On the Compofttlon of Pottery. 263 



the heat {hould be fuch as to expel the moifture, and agglutinate the parts which enter into 

 the compofition of the pafte ; but incapable of efFetSting the fufion, which if too far advanced, 

 will render the ware of fo homogeneous a texture, as to become brittle. The fame efFedl hap- 

 pens with regard to the fine pottery, becaufe the extreme divifion of the earth puts them 

 nearly in the fame Urate as if the materials had been fufed. Hence it is that porcelains which 

 have been flrongly baked are more or lefs brittle, and cannot well be fubje6ted to chanties 

 of temperature : and hence aifo it is that coarfe porcelains, into which a certain quantity 

 of baked clay is mixed, retorts, crucibles, porcelain tubes, and common coarfe pottery, 

 are much lefs brittle than plates or difhes formed of the fame materials in a more divided ftate. 



The general and relative dimenfions of the different parts of pottery are alfo of o-reatcon- 

 fequence to the manner in whicli they fupport the fire. In fome cafes the glaze, particular- 

 ly when too thick, and of a very different nature from the body of the pottery, likewife oc- 

 cafions it to break. 



It is therefore effential in all cafes in the fabrication of pottery, ift, To obferve the befl 

 proportions of the ingredients ; 2d, To divide the materials more or lefs finely, according to 

 the ufe to which the ware is to be applied, and to give all the parts as nearly as poffible the 

 fame dimenfions ; 3d, To bake with as ftrong a heat as can be given without fufion ; and 

 4th, To apply a thin coat of glaze, the fufibility of which ought to be as nearly as poffible 

 approaching to that of the body, in order that the combination may be more intimate. 



From a convidtion that the goodnefs of pottery principally depends on the due propor- 

 tions of earths which enter into their compofition. Citizen Vauquelin has thought it might 

 prove interefting to thofe who are employed in this important manufacture, to be acquainted 

 with the analyfis of the different natural clays employed for this purpofe, and of the pottery 

 made from fome of them, in order that when any new clay is difcovered, it may be known 

 by fimple analyfis which kind of pottery it may the neareft refemble. 



Crude kaolin 104 parts contain, — filex 74, — alumine 16,5, — lime 2, — and water 7. — One 

 hundred parts of this earth afforded eight of alum, after treatment with fulphuric acid. 



Waflie-d kaolin 100 parts afforded, — filex 55, — alumine 27, — lime 2, — iron 0,5, — water 

 14. — This kaolin, treated with the fulphuric acid, afforded between 45 and 50 per cent of 

 alum. 



Petuntze afford-'d filex 74, — alumine 14,5, — ^lime 5,5; — ^lofs 6. — One hundred parts qf 

 this fubftance treated with tlie fulphuric acid, afforded 7 or 8 parts of alum. But this 

 quantity does not account for the lofs which was fuftained. 



Porcelain retorts, filex 64, — alumine 28,8, — lime 4,55, — iron 0,50 j — lofs 2,77. — When 

 treated by the fulphuric acid, this porcelain did not afford alum. 



Defcrlptian 



