a6a On tht Campsfitlcn of Pottery, 



VII. 



Reflexions en the ^alities of Pottery, with the Refult offome Analyfis of Earths, andof Com- 

 mon Pottery. By Cit.Vau^elin*. 



T* 

 HE quality of pottery may be influenced by four things ; i. The nature or compofltion 

 of the material; ad, The preparation; 3d, The dimenfions of the veflels ; and, 4th, The 

 heat to which it is fubjefted in baking. 



By the compofltion of the material, the author underftands the nature and proportions of 

 its elementary parts ; and thefe elements in moft potteries, whether valuable or common, 

 are filex, alumine, lime, and fometimes a fmall quantity of the oxide of iron. Hence it is 

 evident that good pottery differs from bad, lefs in the diverfity of its elements, than in their 

 proportions. 



Silex, or quartz, conftantly forms at leaft two thirds of moft potteries ; alumine from 

 2. fifth to one third ; lime from five hundredth parts to twenty hundredth parts ; and iron from 

 the minuteft quantity, or none, to twelve or fifteen per cent. 



Silex gives hardnefs, infufibility, and unchangeablenefs ; alumine communicates tenacity, 

 and ductility to the pafte, fo that it may be kneaded, moulded, and turned at pleafure. It 

 likewife undergoes a commencement of fufion, by the heat which connetSs its parts with thofe 

 of the filex; but its quantity muft not be too abundant, becaufe it would render the pottery 

 too infufible, and difpofed to break by heat. 



Experiment has not yet fliown that lime is a neceffary ingredient in pottery; the traces 

 which are conftantly found of its exiftence in this manufadture arife from its forming part of 

 the ingredients from which it has not been waftied or properly feparated. When this earth does 

 not exceed five or fix per cent, it appears at leaft to do no harm to the quality of the pottery ; 

 but when more abundant, it communicates too great a degree of fufibility. 



The oxide of iron poffefles the inconvenienceof affording a red or brown colour according to 

 the degree of heat, and has beiides the property of being very fufible ; more fo even than lime. 



Among the various kinds of pottery, fome of them being applied to contain very pene- 

 trating fubftances, fuch asfalts, metallic oxides, glafTes, &c. in a ftate of fufion, require to be 

 formed of a fine pafte, which is only to be obtained by mechanical divifion of the earths ; 

 others intended for the fufion of metals and other matters which a£t very little upon them, 

 and being required to fupport fudden and extreme variations of temperature without break- 

 ing, require to be formed of a mixture of ciment, or burned clay, with the raw clay. By 

 this means a kind of pottery is obtained, the coarfe texture of which, in fome meafure, re- 

 fembles the breccias or pudding-ftones, which eafily undergoes thofe rapid changes of tem- 

 perature. 



The baking of pottery is likewife an objedof extreme importance. It is neceffary that 



S 

 » Communicated to the Philoroathic Society at Paris, and inferted in their buUetia, No. 26, in theyear vii. 



the 



