Oxidation & Reduction Firing

GLAZE OF THE WEEK 9

IT’S AN AIRY SUBJECT: OXIDATION / REDUCTION FIRING

• Definition

• Methods of firing

• Final effects, eg best glaze for each effect

Fire and living things require oxygen all the time, at least on our planet. Let’s take some of the mystery out of what is happening when glazes are oxidized or reduced in firing.

First, any firing requires a long period of time, at least 12 hours. The kiln is brought to temperature somewhat gradually for the first two or three hours, then boosted so that the temperature is increased gradually (60 to 100 d per hour) until the final temperature recommended for the maturity of the clay and glaze. HPG fires to about cone 10, or 1305 d C , 2381 d F. In order to save fuel, and because it has been shown not to be necessary, HPG does not “candle” the kiln, that is, heat very slowly for 12 hours.

The “cone” simply means the triangular shape of a clay stick made by Orton, Inc. used to indicate firing temperature by “melting down” at a specified temperature. They are placed in
the Top, Middle and Bottom of the kiln shelving with observation ports made in the door. (Refer to Glaze of the Week – 1 for some basics) A pyrometer, or temperature gauge, is also a helpful tool. During the firing, the HPG gas kiln atmosphere must be controlled for desired results.

Oxidation means that conditions are produced in the atmosphere of the kiln so that there is plenty of oxygen during the firing. Certain clay bodies, under glazes and glazes which are formulated for oxidation tend to have strong colors from the stains made of oxides and carbonates which retain stability in an oxidation firing. Highly decorated ware depends upon an oxidation atmosphere in the kiln. Red and yellow stains in particular require oxygen and less heat, or colors do not appear.

An electric kiln has elements to continually supply heat without added fuel. There is a free flow of oxygen throughout the kiln, from the room outside and the blower, as the pieces come to maturing temperature. HPG does not use electric kilns for glaze firing.

By contrast, in gas oxidation firing, oxygen has to be supplied constantly to keep the gas burning because there is no other source of heat. Oxygen in high amounts throughout the kiln is supplied by opening all the kiln damper until cone 10 goes down. The presence of oxygen continually burns the residual materials forming gaseous carbon dioxide which goes out the chimney and mixes with car exhaust on the freeway. The result is bright colors in the decorations using underglaze or overglaze stains with transparent glazes. Oribe becomes a bright green shiny glaze. Most other HPG glazes are not formulated for oxidation and become dull and opaque. Many of the commercially prepared stains and oxides work well in oxidation even at high temperatures. Kazuyo Sato fires her charming, flowery decorated ware in oxidation.

Reduction simply means the managing of the atmosphere in the kiln to reduce the amount of oxygen as much as possible at a certain critical times. A gas-fueled kiln is always a reduction kiln to some extent because the burning fuel consumes oxygen. Just enough oxygen is supplied into the kiln to keep the gas fuel burning so that the desired temperature is reached. Fuels are natural gas, propane or wood. Reduction is impossible in a standard electric kiln.

HPG’s kiln is a “down draft” kiln which uses natural gas fed into two “Venturi” burners at the bottom sides in the rear so that the flame shoots forward to the front, heating both sides of the kiln, along a low wall of firebricks, the bagwall, which guides the flames and protects pots. To save fuel costs, the bagwall which requires extra heating is not used.

The heat rises forcefully from the bottom front of the kiln up to the top where it circulates back down again and out the back flues at the bottom. Along the way it encounters many

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obstacles: the shelving and the mixture of pots. Circulation patterns of air and heat are inevitabley different, but can be controlled to get a good firing. The air intake on the burner, “primary air”, can be opened or closed to feed air into the gas fuel. Kiln air is provided by a damper, a sliding gate in the back of the kiln which when pulled out allows air to be mixed into the kiln atmosphere. This is called “secondary air” which also comes in from other open spaces between bricks or around the burner ports. If the damper is almost closed and the air intake valve opening at the burner is decreased, any oxygen in the kiln is burned off and not replaced, creating the reduction atmosphere required at two stages during the firing. This is a difficult step to manage evenly throughout the kiln, while still maintaining the right temperatures.

Preliminary Bisque firing: Pots are slowly brought up to temperature, cone 08 (about 1700 d F. in an electric kiln which is set to low temperatures of 212 d F (boiling) for several hours to eliminate “physical water” in the clay which is about 30%. A gradient of increasing heat up to cone 09 (approx 1650 dF) is used to eliminate “molecular bound water” which is part of the clay molecules. The whole process is about 16 hours or more. It is critical to not rush this bisque firing because any water will cause pots to burst if it escapes too fast. Notonly oxygen but also organic compounds of carbon will be driven oout of the body. The proper bisque firing should strengthen the pot enough for glazing and also prevent “black coring”which is over reduction as well as remaining carbon.

The first stage of glost firng is “body reduction”. This occurs at about 930°C, 1706°F (cone 08) for about an hour. (time varies with the fire chief) The gas is burning full blast. Both damper and air valve are closed almost to the maximum, leaving just enough air to keep the flame from going out completely. The fire needs oxygen to burn, so it finds oxygen in the clay and glazes which are made of chemicals containing oxygen chemically bonded to metals like iron oxides or carbonate compounds.

To the extent that body reduction is achieved, especialy in the bisque firing, there is less oxygen in the clay body to react adversly as the glazes start to mature and fit the clay. Getting the oxygen out of the clay body before the temperature is increased for glaze maturation is important to the strength of the fired piece. Overfiring at this stage can cause black “coring” of the clay which weakens the pot by making the materials charred and brittle, therefore timing is important as well as temperature. After this stage, the dampers and air intake valves are partially reopened to allow air into the kiln which causes the temperature to continue to rise.
The kiln is in still kept in medium reduction for the rest of the firing.

The second stage is “mullite formation” This occurs at about 1800 d F. Mullite is a crystalline form of the aluminas and silicates in the clay body. They are needle-like crystals which provide the skeletal strength of the clay. It is a critical step in the firing.

The third stage is “glaze reduction”. When the temperature reaches 1204°C, 2201°F (cone 5), the kiln has to stay in reduction mode for the development of glaze maturation and optimum effects until the temperature reaches 1260°C, 2300°F (cone 8). This takes 3 to 6 hours of careful adjusting of the heat and air in the kiln. Changes in the appearance of the fire can be observed from kiln top to bottom. The flame coming out of the peep holes should show blue and greens indicating reduction. If pure red it is oxidation. Driving out more oxygen in the clay and glazes affects colors and surfaces by changing the chemical state of the interacting metallic compounds. Flames are licking each pot, seeking oxygen to burn, leaving variations in the glaze surface with interesting flame marks. Certain crystalline forms of the glaze components (alpha and beta crystallization) occur.

Glazes mature between cone 8 and cone 9 (1285°C, 2345°F). After cone 10 goes down (1305°C, 2381°F), the firing nears completion, the kiln can finally be shut off, and the fire

chiefs can go home and rest. At HPG the gas is decreased in steps (10 – 6 – 2) then off to ensure a good clean finish. Dampers are opened and air is allowed back into the kiln for just a short time..

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Not many HPG glazes do well in both oxidation or reduction: perhaps Chun and Nelson’s Transparent are the only ones, but the faint celadon green effect is lost if not reduced. Shino comes out creamy white in oxidation or when it is thickly applied. Miller White stays white.

Most HPG glazes are formulated for cones 9 or 10 reduction and appear to have more depth and clarity if reduced in a gas kiln. UH Blue and other glazes require reduction of iron to a particular molecular state to bring out the blue. The blue or green colors of glazes are more developed and brighter; the copper reds are really red.

CR-17 Red will turn light blue if oxidized because there is some cobalt in the glaze mix. Some parts of the same piece will be red or blue or both depending upon the amount of reduction in certain areas. Oxblood is a deeper red if reduced, but gray where thin or oxidized. Tenmoku, Ohata, and Kaki Persimmon are at their best, rich browns characteristic of each glaze. Shino develops the flame orange color expected. Toshiko orange is golden if reduced, fried egg yolk if oxidized. Thickness and thiness of glaze application has an affect because more or less glaze is exposed to fire.

In summary, firing in oxidation and reduction is not that hard to understand, but might be hard to achieve. We ourselves are walking, talking oxidation/reduction robots. The cells of our bodies are constantly clicking oxygen on and off in a finely tuned way to regulate metabolism with the air taken into our own dampers, the lungs, to keep us healthy.

We try hard to balance more crudely the air and heat in our kiln to achieve our lustrous pottery. It is difficult.

PJH 7/17/05 4/14/07 ed April 23, 2009 NOV 2010

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