RERAMIC STUDIO 
grand feu only after the cherry red has completelj^ pervaded 
the kiln. I reach my results in a definite time with a mathe- 
matical precision, which beginners cannot expect to secure, 
but with observation and care in feeding, thej- will soon be 
successful. 
The drjnng and petit fen together generally take less time 
than the grand feu. 
For the grand feu oak is discarded and birch used. The 
latter is not thrown into the fire mouth, but carefully placed 
on top of the latter, on the hopper (Fig. 82, 83 and 83 bis.). 
ha. SI 
hermetically closes all openings and cracks with mortar, and 
stops the air drafts if any should have been established. The 
firing is over. 
vSince the invention of hard porcelain, firers invarial^ly 
used in order to determine the condition of the porcelain, 
pieces placed in the middle of the kiln and called test or trial 
pieces. They are made of little tiles of baked porcelain about 
2 inches long and i inch wide, with a large hole in the upper 
jnirt. This upper part alone is glazed. To allow them to 
stand up, these test pieces have their feet wrapped in a wad 
of lute which gives them a large and solid basis (PTg. 86). 
Sevres and the porcelain industry keep using these pieces which 
alone show the true condition of the ware. I do the same. 
In 1882 the German chemist vSeger conceived the clever 
idea of adding to these trial pieces fusible cones which can 
be seen during the development of the firing and relieve the 
operator from the uncertainties and anguish which formerly 
assailed him during the finishing hours. Simultaneous^ 
discovered at the factories of Berlin and Sevres bj^ chemists 
who were not aware of their similar work, these cones are made 
of fritted material of different fusibilitJ^ They have the shape 
of a triangular p^-ramid (Fig. 87). I will not undertake to 
Every five minutes regularly a handful of about 10 birch 
sticks is placed against the side of the kiln, and all openings 
caused by the combustion must be promptly filled. From 
that time to the end of the firing, the fire mouths must not be 
lost sight of, as they must be constantly fed. The sheet h-on 
plaque over the fire mouth is replaced by a fire brick plaque 
which rests on the hopper and supports the necessary amount 
of wood. 
Once in a while, every two hours, and toward the end, 
every hour, one watches through the spyholes what is happen- 
ing in the kiln. One sees through the white glass the fusible 
cones which successively curve and melt when the temperature 
of the kiln reaches their point of fusion. Behind the cones 
are three trial pieces which are rapidly picked up with an iron 
rod slightly curved at the end (Fig. 84 and 85). This rod is 
^ 
f'V '^^ 
wonfre cU^e- - 
dtir&r- 
f-- 6^ 
f.i. 6C 
introduced through the opening of the spyhole, after the stopper 
of the latter has been removed with the help of wet cloth. 
The trial pieces show the true condition of the kaolinic matter. 
When with the help of these two tests and his experience, the 
firer judges the porcelain fired, he stops feeding fuel, slides two 
large fire brick plaques over the fire mouths to close them, 
7. Forme da^ «-no*thai 
describe their scientific composition; it will be sufficient to say 
that their successive melting indicates clearly if the same 
temperature is reached at the same time in all parts of the kiln 
where these cones are placed. Ceramists have so much 
appreciated the services rendered l:)y these cones that their 
use has l)ecome c^uite general. 
I order the cones direct from the makers, Seger &. Cramer, 
Berlm, at the cost of fr. 5, 65 (about Si.io) per hundred.* 
The standard adopted for the numbering of the cones is 
the melting point of cast iron i I30°C. Tliis is called cone No. i . 
Numbers go from i to 32, which is the melting point of platinum 
1 770°C. Numbers which register temperatures lower than cone 
I are preceded by a cipher. They go as far down as the dark 
red registered by cone 022, or 590°C., the melting point of 
Ijright liquid gold. There is only about 20° difference between 
the successive cones. After many trials I have adopted the 
following cones for my firing: 
For reducing fire For oxidising fire 
Cones 06 Cones 013 
I I 
7 7 
8 8 (fusion of feldspar) 
9 9 
10 10 (Sevres hard porcelain.) 
In reducing fire, cone 06, corresponding to the point of 
fusion of the glaze, tnarks the limit of reduction in the kiln. 
No. 10 indicates the temperature at which porcelain is fired 
and the firing should stop. 
*In the United States, Prof. Edward Ortoii, Jr., Ohio State University 
Columbus, O., manufactures cones and sells them for one cent apiece. 
