672 GEOLOGY OF OHIO. 
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Sesquioxide ofmnronu(HerrOn \ne.accwaresstee sneer eee eee eee eer eee 5.36 
intel (Cas O) Pie eeeidan comesecssice (noraas Coec aa Nreel nce aCe an nT aes 9.88 
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Phosphoriciactdy (PO gp) cssscesseestee tasek ete eee oe eee Soe eee ae 0.19 
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100.00 
Probably not less than 46 per cent. of the silica present is a very fine- 
grained sand. The main trouble to be overcome in the use of slip glazes 
is a tendency to blister or “blubber.” The cause of this is not de- 
finitely known. Every potter has his theory. The opinion of several of 
the most intelligent has been that undue haste in burning is the most 
frequent cause, and that the ware should be heated to a low red heat for 
several hours before any higher heat is attempted, after which rapid 
raising of the heat can do no harm. ‘This treatment has certainly been 
successful under the conditions in which it was tested. When the ware 
is slipped, its capacity is usually stamped on it with a solution of smallt, 
or cobalt glass, which gives its customary blue color on heating. 
The ware is next “set” in the kiln. The kilns in use vary con- 
siderably. The country kilns are usually small square structures, bound 
on the outside by logs of wood ; the fire holes are left above the ground 
and communicate through the various arches of masonry which sup- 
port the floor of the chamber above ; this floor is full of holes, through 
which the fire freely passes, and the arched roof has usually seven vents 
of exit for gases. The chamber is oftenest about 8 ft. in diameter 
and 7 ft. high to the arch top. Such achamber will hold, by nesting the 
ware, about 1,600 gallons. The firing is commenced at night, and is 
made very light indeed at first; in the morning the ware will be 
approximately dry, and the heat then continues from 20 to 30 hours, as 
the conditions vary. Nearly all stone-ware is salt-glazed on the outside 
and slipped inside. ‘The salt is thrown in the fire when the heat is 
highest, and the vapors are seized by the clay and a soda silicate forms 
over the surface of the ware. The heat required to do this is the 
culminating point of the burning. The fuel is nearly always wood ; coal 
is used in some places on a second fire-floor lower down than the one 
used for wood. This method of use will begin to grow as the wood 
