392 Mr \ Wedgwood’s additional Obfsrvatlons on 
fires with the preceding, continued flat, . with the angles regu- 
larly iharp, and without the leaf! fenfible, prominence in any 
part. . 
Some of the moulds, employed for this- ufe, were made of 
plafter, a material more convenient for the workman than me- 
tal, as the pieces part more freely from it, but which contri- 
buted greatly to increafe the above-mentioned irregularity: for 
the plafter, by abforbing a portion of the water from the clay 
contiguous to it, renders the furface at the fame time, even at: 
the inftant of contadt, much more confident , and confequently 
more difficult to prefs into the angles of the mould ; fo that £he 
outfides of thefe pieces were not only more comprefied , but 
formed of clay of a different temper from the inner parts, being 
much drier or firmer; a circumftance which, as will appear 
hereafter, reftrains ftill more their diminution in the fire* 
The moulds were therefore laid afide, and the prefs adopted 
in their ftead ; for as the foft clay, preffed in a cylindrical 
veffel, gives way and efcapes through an aperture made for that 
purpofe (by which means it is formed into long rods), ths 
fides of the piece cannot be fuppofed to receive fo great a de- 
gree of compreffure againft the fides of the aperture through 
which it is delivered in this operation, as it does againft the 
fides of the mould, by which it is confined till every part has 
born a . preffure fufficient to force the clay into every angle, 
which is much greater than even a workman would imagine 
till he comes to try the experiment himfelf. 
But with this change fome new difficulties arofe ; for pieces 
preffed through the fame aperture, and from the fame preff ful 
of clay, and adjufted, when dry, to the fame point in the 
gage, were found, after . palling together through the fame 
.ftrong 
