KERAMIC STUDIO 
77 
CLAY IN THE STUDIO 
(Tenth Paper.) 
Charles F. Binns 
HE critical period has now arrived 
when the chiy is to pass through the 
ordeal of fire, to perish or to persist. 
Before entering upon the details 
concerning kilns and their use a few 
general observations upon fire and 
its application to pottery will be nec- 
essarj^. The following paragraphs 
should be well studied and thorough- 
ly mastered for it is only by an in- 
telHgent understanding of the principles which govern the 
action of fire that one can hope to reach success. 
The heat of fire is produced by the phenomenon of com- 
bustion which is a chemical union taking place between fuel 
and. the oxygen of the air. The composition of fviels need not 
concern us here except to state that the}^ contain carbon and 
hydrogen in varjang proportions. Carbon under ordinary con- 
ditions is a solid. In its pure state charcoal and plumbago 
(erroneously called black lead) may be cited as examples. If 
a fragment of charcoal be heated to redness and supplied with a 
current of air it w'ill glow and burn awaj\ It has combined 
with the oxygen of the air and has passed into a colorless gas 
known as carbonic acid gas or carbon dioxide. If the supply of 
air has been deficient a second colorless gas may have been 
fonned. This is called carbonic oxide or carbon monoxide and 
it contains only half as much oxygen as the first named gas. 
Hj'drogen is a gas, though it exists in combination with car- 
bon as a solid or as an oil. Both pure and combined thus it is 
inflammable and forms a very important constituent of fuels. 
Burned in a current of airhj'drogen forms water vapor but if the 
air suppl}^ be deficient it refuses to bum, retaining its form as a 
gas. In burning gas the constituent parts are in such a form as 
to take fire readily but both liquid and solid fuels must be so 
heated as to become gaseous before they will burn. Thus 
while we visually speak of burning coal or oil it is really the gas 
which is evoh^ed from these substances by heat which actualh^ 
burns. In elementary text books of science there are gi^^en 
many pretty and interesting experiments to prove the truth of 
these assertions. We have not space to deal with the matter at 
further length. The point upon which these facts bear is the 
condition of the atmosphere inside the kiln upon which all suc- 
cessful burning depends. 
It will be seen from the above statemejits that the supply 
of air to the fuel is of the first importance. A heated chamber 
n\3.y contain gases which are characterised according to their 
chemical composition b^- one of the following terms: 
I. Oxydising; 2. Neutral; 3. Reducing. The first term 
almost explains itself. It means that there is more oxygen pres- 
ent than the fuel actualh^ needs for perfect combustion, or in 
other words, that more air has been admitted than was neces- 
sary. Generally speaking this does no harm except for the loss 
of heat occasioned bj^ a rush of cold air. The oxydising gases 
are the simplest to manipulate and give as a rule the best re- 
sults. The meaning of the term oxydising is that the atmos- 
phere is ready to supply oxygen and therefore to oxj^dise any 
substance which may come within its power. This is not an 
objectional feature because almost all colors and glazes are 
alreadj^ oxydised, they have all the oxj^gen they need and can 
take no more. 
The term "neutral" is likewise easily understood. This 
condition also would be harmless but is extremely diificult to 
regulate. In fact ita an ordinary burn one cannot be sure of a 
neutral atmosphere and therefore it is best to work for oxidis- 
ing conditions. 
The expression "reducing" needs some explanation partly 
because the word has often been wronglj^ vised and partly be- 
cause the efi^ect of a reducing fire is usually disastrous. The 
word "reducing" in this connection has nothing to do with 
temperature, nor with the rise or fall of the fire. It does not 
mean " decreasing. " It has to do with the chemical nature of 
the kiln atmosphere and is the opposite of oxydising. 
If combustion be allowed to take place with a shortage of 
air the gases produced consist in part of carbonic oxide and hy- 
drogen. Both these have a strong appetite for ox5^gen and if 
thej'^ cannot draw it from the air they will rob the glaze, the 
colors and even the body of the pottery. None of these has any 
more oxvgen than it needs and yet thej' are forced to part with 
it to the hungry gases. In doing this thej^ are " reduced" from 
higher to lower oxides and hence the action is called reducing. 
A simple illustration will help the student to understand 
the action. Iron oxide exists in several forms. One of these, 
the common red oxide, has two parts of iron to three of oxygen, 
another, the black oxide, has equal parts of iron and oxygen. 
The parts i^eferred to are not calculated by weight but by atoms, 
iron weighing 56 and oxygen 16. Now if the red oxide of iron 
be heated in a reducing gas it loses one of the parts of oxygen 
and turns black, being reduced to the black oxide. If the 
action be reversed and the black oxide be heated in an oxydis- 
ing gas it absorbs oxygen and turns red.. 
Most of the substances used in potter3^ are capable of losing 
oxj^gen and hence the reducing fire is objectionable. 
Hard porcelain is purposely burned in a reducing atmos- 
phere btit we are not now dealing with hard porcelain though 
this may come later. 
The conclusion to be drawn from the foregoing explana- 
tions is that, in firing, care must be taken to secure perfect com- 
bustion. A good draft and a plentiful admission of air are in- 
dispensable. 
Kilns are of two types, the open kiln and the muffle. In 
the former the flames pass through the firing chamber and the 
ware is usually protected by saggers; kilns used in most potteries 
are of this type. In the muffle the flames pass around and over 
the chamber but not through it. There the ware is heated by 
radiation from the walls. Kilns used for overglaze painting 
belong to this class and also the large kilns used by makers of 
enameled bath tubs. 
The Caulkins kilns are muffles and many of them are doing 
good work. For portable studio kilns there are no better to be 
bought. They have their faults — no kiln that was ever built 
has none — the capacity is small, a trouble inseparable from a 
portable kiln and they need a veiy powerful draft. 
In the matter of inexpensive masonry kilns a state of 
evolution exists. Many have been erected and torn down, but 
little by little improvements are being made. The simple 
description of such a kiln and the rough sketch here given may 
aid some experimenters but local conditions vaiy so greatl}^ 
that one must be prepared to deviate from the lines laid down. 
The kiln is of the muffle type. The firing chamber consists 
of a length of flue lining which can be procured through a hard- 
ware dealer. It is made of earthenware and has walls about 
one inch in thickness. A length should be procured of which 
the section is as nearly square as possible or, two lengths may be 
laid one on the other as indicated bj^ the dotted lines. If this 
be done it must be remembered that the upper chamber will be 
somewhat hotter than the lower. One end of the flue lining is 
filled up with fire brick set in clay. The front is left open but 
bricks are prepared so as to close it in the same way when firing. 
Only in this case the bricks are set together drj- and the front is 
