4 
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4 
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Reflections on the Quality of Earthen-Ware. 289 
aking, to the veffels in which it forms apart, has the pro- 
perty of rendering theni fufible, and even in a greater degree 
than lime. 
As fome kinds of pottery are deftined to m:It very pene- 
trating fubftances, fuch as falis,- metallic oxyds, glats, &c. 
they require a fine kind of pafte, which is obtained only by 
reducing’ the earths employed to very minute particles. 
Others deftined for melting metals, and fubftances not very 
penetrating, and which mutt be able to fupport, without 
breaking, a fudden tranfition from great heat to great cold, 
require for their fabrication a mixture of calcined argil with 
raw argil. By thefe-means you obtain pottery, the coarfe 
patte of which refembles dreche, or fmall-grained pudding- 
fione, and which can endure fudden changes of tempera- 
ture. 
The baking of pottery is alfo an object of great importance. 
The heat mutt be capable of expelling humidity, and agelu- 
ténating the parts which enter into the compofition of the 
‘pafle, but not firong enough to produce fufion ; which, if too 
far advanced, gives to pottery a homogeneoulnefs that renders 
it brittle. "The fame effect takes place in regard to the fine 
pottery, becaufe the very minute divifion given to the earths 
reduces them nearly to the fame fiate as if this matter had 
been fufed. This is the reafon why porcelain ftrongly baked 
is more or Jefs brittle, and cannot eafily endure alternations 
of temperature. Hence coarfe porcelain, in the compofition 
of which a certain quantity of calcined argil is employed, 
porcelain retorts, crucibles, tubes, and common pottery, the 
pafie of which-is coarfe, are much lefs brittle than difhes and 
faucers formed of the fame fubitance, ground with more la- 
~ bour. 
The general and refpective dimenfions of the different parts 
_ of veffels of earthen-ware have alfo confiderable influence 
on their capability to ftand the fire. 
In fome cafes the glazing or covering, efpecially when too 
thick, and of a nature different from the body of the pottery, 
alfo renders them liable to break. Thus, in making fome 
kinds of pottery, it is always effential, 1ft, to follow the beft 
proportion in the principles ; ad, to give to the particles of 
VoL. V: , P p the 
