autres articles liés :
Carbon Monoxide Toxicology-----------------------Version française
REDUCTION and Ceramic Firing

by Smart2000

Foreword :
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...).
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.
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.
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.
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.
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.
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).
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).
How to produce reduction ?
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).
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...).
Example with propane :
In complete oxidizing combustion :
C3H8 + 5(O2+4N2) ===> 3 CO2 + 4 H2O + 20 N2
Propane + air ===> carbon dioxide + water + nitrogen
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
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.
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.
The range of reducing firing relates especially to the high temperatures (from 1100°C to 1350°C) for stoneware and porcelain.
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!!
Effects of reduction on colours :
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...
Copper and iron oxides are very sensitive to reduction.
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.
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.
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.
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.
Reduction at low temperature :
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.
Smoking :
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.
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.
Blueing :
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.
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.
Fe2O3 + CO ===> 2FeO + CO2 (FeO, ferrous iron oxide)
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.
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.
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
It burns in oxygen or air forming a blue flame : CO + ½ O2 ===> CO2
In the past, CO was used as a fuel named " Poor gas ".
It is an energetic reducer at high temperature.
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
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.
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.
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 !!!
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.
(*)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.
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.
In all cases it is necessary to rightly proportion the effects of oxidation or reduction, because they are energy expensive.
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)
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.
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%.


Chemical symbol

% in dry air

Molar mass















18 x 10-4




2 x 10-4




1 x 10-4




5.24 x 10-4


Carbon dioxide


3 x 10-2




5 x 10-5




8 x 10-6




6 x 10-8




1 x 10-6


Carbon monoxide


variable traces


Translated by Edouard Bastarache
Tracy, Québec, CANADA © Novembre 2004 - FRANCE
Écrit et documenté par le propriétaire du site // Contact :
Document pour CONSULTATION PRIVÉE uniquement
Toute reproduction totale ou partielle est interdite le site dédié aux passionnés de céramique
Smart2000 - FRANCE sur
This entire page Copyright © 2004-2023, All Rights Reserved.
Les textes et les photos restent la propriété de leur auteurs, ils ne peuvent être réutilisés sans un accord préalable. Nous consulter.
Summary --------------------------------- Version française