CLAY WORKING INDUSTRIES. 177 



No intelligent objection can be raised against salt glaze, except from 

 those makers who are trying to make paving blocks and building brick 

 in the same kiln at the same time. In this case, the glazing of a few 

 courses of the building bricks is likely to happen and is undesirable, for 

 in building material a certain amount of porosity is essential and a glaze 

 interferes with it. 



The Kilns in which the burning of vitrified clay wares is accom- 

 plished, do not vary much in reference to the kind of the ware to be 

 burnt, consequently the kilns in use are most of them used indiscrimi- 

 nately by the makers of vitrified wares. 



The conditions which the satisfactory drying of clay wares demand 

 have been seen to be definite and concise. Any radical departure from 

 these conditions is fatal to the success of the operation, and that form of 

 dryer in which these conditions can be most easily brought about, or has 

 the most power to vary these conditions slightly to suit the peculiarities 

 of the clay under treatment, is the one which meets the surest and widest 

 success. 



In burning vitrified clay wares, also, the conditions under which 

 success can be obtained are comparatively sharply marked; to produce 

 and obey these natural conditions must be the aim of any successful kiln 

 constructor or operator. 



The first condition of success in a kiln for burning paying material 

 is the use of fireplaces which not only admit of regulation and control, 

 but enable the burner to consume large amounts of fuel without making 

 poor combustion, and which will allow the cleaning of the fires without 

 the loss of great heat or time in the operation. 



In burning paving material the heat, after being carefully and grad- 

 ually raised to the vitrifying point must be held there for some time, to 

 allow it to " soak in " or permeate the entire body of ware, also the 

 various parts of the kiln are not likely to obtain the proper heat just at 

 the same time and the production of heat must therefore be under 

 accurate control. The vitrifying action takes place over quite a range of 

 temperature and it is the aim of the burner to hold the heat at a point 

 where the vitrifying action will take place slowly and without failure of 

 the shape of the ware. Also, the burner must be able to produce and 

 maintain high temperatures for long periods. 



If the fireplace be of a type that is difficult to clean when under full 

 fire, the loss of fuel and time in clearing fires becomes a serious item in 

 the success and cost of the operation. 



There are a number of types of fireplaces in use: — The most impor- 

 tant ones will be described. 



1st. The outside furnace is used in a few kilns. It consists usually 

 of a square brick structure with flat arched roof provided with firedoor 

 and ashdoor in front and with a flue leading into the lower part of the kiln 



12 G. O. 



