72 PIPE CLAY. 
that are made of it more firm and strong. Those that 
are applied to culinary, and other uses in which it is 
requisite for them not to be penetrable by water, are 
covered with a glazing. This glazing, for coarse ware, 
is sometimes made with lead, and sometimes by 
throwing a certain portion of salt into the furnace. In 
the formation of the better kinds of earthenware, the 
clay is made into a paste with water, moulded into the 
requisite shape upon an horizontal wheel, the inside 
being formed by one hand of the potter, and the out- 
side by the other, as the wheel turns round. When 
the pieces have been baked, they are dipped into a 
glazing mixture, consisting of white lead, ground flints, 
and water, and are exposed a second time to the fire. 
The . different colours of earthenware are obtained by 
means of various kinds of metallic oxides (21). 
The coarser kinds of clay are manufactured into 
bricks for the building of houses, and tiles for the co- 
vering and paving of them. These are formed in 
moulds of the requisite shape, afterwards dried for 
some time in the sun, and finally piled in kilns, and 
there baked to a proper degree of hardness, The earth 
for bricks ought to be sufficiently fine, free from peb- 
bles, and not too sandy, which would render them 
heavy and brittle ; nor ought it to be entirely free from 
sand, as this would make them crack in drying. 
Clay is a substance of inestimable value for forming 
the bottoms of ponds, and the bottoms and sides of 
canals and reservoirs, to prevent the water from drain- 
ing away. It also composes, in a great measure,, those 
tenacious earths called arable soils. What is peculiarly 
denominated clay land is known by its holding water, 
and not scon drying when wetted. f Such land requires 
much labour from the husband mgfe, before it can be 
sufficiently pulverized, or brought to a fit state for 
being productive of corn or grass. 
116. PIPE CLAY is a Jine and yellowish white 'variety 
of common clay* It is very plastic, adheres strongly to 
