688 GEOLOGY OF OHIO. 
The impurities which would thus escape are potash and soda, from such 
compounds as feldspar and mica ; yet the decomposition of these minerals 
having been effected by weathering, the mechanical conditions which 
would aid in the escape of the impurities are seldom found. A strong 
slant to the floor of the clay pile, so that water would drain away quickly 
and well after raining, and only a thin layer of clay on the floor so that 
impurities from the top layers might not lodge in the bottom, would 
favor the escape of impurities, but the exact opposite of both of these 
conditions as a rule prevails. Iron sulphide and carbonates of lime 
and magnesia would also tend to decompose and leave the clay, but 
their action would be very gradual. But the mechanical subdivison of 
the clay which takes place is undoubtedly advantageous; alternate frost 
and heat has long been reckoned as a valuable agent in increasing the 
‘plasticity of hard clays. From the pile the materials are selected; the 
mixture of so much of each factor is made by counting the shovelsful 
with which the charging barrow is loaded. In no case in the State are 
the constituents weighed; no closer proportion is kept anywhere than 
careful shoveling will make. The barrow being loaded, the clay should 
be washed; in only one place in Ohio is this precaution taken. That 
is at the works of the Portsmouth Fire-brick Co. The washing there 
is accomplished by running the barrow over a sink or drain and drench- 
ing from a hose above. ‘The barrow being perforated on the bottom 
speedily drains dry again. ‘This treatment, though not thorough, tends 
to free the clay from dust, mud and dirt which stick to it from the 
diggings. The best method of washing seen was at the excellent works 
of Harbison and Walker, Pittsburgh. Their machine consists of a 
cylinder, revolving in a slightly inclined position in a trough of water. 
The wall of the cylinder is made of coarse iron gauze or netting, and 
on the inside is bolted a spiral flange beginning on the upper end and 
running to the other extreme. A charge of clay is introduced into a 
hopper at the upper end, and by the flange is slowly carried down the 
length of the cylinder, being agitated in water, which is about 6 
inches deep in the lower part of the cylinder. This machine is only . 
used to wash hard, uncaleined clays, for the plastic grades would not 
stand so severe a treatment, and calcined clays do not need it. Washing 
is only useful or advisable where the hard clays in use are mined by 
benching or stripping and comes to the works covered with mud or dirt. 
The next step is grinding the charge; as before stated, this is the 
