BAD QUALITY OF EARTHENWARE $Q^ 



diices a green, with oxide of iron, which caufes a red, or with 

 oxide of manganefe, which gives a brown. 



Great im perfections are produced in pottery, from the in- pg^fe^ion in' 

 judicious ufe of glazing over earlhs of an unfuilable nature, pottery. 

 and (his is more remarkable when the earths are not fo well 

 prepared for their glazings as ihey are for thofe of the finer 

 vs'ares. Tiie articles of common pottery are lefs carefully pie- 

 pared both in their rijalerials and baking. This la(t is ufually 

 performed at a fingle operation, and with lefs fire. 



The means of producing good pottery and earthen- ware^ Cautions 

 confifl: in carefully chufing the earths for forming the body : j^fy^e its good* 

 in producing an exad coincidence of expanlion by heat be- nefs. 

 tween ihem, and the vitrifiable glaze with which they are to 

 Jbe covered, and in baking Ihem by a proper degree of fire, 

 produced from combuftibles not capable of changing the nature 

 of the glazing. 



The neglect of thefe attentions occafion defe<5ts in the manu- 

 'ijctured articles, which are either unfighlly and nothing more, 

 or both unfighlly and dangerous. 



The unfighlly defefls which a^re found in ill condilioned Defedls o< 

 pottery or earthen- ware, are, fcalin^; ihe dropping or ^''^P'^i ^n^mtr^t\ 

 finoke; drying of the ware, -dnd jiaxvs or crach. 



The fcaling is the appellation u(ed when the glaziiig of a Scaling of the 

 piece detaches ilfelf in fcales, by the adion of moift air, or on S^^^^* 

 the leafl touch, and leaves the bifcuit uncovered. 



The dropping or drops t»ke place when the moifiure of the Dropping or 

 fuel having ftruck the pieces during the baking, the enamel is "''°ps« 

 coilefled in drops on the furlace, and remains vitrified in 

 thai form, inftead of being equally fpread. 



The fmoky appearance happens when a piece has not been Smoky tinge= 

 purified by a clear flame, but remains blackened or ftained- 



The drying happens when the pieces are, as it were, roafte'd Drying, 

 in the firing, and come out rough Irom ihe abforplion of the en- 

 amel into their .^ubfiance. 



The flaws happen, when the earth or the bifcuit, having pja^j, 

 a different pyroraelncal expanfibility from that of the enamel; 

 the body contrads in cooling more if.an the glaze which is 

 therefore fplit, or which is divided into an infinite nua)ber of 

 fmail parts, fonielimes nol perceptible to the eye when the 

 pieces are new, but which become very vifihie, when the 

 goods have imbibed any greafy fubltance in uling. 



All thefe defects, though difagreeable to ihe eye, have The coarfe pofc- 



reaiiv ^^^X 'S moft 



