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REDUCTION
and Ceramic Firing
by
Smart2000
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- Foreword
:
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- A product put in the kiln
will undergo transformations under the effect of heat.
It will loose gases by dissociation of its compounds
(dehydration, decarbonation, etc...), some chemicals
will melt and form vitreous binders which will harden
the product during cooling or will form crystals,
others will combine and form new compounds, etc... All
these modifications will take place according to a
process specific to the materials put in presence, the
temperature reached and the speed of firing. Up to
that point all seems to go as one generally means when
speaking of firing.
-
- However, one should not
neglect another very obvious parameter but so present
in our environment that we end up forgeting about it :
the gaseous atmosphere in which the process takes
place.
-
- We lived completely immersed
in this element, which is the gaseous mass of
atmospheric air just like fish live in water. We are,
as well as the other living beings (animals and
vegetable), and the elements of the surface of the
planet, sensitive to the components of the air in
which we draw a part of our vitality.
- Oxygen, the essential
gaseous element of respiration in mammals to which we
belong, is also a principal component of the majority
of earthy materials used in ceramics (note that only
0,01% of oxygen is found in gaseous form, the
remainder is combined in a stable way with the various
elements of rocks, water, organic
matter...).
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- When we heat a non-oxidized
product in our atmosphere, it becomes oxidized by
contact with the oxygen in the air (except for hard to
oxidize elements such as gold, platinum and other very
stable elements). This oxidation takes place according
to a certain affinity for oxygen and according to
temperature, thus certain elements will become
oxidized as of the ambient temperature and others will
require an important heating... while already oxidized
products heated together in the air will remain
stable.
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- If one heats together,
sheltered or inside a kiln low in air, elements of
different affinity for oxygen and having an intimate
contact between them (for example a gas and a
solid...), the most avid one if insufficiently
oxidized will be able " to take " oxygen from the
other. The compound which yields its oxygen is then "
reduced " and the one which collects it is "oxidized
".
- The exchange of oxygen of
the solid phase (surface of the glaze or of the
ceramic product) towards the gas phase insufficiently
oxidized but more avid for this one (oxygen)
characterizes what ceramists call reducing
firing.
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- Oxidizing or
reducing Atmosphere ?
-
- These terms which always
question the beginners in ceramics refer to the
composition of the gaseous atmosphere present in the
kiln during firing.
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- Firing known as " oxidizing
" or " in oxidation " relates to an atmosphere partly
or entirely made up of air (see
composition of air).
This gaseous atmosphere thus contains free oxygene.
Oxidizing firing can take place throughout the whole
of the firing cycle.
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- When one speaks of "
reducing firing " or " firing with a reduction phase
", it means that a gaseous atmosphere avid for oxygen,
and deprived of this element, is produced inside the
kiln at a particular time, but not during the whole of
the firing. The phase of reducing firing thus
represents only a part of the firing cycle while the
rest of the firing is done in oxidation. It generally
occurs towards the end of the rise in temperature and
during cooling.
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- It is also said that the
atmosphere is " neutral " when the lack of air is null
and that there is no excess of oxygen. This type of
atmosphere is purely theoretical, in reality it is
very rare to come " to fix " a combustion under these
conditions, it can only be an intermediary passage
obtained while going from oxidation to reduction for
instance. Even in an electric kiln where heat is
produced without the use of combustion, the little
amount of air present contains nearly 21% oxygen, that
is much more than in the richest of atmospheres of
oxidizing combustions (unless consuming this oxygen
with the combustion of organic matters and the
kilnbeing tightly closed).
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- It is possible that all
these types of atmosphere are simultaneously present
inside the kiln when this one presents turbulences and
bad gas mixtures (case of kilns heated by
fuels).
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- How to
produce reduction ?
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- In ceramics, reduction is
the result of the extraction of oxygen contained in
some compounds of the glaze, the clay or the
colourants (colouring oxides ) by the gaseous
atmosphere produced in which this element (oxygen) is
missing. This reaction cannot occur at ambient
temperature, it must take place at the time of firing,
when the level of heating allows the oxygen links of
the compounds to be reduced to break under the
attraction effect of a gaz known as " reducer ". This
level of heating can be during the rise in temperature
or at the time of the descent during
cooling.
- The material to be reduced
must be sufficiently permeable to the reducing gas
that the reaction is done in the totality of its mass,
if not, the effect will be active only on the surface
of the product.
- The kiln atmosphere must
thus be controlled so as to produce a gas flow avid
for oxygen at the right time, when the links of this
element weaken in the compounds having to be reduced.
Each one of the oxidized compounds has its own level
of reductibility. The action of reduction can be
effective only if it is targeted on a level of energy
corresponding to one or more weakened compounds that
can yield completely or partially their
oxygen.
- The type of reducing gas
produced, its content in the kiln and the temperature
characterize the reduction capacity of the
atmosphere.
- For the majority of
traditional ceramic firings with phases of reduction,
the principal reducing gas used is carbon monoxide
(but hydrogene can also be produced during the phase
of reduction at the time of incomplete combustion of
hydrocarbons).
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- Carbon monoxide is
produced from the incomplete combustion of carbon
present in a gaseous combustible (natural gas,
propane...), a liquid combustible (fuel...), or a
solid one (coal, wood...). Carbon monoxide is the
reducing gas which is easiest to produce in the range
of temperature of traditional ceramics. One manages to
create a reducing atmosphere in a kiln by reducing
combustion air, or by increasing fuel flow. The main
point being to arrive at the good moment, at the good
temperature, and with a reasonable rate of unburn
combustible (rate of C, CO, H2...).
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- Example with propane :
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- In complete oxidizing
combustion :
- C3H8 + 5(O2+4N2)
===> 3 CO2 + 4 H2O + 20 N2
- Propane + air
===> carbon dioxide + water + nitrogen
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- In combustion with a 20%
lack of air :
- C3H8 + 4(O2+4N2)
===> CO + 2CO2 + 3H2O + H2 + 16 N2
- Propane + air
===> monoxide of carbon + carbon dioxide +
water + hydrogen + nitrogen
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- A reaction of reduction is
also possible from a smoky atmosphere charged with
very fine and very avid carbon particles for oxygen
which act by contact with the compounds to be reduced
present at the surface of the pieces. It is partly the
case at the time of the reduction of copper oxide to
copper metal during fuming carried out at the end raku
firings. It is necessary at the end of this treatment
to cool the pieces very quickly, if not copper metal
will reoxidise in the air and the metal effect lasts
only a few moments.
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- Reducing firing is generally
produced in a gas kiln or in a kiln heated by other
fuels (wood, coal, fuel...). It is not appropriate in
electric kilns beacuse it will damage the elements, or
then these must be isolated from the atmosphere by a
sheath or a saggar.
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- The range of reducing firing
relates especially to the high temperatures (from
1100°C to 1350°C) for stoneware and
porcelain.
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- Caution
: Thermocouples of the " S " type; made
of rhodium platinum-platinum do not withstand
reduction if they are not protected by alumina sheaths
(tubes). A naked thermocouple exposed in a reducing
atmosphere volitilizes in a few
seconds!!
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- Effects of
reduction on colours :
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- In oxidizing firing,
therefore in the presence of oxygen in the atmosphere,
metallic oxides which compose the colours of the
glazes are maintained at their highest state of
oxidation and are thus very stable. The colors which
are developed in this state of oxidation are uniform
and easily reproducible. Reproducibility will be best
in an electric kiln, where the oxidation state is
ensured in a homogeneous way, there is no combustion
contributing to the production of heat (except in
the case of firings of decorations containing organic
matters).
- In the case of firing in an
atmosphere deprived of oxygen, where combustible
compounds are incompletely oxidized, the metallic
oxides of the glaze undergo variable levels of
alteration according to their reductibility, their
contact with the reducing atmosphere, the thickness of
the glaze, the reduction of the atmosphere, the
temperature, the selected moment and the duration of
the reduction, the location of the kiln, etc. This
produce very subtle variations of colours which are
not identical from one piece to the next.
- Reproducibility works only
in one pallet of effects with broad variations, and
makes this type of firing rather unsure. This is what
contributes to the mysterious and surprising side
causing the passion for this type of firing,
guaranteeing strong emotions with each opening of the
kiln...
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- Copper and iron oxides
are very sensitive to reduction.
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- Thus, for example, ferric
oxide (Fe2O3) or red iron oxide can produce greens,
blacks or blues when it undergoes the effects of
reduction. These colors are due to its transformation
into ferrous oxide (FeO). These reactions happen above
900°C.
- The " Celadon " glazes are
glazes coloured by iron oxide fired in reducing
atmosphere. The transformation of ferric oxide into
ferrous oxide gives the green-blue colour to the
glaze, if not it would be yellow.
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- As for copper oxide (CuO) or
cupric oxide which gives a green color, it turns dark
brown by transformation into cuprous oxide (Cu2O) or
rather easily becomes metallic copper (Cu) below
800°C.
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- Copper red high fire glazes
(as " Ox blood ") obtained by reduction are among most
difficult to produce. The red colour results from a
dispersion of very fine metal copper particles in a
metallic state within the glaze.
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- The reduction of the
majority of glazes must be done towards the end of the
firing when these are in a fluid state. It is
necessary to continue reduction during cooling until
the glaze is very viscous, and when reoxidation cannot
act any more. One can also stop firing and close all
the openings to prevent air from entering the
kiln.
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- Reduction at
low temperature :
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- In raku firing, the
reduction is obtained outside the kiln, in a tight
container where the incandescent pieces are introduced
at the same time as organic matter (dry leaves,
sawdust, rags...). While wanting to burn under the
effect of high temperature, these organic matters
consume completely the oxygen present in the container
and produce carbon monoxide (CO) and carbon (C) which
actively seek to extract the oxygen contained in the
glaze and the colours present on the surface of the
pieces. The produced effect is at the same time a
surface reduction and smoking.
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- Smoking
:
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- Smoking is produced by a
period of extreme reduction at the end of the firing,
so that the nearly total deprivation of oxygen
saturates the atmosphere with unburnt carbon in very
fine particles, which produces black smoke that
invades the kiln and impregnates the surface of the
unglazed pieces which will have after cooling a colour
ranging from gray to black. The colour is thus due to
the fuel and not to a transformation of the shard.
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- In the case of smoking of
glazed pieces carried out before and during the
vitrification of the glaze, the carbon trapped by the
glaze not being able to enter more in combustion even
if the atmosphere becomes oxidizing again, will give
it a more or less dark gray colour.
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- Blueing
:
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- This operation is used to
give a silvery glare, metallic gray to the pieces
fired by the introduction of strongly carbonaceous
combustible materials at the end of the firing. The
hydrocarbons or the carbonaceous matters breaking up
at high temperature create an amorphous carbon deposit
on the fired pieces. According to certain authors, in
presence of ferric oxide, the decomposition of the
fuel at high temperature can produce iron carbide
(Fe3C), and also sometimes graphite. These are the
compounds which contribute to blueing. The presence of
water vapor during the decomposition of the fuel and a
fast cooling without air at the end of the treatment
generally ensures good results.
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- Reduction of
clay :
-
- This process is used in
particular during the firing of porcelain to make the
clay whiter. The reduction acts mainly on ferric oxide
(Fe2O3) present in the raw materials (kaolins and
feldspars), it prevents this one from yellowing the
clay. The reduction of the ferric compounds of iron to
ferrous compounds gives a pale blue green colour which
contributes to the whiteness of the clay after
firing.
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- Fe2O3 + CO ===> 2FeO +
CO2 (FeO, ferrous iron oxide)
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- The reducing phase is
produced from 900°C to 1400°C and sometimes
beyond, the end of firing being done in oxidation.
- It is important that the
glaze and the paste are still sufficiently permeable
to allow the penetration of the reducing gas (mainly
CO), but also to allow formed CO2 of escape. The speed
of rise in temperature must be adapted
consequently.
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- In a stoneware clay, the
transformation of ferric oxide to ferrous oxide by
reduction above 1200°C involves the formation of
iron silicate or fayalite FeSiO4 (2FeO + SiO2), whose
clay fluxing eutectic is rather brutal if the initial
content in Fe2O3 is high. FeO thus acts like a
fast-acting flux for silica. If the reduction
penetrates deeply into the clay containing Fe2O3 in
great amounts, it is likely to create deformations.
- Without the reduction
effect, ferric oxide Fe2O3 does not react directly
with silica.
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- The reaction Fe2O3 + SiO2
===> 2FeO.SiO2 + 1/2 O2 takes place at high
temperature, the formation of fayalite is accompanied
by a release of oxygen which can produce blebs and
bubbles if the fluxing effect is too
brutal.
-
- Carbon
monoxide (CO) :
-
- This gas is very toxic,
colourless and odourless. Its presence in air at the
0.5% level causes asphyxia. CO is the result of
incomplete combustion of carbon.
- See
E. Bastarache's article on the toxicity of carbon
monoxide
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- It burns in oxygen or air
forming a blue flame : CO + ½ O2 ===>
CO2
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- In the past, CO was used as
a fuel named " Poor gas ".
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- It is an energetic reducer
at high temperature.
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- This gas dissociates easily
towards 400-450°C in the presence of iron
componds and produces the following type :
- 2 CO ===> CO2 + C
with the production of smoke (or carbon deposit) and
carbon dioxide CO2.
- Above 1000°C and in the
absence of oxygen, the reaction reverses, carbon burns
by reducing carbon dioxide to form carbon monoxide :
CO2 + C ===> 2 CO
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- Thermal
efficiency of combustion :
-
- The maximal thermal
efficiency of a fuel kiln corresponds to complete
combustion producing a neutral atmosphere. It means,
when fuel is burnt with the right amount of air so
that there is neither reduction nor oxidation, it is
what we call also " stoechiometric " or neutral
combustion.
- This applies to the kilns
known as " atmospheric ", those which use air as
combustive bringing the oxygen necessary for
combustion.
- Air contains approximately
21% oxygen and 79% nitrogen. Nitrogen behaves in
combustion as an inert gas, it does not undergo any
transformation. Its high proportion in the air makes
it that it occupies also a large volume in the gaseous
flow resulting from the combustion which travels
inside the kiln. Nitrogen heats up by absorbing a
great part of the heat produced by combustion and acts
like a thermal shock absorber of the thermal
effect.
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- The output can thus be
increased considerably if in place of air one uses a
mixture of air and oxygen or pure oxygen. The heat
produced by neutral combustion will be concentrated in
a smaller volume of combustion gas containing little
or no nitrogen at all, the flame will be much
hotter.
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- Example of the
combustion complete combustion of methane
:
-
- 1) In air :
CH4 + 2(O2 + 4N2) ===> CO2 + 2H2O + 8 N2
- 1 Nm³(*) methane burns
completely by producing 11 Nm³ combustion gas
containing 72,7% nitrogen.
- The temperature of the flame
can maximally reach 1940°C.
-
- 2) In pure oxygen
: CH4 + 2O2 ===> CO2 + 2H2O
- 1 Nm³ methane burns
completely by producing 3 Nm³ combustion gas
only.
- The temperature of the flame
can maximally reach 2760°C !!!
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- Combustions with pure
oxygen are used in ceramics for the firing of special
materials, such as certain refractories like
"recrystallized silicon carbide" and other special
products.
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- (*)Nm³ :
Normal cubic meter, it is a volume of 1 m³ of a
gas considered at the temperature of 0°C under
atmospheric pressure (1013 mbar).
-
- Thermal efficiency decreases
with oxidizing firing, the excess of air cools
combustion gases. Thus, a badly regulated fuel kiln
where excess air is too important cannot manage to
reach the desired temperature... An excess of air of
100% in a natural gas combustion does not make it
possible to exceed 1150-1160°C, if it is 140% the
temperature will reach a maximum of 1000°C... and
for 200% it will have difficulty to reach
800°C.
- It is the same for reducing
firing, this one affects still a little more the fall
of the output by the reaction of incomplete combustion
which releases less heat.
- For the same amount of fuel,
the heat produced in reducing combustion is lower than
in oxidizing combustion.
-
- Attention, at the time of a
change of atmosphere, going from oxidizing combustion
to reducing combustion in a fuel kiln, a change
through neutral combustion is inevitable, it must be
as short as possible because the high output of
neutral combustion risks to raise strongly the
temperature.
-
- The temperature of the air
also plays an important role in thermal efficiency.
Air heated by an exchanger using the heat recovered at
the level of the chimney of the kiln or in its walls
allows to increase the thermal efficiency of the
kiln.
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- At the beginning of the
firing, an important excess of air is always necessary
to avoid a too fast rise of the temperature. The
excess of air is the only means making it possible to
act on the behaviour of heating, either by the
addition of air or or by the reduction of
fuel.
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- In all cases it is
necessary to rightly proportion the effects of
oxidation or reduction, because they are energy
expensive.
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- Reduction
glazes with silicon carbide (SiC)
:
-
- In oxidition firing, finely
crushed (40 to 80 µ) silicon carbide (SiC), and
properly mixed in a glaze produces an important
reduction effect on metallic oxides such as copper or
iron.
- Be careful not to introduce
more than 1 to 2 % SiC, because a strong boiling could
occur if the oxidation of carbon takes place after a
too advanced softening of the glaze (case of lead
and/or boron glazes). This type of reduction between
solid phases is better appropriate for high
temperature glazes. The boilings are however always
latent with the use of this compound and it is
consequently also used to produce bubbled glazes
(volcanic glazes).
-
- SiC (solid) + 2 O2 (taken
from metallic oxides) ===> SiO2 (solid) + CO2
(gas)
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- Ex: The addition of 0.5 %
very fine SiC in a glaze containing 1 to 1.5 % of
basic copper carbonate can produce a red color or a
green color with coppered metal particles or red dots
(according to the fineness of particles and the
quality of their dispersion). The presence of tin
dioxide in the glaze favors the development of the red
copper colour (oxblood, sang de bœuf). A rather
long level at the highest temperature will allow the
glaze to heal over if CO2 outbursts produced craters
during reduction.
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- Composition
of dry air :
-
- Terrestrial gas atmosphere
is composed mainly of two elements: nitrogen and
oxygen. The first represents approximately 78 % of the
air we breathe, the remainder is composed of oxygen at
the 21% level, and of ten other gases in very small
amounts accounting for the remaining 1%.
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|
Components
|
Chemical symbol
|
% in dry air
|
Molar mass
|
|
Nitrogen
|
N2
|
78.0900
|
28.016
|
|
Oxygen
|
O2
|
20.9500
|
32.000
|
|
Argon
|
A
|
0.9300
|
39.944
|
|
Neon
|
Ne
|
18 x 10-4
|
20.183
|
|
Methane
|
CH4
|
2 x 10-4
|
16.042
|
|
Krypton
|
Kr
|
1 x 10-4
|
83.070
|
|
Helium
|
He
|
5.24 x 10-4
|
4.003
|
|
Carbon dioxide
|
CO2
|
3 x 10-2
|
44.010
|
|
Hydrogen
|
H2
|
5 x 10-5
|
2.016
|
|
Xenon
|
Xe
|
8 x 10-6
|
131.300
|
|
Radon
|
Rn
|
6 x 10-8
|
222.000
|
|
Ozone
|
O3
|
1 x 10-6
|
48.000
|
|
Carbon
monoxide
|
CO
|
variable traces
|
28.010
|
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- Translated by Edouard
Bastarache
- Tracy, Québec,
CANADA
- edouardb@colba.net
-
-
-
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- Smart2000.fr
©
Novembre 2004
-
FRANCE
- Écrit
et documenté par le propriétaire du site
// Contact : Smart2000@wanadoo.fr
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