30 
KERAMIC STUDIO 
GRAND FEU CERAMICS 
X— FIRING— Neutral, Oxidising, Reducing 
Taxile Doat 
(CONTINUED) 
THE regulation of the firing may be done in three different 
ways: ordinary or yieutral, oxidising and rcdticing. 
Neutral firing has always and is still nsed by all porcelain 
manufacturers. It is the "go as you please" of empirics. It 
is easy to imderstand that if one throws wood into the fire 
mouths, little at first, more towards the end, the point of firing 
of kaolinic matter will sooner or later he reached. At Limoges, 
in Saxoiw, at Sevres and everywhere that porcelain Avas made, 
the main result sought for was the brilliant whiteness of the 
ware. (This whiteness caused the public to prefer porcelain 
to earthenware against which porcelain makers had made a 
bitter fight. The whiter the porcelain the better it was 
adapted to decoration with the only palette known at first, 
that of the muffle fire.) 
Manufacturers had observed that when, dining the firing, 
there was an excess of smoke the porcelain had an un^Dleasant 
grejash tint. It was therefore absolutely necessary to avoid 
this smoking of the kiln. On the other hand the time of firing, 
with the labor and expense involved, always seemed too long, 
and it was important to push the fire so as to reach the end as 
rapidly as possible. Tossed between these two extremes, 
porcelain makers had succeeded in regulating the firing in an 
abnormal and empirical way, by selecting the wood, dividing 
the feeds, forcing into large fire mouths all the fuel they could 
stand, but at the same time airing the kiln everywhere with 
strong drafts, as if it was burning in the open air, with a violent 
wind. And as with this excess of fuel, the live coal accunui- 
lated in the fire mouths, and formed smoke, everj^ two houi's 
they were cleaned cmt. This severe operation, the taking out of 
the coal which had accumulated and obstructed the openings, 
scorched the skin of the firers, but they did the work bravely, 
as this coal was one of their perqiiisites. As soon as the fire 
mouths were clean, they became active again and the smoke 
disappeared to give place to the oxidising atmosphere. With 
this regular cleaning of the fire mouths, porcelain makers 
succeeded in obtaining very pure white after 30 to 34 hotn's 
firing. The oxidising action was strong but not complete, as 
it was active only at times. When the palette of pates sur 
pates was created at Sevres, the cleaning of the coal was 
practiced regularly to preserve the oxidising atmosphere so 
necessary to the development of certain colors, such as the 
yellows of uranium, the pinks, mauves, turquoise and blades 
which owe their brUliancj^ to this atmosphere. As the oxida- 
tion was insufficient this was remedied by the creation of air 
drafts through fire brick tubes X passing through the walls 
of the kiln (Fig. 49, p. 228, Feb. 1904). These tubes conveyed 
an excess of oxygen to the inside of the saggers where the 
colors were, which certainly, when it worked well, improved 
the tones wonderfully. But success depended on the stability 
of these tubes. If in their moving surroundings, thej^ became 
displaced bj^ a movement of the bungs or were obstructed by 
some rmexpected cause, the flow of oxygen being imperfect, 
the colors did not come out with aU their brilliancy, the decora- 
tion remained gre\- and dirty. I have seen manj^ of these 
finely decorated pieces on which the air draft had failed to act. 
The air draft, led into the chamiel at the bottom of the 
bung, follows it without any break to the top, ending outside 
after crossing the vault of the firing chamber. The points 
of entrance and exit of the air being fixed, at each firing these 
air passages must be carried between the bungs in the same way. 
This empirical firing could not stand the examination of 
serious and learned minds, trained in the logic of exact sciences. 
The Sevres chemists, applying to ceramics the scientific pro- 
cesses wliich were used in metaflurgy, solved the question of a 
purely oxidising or a purely reducing atmosphere by the rela- 
tion of the sections of the chimney to the fire mouths. 
Oxidising (Fig. 89) — If a narrow fire mouth corresponds to 
a large opening for the exit of the flame, the fuel is completely 
consumed under the action of the strong draft which the 
narrow fire mouth produces. As wood consumes only the 
amount of air necessary for its complete combustion, there 
rushes into the kiln an excess of air which devours the unburnt 
gases which may have been introduced. The atmosphere of 
the kiln is then called oxidising. As it is necessary to con- 
stantly increase the feeding of fviel in order to reach the high 
temperatures for the firing of porcelain, one has to face the 
following problem: the temperature wiU be lowered if, when 
feeding little wood, too much air is left to penetrate the kiln, 
or the oxidising atmosphere will be lost, if with an excess of 
fuel the smoke irn-ades the firing chamber. There is a happy 
medium which practice alone will teach. It would be impossi- 
ble to finish a firing if one tried to have an absolutely oxidising 
atmosphere all the time, wliile the contents of the kiln would be 
irreparably damaged. if the reducing atmosphere was kept from 
beginning to end. The characteristics of an oxidising fire are, 
inside of the kiln, a brilliant white light, without an3' trace of 
flame or smoke, and outside, the absence of flame on top of the 
chimney. When a rush of flames is produced by the fall of 
a burning load of wood in the fire mouth, these flames should be 
blueish. 
Dujing an oxidising fire, the neutral firing which allows a 
rapid increase of heat, is adopted until the fall of cone 01 3, 
vv^hen it is necessary to begin oxidising, because unburnt gases 
which would be deposited on the pieces, woUld prodiice with 
the colors a pyrochemical combination which wovild injure or 
destroy them. It wiU be easily imagined that this kind of 
firing is much, slower than the neutral or reducing ones. It 
lasts 3 or 4 hours longer. 
Crystalline glazes are obtained only in a purelj' oxidising 
fire. Oxidising firing gives a sHghtly ivorj' tint to white 
porcelain. 
Reducing (Fig. 90) — If on the contrary' to oxidising, a 
large fire mouth corresponds to a narrow openitig for the exit 
of the flame, the draft is insufficient and the wood which has 
accunmlated in the fire mouth does not bum completely; 
carbonic oxide and carburets of hydrogen escape from it, and 
in the shape of a sooty smoke whirl in the kiln, mixed with the 
flame which then takes a reddish tint, l^his is the reducing 
atmosphere. It gives a greyish tint to porcelain. It would 
be dangerous to maintain the flame in this condition of satura- 
tion with smoke during the whole firing. It is necessary to 
keep tliis atmosphere only up to the fafl of cone 06 inclusively. 
In this case the unburnt gases help the colors. They are de- 
posited on and combined with them at the time of the fusion of 
the glaze. As soon as this vitrification is obtained, the fall 
of cone 06 occuring at about § of the time of firing, one must 
change to oxidising to clean the kiln of aU the carljurets, which 
cannot have a good action any longer. If the reduction was 
continued much longer, the colors would lose their brilliancj' 
and become blackish. 
One wifl understand how important it is to handle well 
these two firings, the same colors developing in an entirely 
different way according to the condition of the atmosphere. 
So celadon of iron, a delicate jade green in reducing, remains 
greyish in oxidising. Red of copper, of crimson color in re- 
ducing, becomes opale or spotted green in oxidising. Uranium 
