CLAY MANUFACTURES. 673 
supply fails. The heat applied to stone-ware burning is rather severe, 
as there is always danger of not getting a uniform glaze in different 
parts of the kiln. The kilns in use at Akron for burning stone-ware 
are examples of the different scales on which the manufacture is con- 
ducted. They all hold 6,000 gallons, and many hold as much as 8,000. 
They are oblong kilns about 32x16 ft. and 12 ft. high; the fireplaces 
are at each end, and there are six of them; they are usually down draft 
with only one center flue ; they are set and drawn from lateral.doors, and 
require 70 hours to burn. A sample is usually withdrawn to test the 
efficiency of the glazing. Coal is in large use, though wood is still 
retained in small part. The cooling ofa kiln should occupy as much 
time as the burning. The ware when drawn is piled in storehouses for 
shipment. In viewing a large amount of ware at once, of course the 
imperfections can be noticed more readily than one piece at a time. One 
of the most common troubles besides the “ blubbering” of the slip, is 
the ‘ pitting” of the ware in spots, due to iron. It is a peculiarity of 
stone-ware clay that while it needs iron to give it color, by far the 
largest part of the iron present is collected in grains and has no favorable 
effect on the color. These grains, if the heat is high enough to vitrify 
. them, make pimples on the surface, or break out in rough spots which 
are black and ill-looking, from the silicate of iron formed. If the heat 
is too low to vitrify the grain, some change in its volume on heating 
causes a scale to separate from the surface of the ware, showing in the 
light colored cavity the piece of iron oxide at the bottom. 
The failure of the ware to glaze is another source of trouble. This 
may be considered as due to ill-regulated burning usually, but one other 
cause is thought to act at times. Many clays exhibit collections of 
fine crystals of a whitish color, which prove on testing to be sulphate 
of lime or land plaster. This substance when exposed to vapors of salt at 
high heat, suffers a common chemical transformation and changes to 
chloride of lime, leaving the soda as a sulphate, which it appears either 
will not displace silica from combination, or will not do it at the tem- 
perature the ware will stand without losing shape, and hence there is. 
no glaze. The heat, besides that necessary for the ware, must be high 
enough to make both slip and salt operate in glazing.- The failure of ’ 
the slip to melt leaves the ware of a rusty red color from the{uncom- 
bined oxide of iron. 
43 G. 
