Tip #71:  All About Kiln Vents

We often get questions about Kiln Vents.  The most common being, "Do I need one?"  Any kiln that is located in a room where people are, should definitely be vented to the outside for safety reasons.  It is also a good idea in rooms that are attached to living spaces, such as an attached garage.  But beyond that, a vent makes it much easier to fire the kiln and provides better results.

1. Safety and Comfort:  

Removing fumes from your work environment is important, since these are unhealthy to breathe on a regular basis.  All products fired in electric kilns contain organic materials that make carbon monoxide and other fumes when they are burned. These include clays, glazes, decorating products, decals and the like. Fumes released may contain sulfur oxides, hydrogen fluoride and metal vapors, depending on the products fired.  Carbon monoxide can cause headaches and nausea.  Other fumes can also potentially cause health problems.  For example, glazes may contain chemicals such as lead and manganese which are not safe, at least in large quantities or on a regular basis.      

Sometimes you may actually notice a smell when you fire your kiln.  Usually this comes from organics burning out of the clay.  And if you use newspaper or wood or other structural supports which burn out in the kiln, the smells will be very pronounced.  

2. Ease of Firing:

If you don't use a vent, you have to prop the lid of the kiln at the early stages of bisque firing to allow the moisture to escape.  Since most kilns now are sold with electronic controllers, having to manually do this step kind of defeats the purpose of having an "automatic" kiln.  It is much easier to start the vent when you start the kiln, and not have to remember to close the lid after a few hours.

3. Improving your Fired Product:

Glazes work best when the organics have been burned out of your clay at the bisque stage.  The best way to ensure that is to do a slow firing with a lot of air flow.  The vent helps remove the organics as they are burned out.  You will usually find you have fewer glaze defects when you have fired your bisque slowly, with a vent.

Glaze colors can also be affected by these fumes.  Carbon monoxide and other fumes can affect the color and properties of the fired product if they are not removed from the kiln earlier in the firing. Red, green, gold and similar products are particularly affected by carbon monoxide at temperatures where the color develops.  

Sometimes colors that are fired close to each other will affect each other (cross contamination).  This is significantly reduced with a vent.

Many bright glazes, like reds, need a lot of oxygen to develop their color.  One customer used a lot of reds and was often having bad results where the glazes would came out pink.  He added a vent to his kiln and his reds became very vibrant and predictable.

4. Extending the Life of Your Kiln

The same fumes that are harmful to you also cause corrosion on your elements, kiln sitter parts and thermocouples.  Pulling those fumes out of the kiln reduces the wear and tear to those parts.     

Orton Vents are 20% off through February 06

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Q&A

Q.  If I live in a residential area and I vent outdoors, am I harming or bothering my neighbors? 
A.  Once mixed with outside air, the fumes are very small concentration, probably not enough to be detected and certainly not enough to be a health hazard.  Remember that by the time the exhaust gets outside, it is already mixed with room air by the vent.

Q.  What will happen if my exhaust ducting leaks?
A.  All major vent manufacturers now have the fan motor mounted away from the kiln.  This allows the fumes to be pulled from the kiln, rather than pushed.  These are called "negative pressure" systems.  The main safety advantage of this is that should you have a leak in the duct, the fumes do not leak out into the room.  Instead, additional room air leaks into the duct further diluting the fumes before it is exhausted outside.

Q.  How do I install the vent exhaust?
You drill a few holes, usually at the top and bottom of your kiln.  The vent exhaust connection usually goes under the kiln in the space where the stand is, although it is possible to mount it on the side.  Ducting runs from the kiln to the fan-motor, then from the fan-motor to the outside.  The ducting can be aluminum or galvanized steel dryer ducting, or PVC pipe.  Over time metal ducting will corrode from the fumes, but it is inexpensive to replace.  For a permanent installation, you can cut through the outer wall and install an exhaust duct, like those used on clothes dryers.  However significant improvements in air quality can be obtained by simply running the exhaust ducting out or under a door.  My kiln is on a wheeled stand, so I hang my ducting on the wall when I am not firing.  When I am firing, I connect the ducting with clamps, and run it out my garage side door.  There is a very noticeable difference in the comfort of my studio (reduced smell and heat) and the glaze results when I use my vent vs. when I don't.

Q. Is it hard to drill the holes?
A. No.  Most manufacturers will drill the holes for you if you purchase the vent with your kiln.  However, it is also very easy to do yourself.  Kiln brick is so soft that you don't even need a drill.  You just twist the drill bit between your fingers. 

Q. Won't I waste a lot of energy by pulling the heat away?
A. Actually the amount of air that flows through is not very high, and very little heat is lost.  Kiln manufacturers consider this heat loss negligible.

Q. Do I have to order a vent at the time I order my kiln?
A. No.  A vent can be added to any kiln at any time.  

Q. How does a Downdraft Vent work?
A. The vents we have been describing are downdraft vents.  With downdraft venting, gases are pulled out of the kiln near the bottom of the kiln and then mixed with room air and exhausted to the outside through a duct.  All the vents we sell are Downdraft Vents: Orton, Skutt, L&L and Cress (which actually is a side draft not a down draft.)

Q. What about those vents I see that hang above a kiln?
A. Those vents (called hoods) remove some, but not all the fumes from the kiln.  They are typically used with gas kilns where downdraft isn't a good option. 

Q. Do I need a certain brand of vent?
A. Other than the Cress Clean Air System which requires that the kiln be a "vent ready" Cress kiln, any brand vent can be used on any brand kiln.