Tennessee Tech University - Appalachian Center for Crafts - Clay Studio

Vince Pitelka, 2006

Firing the Downdraft Kiln

Always keep an accurate kiln-log when firing this kiln.  It is the only way you will get to know the kiln and its firing characteristics.  On the kiln log note the outside temperature and humidity, and any other unusual conditions such as high winds. 

General Information

Gas Pressure at the Power Burners
This kiln is equipped with oversized power burners which are capable of firing a kiln twice this size.  The primary air for a power burner is supplied by an electric blower, and the power output is dependent on the amount of gas that can be fed into the airstream.  With sufficient blower and gas supply, even a small power burner can be very powerful.  On all of our indoor gas kilns, the mainline gas pressure is regulated down to 9 WCI (water column inches), but that is far more pressure than is needed to fire this kiln.  The gas pressure need never be turned up higher than 3 or 4 WCI, and as we know from an incident some years ago, firing at higher pressure can be very destructive to the wares, the shelves and the kiln hotface.  After an appropriate preheat, you can do a 10-hour cone 10 firing without turning the gas above 3 WCI, and it will give better glaze results throughout the kiln than could be achieved with a faster firing.  The time and fuel saved in a shorter firing are poor compensation for inferior surfaces on the wares.  It takes time for glaze surfaces to develop in the firing, and you must allow for this in your firing schedule, especially approaching maturation temperature. 

Power burners of the type found on this kiln are equipped with inexpensive rheostat-type blower speed-controls which are often inaccurate and unreliable, and all adjustments of the primary air should be accomplished with the pivoting primary-air-shutter on the blower intake, using the large spring clamps to hold the shutter in place after each adjustment.  Make sure that the spring clamp is placed so that it will not slip off.  When first starting the kiln, turn on the main power switch in back of the chimney and then turn both blowers to high, which is right next to off.  Leave them on high for the duration of the firing, and do all primary air adjustments with the air shutters. 

Please note that power burners are never perfectly matched, especially in terms of blower performance and WCI gauge readings. Your objective is to get the same quality of flame from both burners, rather than to duplicate the same mechanical settings.  Each time you adjust a burner, observe the appearance and sound of the flame, and try to duplicate that on the second burner. 

When working with low pressure gas burners, excessive noise generally indicates a cool, inefficient flame, where an unnecessary amount of air is being introduced.  A healthy gas flame should be fairly quiet.   If the burners sound especially noisy, the shutters should be closed a bit. 

Before and after body reduction you will adjust the burners to change the kiln atmosphere.  In such cases, after adjusting both burners as described above, adjust the damper opening to get the atmosphere and back-pressure (desired flame at the spy-holes) that you want.  If the damper adjustment alone does not give you the atmosphere and back-pressure you seek, you may also have to readjust the primary-air-shutters.

NOTE: in order for this kiln to fire evenly, the flue-opening in the bottom back of the kiln interior must draw primarily from beneath the bottom shelf.  The bottom layer of shelves normally always stays in place from firing-to-firing (unless they need cleaning and re-washing), and should be spaced one brick (4 1/2") off the kiln floor.  Always leave the softbrick pieces in place beneath and behind the rear bottom shelf to direct most of the flue draw under the bottom shelf. 

These Damn Idiosyncratic Burner Valves
The valves on these burners are a bit problematic.  You adjust them by turning them in a conventional fashion (righty-tighty, lefty-loosey), but there is some slop in the valve shaft when you pull or push on the valve handle, and the burner pressure will change significantly when you do so.  The safest practice is to make all your adjustments while pulling on the handle.  That way, if vibration causes the handle to shift at all, it will result in lower pressure at the burner rather than higher.  

The Chimney and Damper
This kiln has a very tall chimney, producing powerful flue-draft, so it is rarely necessary to open the damper more than 1” overall (1/2" on each side) at any point during the firing.  The damper on this kiln features horizontal sliding slabs on both sides of the chimney.  Whenever adjusting the damper, move both slabs an equal amount.  Note the vertical black line on the rear edge of each damper slab.   When this line is flush with the surface of the chimney on both sides, the damper is completely closed.  To open the damper 1”, pull out each slab 1/2”.  The damper slab on the window side of the chimney has had a crack down the center of it for the twelve years I have been here, but that is not a problem as long as you grasp it with both hands when adjusting it.

In certain weather conditions, there may be a strong reverse draft of cool air coming down the chimney.  As long as you have the blowers set on high with the primary air shutters open, the heat from the pilots or burners will fill the kiln and enter the flue, and things will start to flow properly.  On very rare occasions it has been necessary to remove several bricks from the back side of the base of the chimney and build a small kindling fire inside the chimney, but with patience, the burners will usually take care of this on their own.  

Controlling Atmosphere Before Body Reduction
It is critically important to maintain a clean oxidizing or neutral atmosphere up until body-reduction temperature.  Any reduction taking place earlier in the firing can trap volatile carbon and sulfur within the claybody, causing discoloration (carbon coring) and possibly serious bloating and blistering of the claybody later in the firing.  In the very early stages of the firing, before any red heat is present, the only indication of excessive reduction is the unpleasantly sweet smell of hot, unburnt hydrocarbons.  Remove the top spyhole plug, and wave a gloved hand in front of the spyhole towards your face, and sniff the gases.  If the kiln is oxidizing, you will smell nothing, because no gases will be exiting the spy holes.  If the atmosphere is neutral, it will smell like a clean-burning gas stove.  An excessively oxidizing atmosphere is inefficient, because too much cold air is being drawn into the kiln.  If you smell nothing at the spyholes, close the damper a little at a time until you detect the smell of a healthy gas flame, like a gas stove.  If you smell reduction, check the damper and increase the opening if necessary, and check to make sure that the primary-air-shutters are properly adjusted. 

Once you are well into red heat, you can simply check for a flame at the top spyhole.  If there is no flame, there is no reduction, but you do want to make sure there is some back pressure, indicated by the smell of a clean-burning gas flame at the top spyhole.  As long as there is a little back pressure, you have the kiln adjusted for most efficient heatwork.  At any point before body reduction, as long as the primary air shutters are open and you maintain correct gas settings and a damper opening of at least 3/4”, you should not have any trace of reduction atmosphere. 

Reduction Glaze-Firing in the Downdraft
The downdraft kiln is specifically designed for reduction high-firing, and it is the best kiln we have for this purpose.  It is easy to high-fire in this kiln, and it is far easier to achieve even reduction than with an updrafts kiln.  Reduction depends on back-pressure within the kiln, and in updraft kilns the heat naturally tends to rise through the flue-opening in the top of the kiln, making it difficult to achieve adequate back-pressure and even reduction.  In a downdraft kiln the flue opening is located at the bottom, and since heat rises, there will naturally be back-pressure unless you open the damper wide enough to create negative pressure within the kiln.  An updraft kiln oxidizes easily, and it takes specific and precise adjustments of the damper to make it reduce efficiently.  A downdraft kiln reduces easily, and it takes specific and precise adjustments of the damper and burners to make it oxidize efficiently. 

Cone Packs for a Cone 10 Firing
Each cone pack for a normal cone 10 reduction firing will include a cone 010 or cone 08 body-reduction cone with melt-basin, a cone 9 warning cone, a cone 10 firing cone, and a cone 11 guard cone.  Be sure to consult the section of the text on making proper cone packs - use as little clay as possible, and perforate all thick areas thoroughly with needle-tool holes.  Place your conepacks carefully on very stable arrangements of brick or kiln furniture, and make sure that you can see the entire conepack clearly through the spyhole with the door closed.  You will need a flashlight to properly check conepack position. 

Once your cone packs are in place, close and latch the door, making sure to place the small steel shims behind the latch cams.  

Firing Schedule for Cone 10 Reduction High-Fire

Overnight Preheat
It is always best to do an overnight preheat, because it will insure that you have a short firing the following day.  It is also much easier on the wares, the shelves and furniture, and the kiln itself.  It usually works best to do the overnight preheat with both pilots going but just one burner lit, and the burner closest to the Norman updraft kilns is the most reliable at low settings.  With the door latched shut, open the damper 3/4” (3/8” on each side).  Turn on the main power switch on the vertical electrical conduit behind the chimney.  This also opens the electromagnetic valves on the gas line.  Make sure the burner valves are closed (handle screwed in clock-wise until snug.  Do not over-tighten).  If the pilot valves are closed, turn the handle on each one three full rotations counter-clockwise. 

On the burner closest to the updraft kiln, turn the blower rheostat to “high” (one click from “off”).  Press down the red button on the Baso valve and light the pilot.  Hold the button down one minute and release.  If the pilot goes out repeat this step.  Do the same for the other burner, but do not turn on the blower rheostat. 

Leave the primary air shutter open.  Open the main burner valve slowly until the flame ignites, and then open it until gauge reads 1.5 WCI. 

The Next Morning After a Preheat
Leave the damper at 3/4” total opening.  Make sure the air shutter is wide open on the second burner.  Turn the blower rheostat to high (one click from “off”), slowly open the gas until the burner lights, and turn the valve up until the gauge reads 1.5 WCI.  Re-check the first burner and make sure it is still set at 1/5 WCI with the blower rheostat on high and the air shutter open. 

You do not want any reduction before body-reduction temperature, and it is a good idea to check the spyholes for reduction at this point.  There will be no visible flame at this temperature, but you can check for reduction by smell of the fumes coming out of the top spyhole.  Remove the top spyhole plug, and standing off to the side, with a gloved hand wave the fumes towards your face.  You should sense only the familiar smell of clean-burning gas.  If you smell the sweet-acrid odor of reduction, open the damper by 1/8” increments until the reduction smell is gone. 

Without an Overnight Preheat
As mentioned, a preheat is nice, because it gives you a good head-start on the firing the next morning, and it gently preheats the wares, furniture, and the kiln itself.  However, it is not absolutely necessary, and when firing on a tighter schedule you can do without it.  But if you fire without any preheat, be sure to make proper cone packs, thoroughly pierced with a needle tool.  Also, if the set includes a lot of freshly glazed ware, you should do a lower preheat (as described above) for at least a few hours before turning the kiln up to this setting.

To fire without a preheat, light both pilots and turn both blower rheostats to high, with the air shutters fully open.  This will circulate warmth throughout the kiln and get the stack convection currents going.    After two hours, light both burners and set at 1.5 WCI as described above. 

After One Hour at the Above Setting, With or Without a Preheat
Turn the burner valves up to 2 WCI on the gauge, and readjust the primary-air-shutters to get a blue flame with a slight flickering of yellow inside the burner port.  Readjust the damper to achieve slight backpressure with no reduction.  With an overnight preheat it should take another thirty minutes to an hour to reach body reduction.  Without an overnight preheat it will take considerably longer, perhaps three or four hours.

When Cone 08 Bends on Top, if You are Doing a Body Reduction
Turn the gas up to 3 WCI on both burners and close the air shutters 1/2 to 2/3 of the way.  Adjust the damper in by 1/8” increments until you get a 3” to 6” flame at the bottom spyhole. 

When cone 04 is down on the bottom, open the primary-air-shutters all the way.  Open damper 1/8” to 1/4” to get a 1” to 3” flame at the bottom spyhole.  This should leave the kiln in a gentle partial reduction, and it can stay at that setting for the duration of the firing. 

When Cone 08 Bends on Top, if You are Not Doing a Body Reduction
Adjust the gas up to 3 WCI on both burners, and adjust the damper to get a 1” to 3” flame at the bottom spyhole.  This should leave the kiln in a gentle partial reduction, and it can stay at that setting for the duration of the firing. 

When Cone 9 Starts to Bend
If cone 9 bends evenly top and bottom, leave things alone.  If it starts to bend on the bottom first, close the damper slightly to give more back pressure, which should push more heat to the top.  If it starts to bend on the top first, open the damper slightly to pull more heat to the bottom. 

Glaze Maturation
The period of time from when cone 9 starts to bend until cone 10 is down is very important in the maturation of the glaze, and should not proceed too quickly.  Under no circumstances should you go through this period in less than one full hour, and preferably 90 minutes to two hours.  You do not want to hold the kiln at cone 10 for very long, because you risk flowing glazes and slumping wares.  The solution is to slow down the firing throughout the falling of cones 9 and 10. If the firing seems to be progressing too quickly at this point, cut back the gas pressure slightly, but always readjust the damper to ensure partial reduction with a 1” to 3” flame at the bottom spyhole. 

Whether to Do a Glaze-Reduction
After a climbing reduction, a glaze reduction is generally, wastes fuel, and can over-reduce the glazes.  Essentially, it is redundant, and a glaze reduction makes the glazes bubble actively right at the end of the firing, which can cause other problems.  If you do decide to do a glaze reduction for some specific reason, when cone 10 is falling, close the primary-air-shutters 1/2 to 2/3 of the way, and close the damper by 1/8” increments until you get a 3” to 6” flame at the bottom spyhole.  The glaze reduction does not need to be smoky.  After 30 Minutes of Glaze-Reduction, if cone 10 is not down, restore a partial-reduction atmosphere as described above, and maintain until cone 10 is down.  When cone 10 is down, initiate a period of oxidation-cleanup as described below.

If You Are Not Doing a Glaze Reduction
Keep the kiln in partial reduction.  When cone 10 is down top and bottom do a soaking period of oxidation cleanup as described below. 

Oxidation Soak/Cleanup and Shutdown
It is always a good idea to initiate an oxidation soak at the conclusion of a reduction firing, except in particular cases with specialized glazes such as oil-spot temmokus.  Otherwise, an oxidation soak will usually clean up the glaze surfaces by healing any flaws that form during the inevitable outgassing that occurs with reduction firing.  The results are smoother, shinier, brighter glazes.  The oxidation soak is often referred to as an oxidation cleanup. 

To initiate oxidation soak, leave the damper in the position it was in for a light partial reduction and reduce the gas pressure to 1 WCI.  Even fifteen minutes of oxidation cleanup will help, but for the shiniest, brightest glazes, you can do up to one hour of oxidation soak.  This soaking period will have no adverse affect on reduced glazes wherever the glaze is thick, but will reoxidize the clay and any thin-glazed areas, giving you more color breaking in the glazes, and a warmer tone to the clay in the unglazed areas.  Once you are done with oxidation soak, close the damper all the way, close the burner valves, turn off the blower rheostats, and close the pilot valves.  In hot weather, leave the exhaust fan on.  In cold weather, shut it off.  

NOTE: The time it takes to go from body reduction to glaze maturity depends on how tightly the kiln is stacked, the arrangement of shelves and wares, the outside temperature and humidity, and your karma.  I have seen this kiln go from body reduction to glaze reduction in three hours at 2 1/2 WCI of gas pressure, and I have seen it take seven hours at 3 WCI pressure.  The latter was a very full firing, and turned out to be a very good firing.  Give it time.  If you care about your work you will be patient and allow the kiln to do its work.  Firing very fast produces lifeless glazes. 

Cooling the Kiln
The surface crystals that give desired results in many glazes form during the cooling ramp from 1900 down to 1700 degrees Fahrenheit, and it helps to slow down the cooling during this “zone of crystallization.”  It is not desirable to slow down cooling from cone 10 down to 1900 degrees, but at such high temperatures the heat tends to dissipate very quickly, so you can safely close the damper right away after finishing your oxidation soak and shutting down the kiln.  It will cool quickly through the first few hundreds of degrees, and then will cool more and more slowly as cooling proceeds.  For best glaze results, keep the damper closed for at least the first three or four hours of cooling, and then you can open it several inches. 

If you wish to maximize crystal growth during the desirable zone of crystallization, use a pyrometer to monitor cooling, reignite the burners on 1 WCI and a neutral atmosphere (back pressure at top spyhole, none at bottom) at 1900 degrees, bring the kiln down to 1700 degrees over a period of four hours, shut off the burners, open the damper two inches, and let the kiln cool on its own. 

Opening the Kiln, Unloading, and Cleanup
Once all red heat is gone, you can crack the door an inch or so, but do not open it any wider without checking the temperature.  To do so, make a newspaper twist, and insert it in the top spyhole.  If it flames or chars (even slightly) then the kiln is still too hot, and should be left with the damper open several inches and the door slightly ajar for another few hours.  Once the newspaper twist does not char, you can open the door, but if there are large pieces in the firing you will still want to open it in stages over an hour or two.