Techno File: Kiln Venting

While fuel-fired kilns are necessarily self venting, electric kilns for ceramic art always require installing some sort of ventilation. With a proper vent system both the work inside the kiln and the workers around the kiln benefit.

Why Vent a Kiln?

Kilns are vented primarily to protect the health of people around the kiln. An additional benefit is that a vented kiln produces cleaner firings, ultimately contributing to brighter glaze colors and fewer glaze faults. A vent system can also be used to cool the kiln more rapidly, but in a controlled manner.

All kiln firings produce fumes by burning off organic material from either the clay body or glazes. The fumes produced in any firing are water, carbon, sulfur compounds, and potentially heavy metals if the firing temperature is hot enough and volatile metals are present. Carbon is mostly carbon dioxide, but also a little carbon monoxide. Other volatile carbon compounds can come off early in the firing. Most carbon compounds come from the organic materials in the clay body, but some carbon dioxide comes from decomposition of carbonates (whiting and dolomite) late in the firing. However, any gum or other organic material added to a glaze also contributes organics to the kiln atmosphere, as do oils used if one is firing overglazes. Sulfur compounds burn off from pyrite, a minor contaminant in clays. Without proper kiln venting, these vapors will leak into the kiln room and become a health hazard.

Sulfur fumes are particularly irritating, producing a burning sensation in the eyes, nose, and lungs. Organic burnout produces compounds that can be harmful for a variety of reasons. The most immediate hazard is the potential for carbon monoxide to be present in the kiln fumes.

Defining the Terms

Black Coring: A fired clay body fault caused by lack of oxygen in a kiln atmosphere below 1650°F (900°C) in the first firing, which reduces iron in the body and prevents carbon from burning out. A broken cross section of the ware will be black in the center.

Centrifugal Fan: A fan arranged like
a squirrel cage which forces air out of
the fan housing at a right angle to the fan inlet.

Kiln Fumes: First air and water vapor, later also volatile organics, at peak temperature oxides of carbon and sulfur and volatile heavy metals.

Oxidizing Atmosphere: The presence of more oxygen in the kiln than the minimum required to burn off organics and sulfides present in the clay body and the glazes.

Propeller Fan: A fan arranged like a boat propeller, which forces air out of the fan housing parallel to the fan inlet.

Reduction Atmosphere: A kiln atmosphere that does not contain enough oxygen to react with combustibles like carbon and sulfur so it obtains that oxygen by breaking down oxides in the clay body and glazes.

Vent System: A chimney or fan-forced mechanical system that draws air out of an enclosed space such as a kiln room or directly from the kiln itself.

How Does Venting, or Not, Affect the Fired Work?

When we say organics and sulfur burn, that means they react with oxygen in the air in the kiln to produce carbon, hydrogen, and sulfur oxide compounds. Those chemical reactions reduce the amount of oxygen in the kiln atmosphere. An unvented kiln can thus go into reduction while these reactions take place. In this situation, the organics and sulfur do not have enough oxygen to burn away. These unreacted materials thus can remain to cause clay and glaze faults (like black coring and pinholing) and murky appearing glaze surfaces. By venting, the oxygen-deficient kiln vapors are removed and replaced by oxygen-rich air from the kiln room.

Direct kiln vents ensure there is an abundance of oxygen inside the kiln, producing clean burnout of oxygen and sulfur compounds which, if not removed completely, can contribute to glaze faults later. In addition, a strong oxidizing atmosphere in the kiln produces brighter glaze colors, particularly with shades of iron red, yellow, and brown. This is why it is important to vent a kiln even if it is located outside. A properly vented kiln assures a more reliably oxidizing atmosphere within the kiln, which is helpful to the finished appearance of the work.

How Kiln Vents Work

There are several excellent kiln venting systems on the market. They can be separated into two simple categories. One is hood-type venting and the other is direct venting.

A vent hood is installed above the kiln and operates in the same way an exhaust hood works above an ordinary kitchen stove. An exhaust fan inside the duct leading from the hood to the outdoors pulls air in from around the kiln beneath the hood.

A hood exhaust collects fumes that escape the kiln. It also removes a significant volume of room air. The efficiency of such a system depends on the volume of air drawn through the hood relative to the volume of the room, and also on whether there are other drafts in the room that can disrupt air flow into the hood. A proper vent hood relies on an adequate amount of makeup air being allowed to flow into the room, either under doors, through open windows or doors, or through inlet vent openings in a room wall or roof.

Vent hoods are most useful at protecting people in the kiln room from harmful gases coming off the kiln during a firing. They are not necessarily completely effective at removing all kiln fumes from a kiln room. Because of their design, they do not pull fumes out of the kiln itself.

A direct vent uses a fan or blower to pull air both from the room and directly from the kiln. This is done by connecting an exhaust fan to the kiln using a duct and a fitting that mounts over one or more very small holes in the kiln wall or, more often, the kiln floor. To pull some air from the kiln requires that the same amount of air has to be flowing into the kiln somewhere else to replace the air flowing out.

A common arrangement involves mounting the exhaust fan on a wall or stand near the kiln. A flexible hose connects the suction side of the fan to a housing over a small vent hole in the center of the bottom of the electric kiln. The exhaust duct is routed outside the kiln room, either through the roof or through a wall away from any windows, doors, or air intake openings.

The housing either has several holes in it to admit room air, or it is mounted with a small gap, typically ¼ to ³⁄8 of an inch, between the housing and the kiln bottom. When the fan is running, it draws a small volume of air from the kiln and a larger volume from the room at the same time. The room air mixes with the kiln vapors to cool them enough so the exhaust duct is not hot.

A squirrel-cage wheel on the fan, also called a centrifugal fan, is most common (rather than a propeller fan, known as an axial fan). That’s because the squirrel-cage fan will move a similar amount of air despite moderate restrictions on the inlet or outlet of the fan. Air flow from a propeller fan, on the other hand, will fall if there are restrictions on the fan inlet or exhaust.

Properly Using the Vent

A properly installed vent, one that draws only a small volume of room air through the kiln, can be left on until the kiln is fully cooled down. The vent may slow an 8 to 10 hour firing by 15 minutes, but it can speed the cooling cycle by as much as a couple hours. However, if slow cooling is desired to permit crystal growth in a glaze, or firing down is done during the cooling cycle, then it makes sense to shut off the vent until faster cooling is desired.

The practice of propping open the lid of an electric kiln began because the first electric kilns had no vents. It’s still not a bad way to get rid of moisture while candling a first firing. However, once the kiln temperature rises above the boiling point of water, the kiln should be closed up so the vent will work properly.

Kiln Vent Care

While current kiln-vent systems are generally well made, they do require attention. Kiln fumes are strongly corrosive and can cause flexible duct materials to corrode. Use of stainless steel duct can avoid this problem. Otherwise the discharge side of the vent duct must be inspected regularly to be certain no leaks have developed. If they have, the exhaust duct must be replaced. Vent fans mounted outdoors or that discharge directly outdoors are least troublesome.

Kiln elements, electrical connections between the kiln wiring and the elements, and the kiln sitter will all last longer if the kiln is vented. Venting particularly extends the life of the electrical connection at the kiln because kiln fumes won’t leak out around the incoming element wires.

To Vent Up or Down?

An indirect kiln vent captures kiln vapors that leak from the kiln and draws them out of the kiln area. The collection efficiency of such a vent depends on the air velocity at the face of the vent hood. Higher velocities capture more kiln vapors. Both illustrations courtesy of Skutt,

A direct kiln vent that draws air out of the kiln itself is shown. Holes in the kiln lid let air in to replace than being withdrawn. Room air is also drawn into the vent duct. That air enters through holes in the cup under the kiln, and mixes with the kiln vapors, keeping the vent duct cool.

the author Dave Finkelnburg is a studio potter, writer, and a practicing professional engineer with a Master’s degree in Ceramic Engineering from Alfred University.

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