720 GEOLOGY OF OHIO. 
special dies made for the purpose. The more artistic chimney-tops and 
all the other kinds of ware are made in molds. In the manufacture of 
terra cotta, the development of the possibilities of molds is better illus- 
trated than in any other line of clay working. In the preparation of 
the clay, which is usually a gritty plastic clay, there is no special precau- 
tion taken, unless it is a little longer grinding than for sewer-pipe. 
The molds are filled by hand. In making statuary and busts the mold 
used consists of an outer shell with proper handles cut in it by which it 
can be lifted off as a cover from the inside part. The mold is filled when 
in an inverted position ; the cavity is seen in its proper shape, and the 
walls of it are made of a great number of small blocks which fit together 
perfectly. The cavity is filled by hand and the mold is then inverted, 
the outer shell withdrawn, and the blocks removed one by one and put 
back into place, leaving the figure desired in clay. Some pieces of 
statuary of small size have more than 100 blocks inside the outer shell. 
Real terra cotta burning should be done in a closed chamber. In only 
one establishment, viz., the Wassall, Fire-clay Works, of Columbus, is 
it done; their kiln consists of an inner kiln, in which the ware is piled 
and secured, and which, is then shut and luted with clay and an outer 
kiln, which carries the heat on all sides of the interior cavity. The 
fuel used to make the interior kiln red-hot is in quite large amount, but 
the pure color and absence of spotting of the ware repays the trouble. 
When terra cotta is burnt in open kilns like pipe it is burnt harder, but 
is not nearly so light-colored or handsome. ‘The clays of the river 
district make an admirable terra cotta, but are all burnt in open 
sewer-pipe kilns; and they have contributed one point of some value 
to the theory of burning of clays. On the light-colored terra cotta 
it is found at all times that the surfaces exposed to the action of the 
down-draft, 7 ¢., all the upper surfaces of the ware are covered with a 
fine red powder which seems deposited on these places, just as snow 
which descends slowly will cover the upper side of every object exposed 
to the weather. This red powder can be partially removed, but at the heat 
at which it is burned part of it sticks to the ware, and will not come off. 
It consists of particles of ash and other debris from the fires, and deposited 
probably during the whole process of burning; but as the wet ware ac- 
cumulates soot an inch thick when it is first heated, it may be that the soot 
is what catches the flying particles, which are left as the soot burns away. 
At any rate the point is proved that the draft of a kiln carries in a very 
