

 is capable of being advantageously combined with a greater 

 quantity of silicious earth or flint than any other kind. The 

 greater the quantity of silica used, the whiter the ware will prove : 

 the limit to the use of flint being the inability of the clay to bear 

 it in combination beyond a certain proportion without cracking. 



Black clay owes its distinctive colour to the quantity of bitu- 

 minous matter which it holds in combination, but which is entirely 

 consumed and dissipated when the clay is submitted to the heat 

 of the oven, leaving the articles produced from it of a colour the 

 whiter in proportion to the original blackness of the clay. Crack- 

 ing clay is so called from an evil property of occasioning the ware 

 to crack while undergoing the first application of fire. To com- 

 pensate in some degree for this irregularity, the goods in the 



