566 GEOLOGY OF OHIO. 
The primitive method of coking in heaps has been to a great 
extent superseded by the use of ovens or closed chambers of different 
forms and construction, by which all sizes of coal become equally avail- 
able. Though the apparatus may be more expensive, the increased 
yield of coke, its better quality, and the less expense of handling the mate- 
rial cause the cost of the coke to be much less than when made in the 
open air. A greater proportion of sulphur may be also expelled in the 
ovens, and coals which would otherwise not coke, or only but in- 
differently, are made to yield an excellent product. This is specially 
marked where the uniform and very dry coals are required to be used 
for coke, and where the expense of the coal requires the greatest economy 
of material and manufacture. Thus in Belgium, France, etc., the use 
of the dry and inferior coals, which are commonly washed, has necessi- 
tated the employment of ovens specially designed for the cireumstances, 
and thus has arisen the large class of ovens known as the Belgian ovens. 
From the excellence of their plan, their economy and efficiency, they 
are now being largely supplied for the better varieties of coal, both in 
England and this country. 
In the production of coke the ovens are almost always heated by 
the combustion of a part of the coal itself, and in only one or two plans 
is the necessary heat produced by burning a separate portion on the 
exterior. When, however, the object in carbonizing the coal is to 
obtain the gaseous products, as in manufacturing illuminating gas, the 
coal is heated in a closed retort by fuel burnt beneath it, the opera- 
tion for obtaining gas solely being conducted in a different manner from 
that when coke is desired; the coke produced is light, spongy, fragile 
and unfitted for metallurgical use. 
The simplest form of oven for producing a metallurgic coke is a 
vaulted and closed chamber, like a baker’s oven, which is first heated 
to a high degree, after which the coal is charged, and the volatile matter 
expelled by the heat of the oven, and being burnt by the admission of 
air into the furnace, maintains the necessary degree of heat. In this 
form of oven the coking operation begins at the surface of the coal and 
descends to the bottom of the charge, and if the heat is insufficient 
from any cause, coldness of oven, small amount of volatile matter in the 
coal, ete., the bottom will not be well coked. ‘To facilitate the process, 
and to use to best advantage the drier or less coking coals, the ovens 
are heated on the bottom or sides, or on both by the combustion in flues | 
