Pottery Kilns

Use of oxygen probes in pottery kilns

Oxygen probes take the guess-work out of firing.

As a potter, you need The right temperature The right oxidation/reduction The least possible fuel consumption.

An oxygen probe lets you check The temperature in the kiln The oxidation/reduction The fuel in the flame

How does the air supply affect the firing?

The right amount of air will give the hottest flame and the fastest temperature rise. This is the most economical heating.

Too much air makes the flame cooler. The flow of excess air can help distribute the heat more uniformly.

Not enough air can still give a hot flame and can give interesting special effects in glaze colours.

An oxygen probe shows how much fuel and air is in the kiln

How do we measure oxygen?

Typical oxygen probe The sensor is an alumina tube with a zirconia tip and a platinum wire attached to the zirconia tip. The cable connects the probe head to a meter, to indicate the oxygen reading. An air connection supplies fresh air to the the inside of the alumina tube. Most probes have a pyrometer built in, and the meter can usually also measure temperature.

The benefits of an oxygen probe

Efficient fuel use
To reach the highest temperature with the least fuel, use a neutral flame. The oxygen probe will tell you if you have a good balance of fuel and air, because the reading will be in the range of 0.1 to 0.2.

There is an even better way to find that perfect balance. In an efficient neutral flame, the reading jumps around erratically, especially in the range of about 0.1 to 0.2.

Glaze control
Some glaze colours are affected by how heavily reducing the flame is. Sometimes there is a difference in colour between a slightly reducing flame with an oxygen reading of say 0.35 and a heavy reduction of say 0.6. In a reducing flame, the oxygen reading is usually much more stable and it is easy to measure the degree of reduction quite accurately. The potter can record the reduction conditions, then obtain the same glaze colours on later burns.

How to install an oxygen probe

Fit the probe anywhere in the kiln or furnace where a pyrometer could be fitted. If the probe is used at temperatures over 1100 deg C / 2000 deg F, put it through the top, so the ceramic tube hangs vertically. If the probe is used horizontally at high temperatures, it will gradually sag.

Seal the hole for the probe well enough to prevent air from flowing inwards and affecting the oxygen reading.

When fitting or removing a probe into or out of a hot furnace, move the probe slowly to avoid thermal shock of the ceramic components.

When the probe is in use, make sure there is a supply of fresh air for the reference air inside the alumina tube. Some probes have a hand-operated puffer, that must be squeezed when a measurement is taken. A small fish aquarium air pump is better, because it provides a continual flow cheaply and reliably.

Once the kiln is over 700 deg C / 1290 F and the reference air is available, the probe is ready to use.

Understanding an oxygen probe

An oxygen probe works by comparing the oxygen level in the kiln with the oxygen level inside the alumina tube. This is why the air inside the tube must always be renewed. If the temperature of the the zirconia tip is over about 700 deg C / 1290 F, it produces an electrical voltage. The less oxygen in the kiln, the bigger the electrical voltage, so the voltage can be used as a guide to the oxygen level. There is no safety hazard - the maximum a probe can produce is less than a battery!

Roughly speaking, a reading less than 0.1 represents oxidising conditions and a reading over about 0.3 represents reduction. Heavy reduction might give a reading of 0.5 or even more. In between 0.1 and 0.3 the temperature must be known for accurate interpretation.

In any flame, the air and fuel never mix perfectly. As burning fuel blows past the tip of the oxygen probe, some of the flame will have excess air and some will be gas rich. This means that the oxygen reading will jump around as the flame flickers past. This is most noticeable when there is just the right amount of air to give a neutral flame.

Air consists of 20.9% oxygen, about 78% nitrogen and some trace gases. In a flame, the fuel combines with the oxygen in the air and burns, forming carbon dioxide and water vapour (steam). Inside a kiln, there is a mixture of fuel, oxygen, carbon dioxide, steam and nitrogen. The amount of each of these depends on the amount of fuel and air in the flame. Oxidising flame With excess air, there is typically over 2% oxygen in the exhaust gas, but it can be almost up to the limit of 20.9%. This is called an "oxidising" flame. Neutral flame With exactly the right amount of air for the fuel, there is a "neutral" flame. Even in ideal conditions, there will be some fuel and some air that cannot find each other to burn completely. A little unused fuel and air will be in the exhaust gas leaving the kiln. There is typically anything from 0.02% to 2% unused oxygen in the exhaust. Reducing flame With too little air, there will be unburnt fuel in the exhaust gas. This is called a "reducing" flame. Many people say that there is no oxygen under these conditions, but there will always be some unused oxygen in the exhaust. It might be less than 0.02%. It might be less than 0.000001%, but it is there and it can be measured. There is no sharp distinction between oxidising, neutral and reducing. There is a smooth variation from one to the next, so the above figures are only guidelines, not hard and fast rules.

Ordering & Information

In Australia

A.I.C. 62 Glenlea Drive, Maroochydore, Queensland 4558 Australia Fax (61) 7 5443 7235

Clayworks 6 Johnston Court, Dandenong, Victoria Phone (03) 9791 6749 Fax (03) 9792 4476

In USA and Europe

Axner P.O. Box 621484 Oviedo Florida FL 32765 USA Fax (1) 407 365 5573

Scandiaovnen A/S Rypevang 6 DK-3450 ALLEROED DANMARK Phone +45 - 4817 4004 Fax +45 - 4817 4005

Rest of World

Australian Oxytrol Systems 85 Wood Street, Eaglehawk Victoria 3556 Australia Phone (61) 3 5446 1530 Fax (61) 3 5446 1215

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