362 



Compofition of 

 the enamel or 



Caufes which 

 occaiion varia- 

 tions in the 

 Slaee. 



Brown pottery. 



Yellow pottery. 



Glaze for brown 

 ware. 



Glaze for 

 yellow ware. 



Mottled ftreaks 

 in foreign ware. 



Cloudy tinges 

 In ihe glaze. 



BAD QUALITY OF K A RTH I! N' U'A K E. 



rough, and as it were dried. If it contains too much "lime, H 

 throws ofF the enamel, which falls fron) it in fcales inflead of 

 adhering to it. 



On the other hand, tiie white enamel is compofed of filicious 

 fand, a little lime, lead and tin oxides, and fome flux, ground 

 together with water in mills. The brown fort is compofed of 

 the fame materials, with the addition of manganefe and 

 perigord ftone *. 



The greater or lefs fufibilit)' of the fand ; the greater or 

 iefs purity of the lead, of the tin, and of the faline fubftances 

 employed as fluxes; the different degrees of heat which the 

 mixture receives in (he glazing; the variations of the finenefs 

 given to the glazing materials by the action of the mill, are fo 

 many circumftances which ,caufe changes in the enamel in its 

 fiate of fufion on the pieces, relative to the fiate in which h 

 finds the bifcuit and to the fulible layer, with which this laft 

 is covered. 



Potteri/. 



The body of the brown pottery is a red clay, more or lefs 

 ferruginous and compact according to the places where it is 

 procured. 



The common or yellow pottery is made of a white clay, 

 which contains a little lime and n.agnefia, and a confiderable 

 quantity of lilicious fand, which may be generally efteemed 

 a fourth of the mafs. , 



The glazing of the brown pottery is formed with a mixture 

 of filicious fand, yellow or red oxide of lead, and manganefe 

 pulveriled together. 



That of the yellow earthen- ware is compofed of a mixture 

 of filicious (and, and red oxide of lead, which, during its 

 baking vitrifies at its furface, and forms a yellow glazing more 

 or lefs tranfparent. To this mixture is commonly added, in 

 France, a little oxide ot magnanele in powder, more or lefs 

 fin«, without grinding ihem together. This is called Ihe graitif 

 becaufe it fufes more difticuhly than the other materials, without 

 mixing with them, and by thai means forms llreaics, fpols, 

 or brown fpecks, according to the coarlenefs of the powder 

 itfelf. 



In fome manufadories they mix oxide of copper with the 

 common glazing, to give it a green colour, and in others they 

 form defigns on the pieces, with oxide of copper, which pro- 



* A black ftone or compaft manganefe, T. 



duces 



