As most of you know, there are MANY different ways to fire kilns. Different shutoff cones, different speeds, different ways to load for different effects. Often the type of firing you can do is determined by the kind of kiln you have. (Barrel kilns are for smoke firing, Raku kilns are NOT for bisque firing, etc). There is a major category of firing that is, typically, limited to fuel-burning kilns. This type of glaze firing can be confusing to people who are just getting started, or who are not taking a college ceramics class. This month’s Tech Tip is about navigating the murky atmospheres that give us those fantastic copper reds, toasty clay bodies, and sublime celedons – The Reduction Firing.
Let me start off by saying that reduction firing IS possible in an electric kiln, but I do NOT recommend it for a couple of reasons. 1) the reduction atmosphere will literally eat-up the protective oxide coating that is on your elements, which will cause them to burn out VERY quickly. 2) K-23 brick is not really good at surviving reduction atmospheres, so your kiln may start to fall apart rather quickly 3) Reduction firing produces waste gasses that need to be vented out of the kiln and kiln room. These are called combustion fumes, and must be vented out of the kiln room via a specific kind of ventilation hood (downdraft vents are not approved by code for this, and most updraft hoods that are for electric kilns do not meet code for this either) Check your local area’s building code for more details. For these reasons, I will focus only on fuel-burining kilns and, to be more specific, kilns that use natural gas or propane burners. I will also not spend time in this post talking about how ‘fast’ a certain rate-of-climb should be.
Now that we’ve got that out of the way, lets talk a little about the kinds of kiln atmospheres that you need to understand:
Normally, when we combust natural gas or propane, we are looking for a very efficient burn. We can see this by that ‘cone-within-a-cone’, blue flame (4 above). Most typically, people encounter this kind of flame on their gas grill, gas kitchen stove, or gas hot water heater. This kind of flame has enough oxygen that every bit of the gas that is burning will ignite and release its full heat potential. In gas kilns, this is the kind of flame that does the majority of the temperature climb, and produces an oxidation atmosphere in the kiln chamber. Since there is plenty of oxygen for the gas to ignite, this kind of atmosphere allows the kiln to climb rather quickly. Since all of the gas is combusting within the kiln chamber, there is usually very little (if any) visible flame present at the flue of the kiln.
A Neutral Atmosphere is the middle ground between Oxidation and Reduction. The flame is typically greenish (see #2 above), especially at the exit flue of the kiln, but the air in the kiln chamber is still clear when looked at through the spy hole. There will be a little back pressure at the spy holes, but you shouldn’t have a lot of flame shooting out (about 1-2″ max). In the Neutral Atmosphere, there is only enough oxygen to completely combust the fuel, and convert the combustion gas to CO2. This can be considered the ‘cruising state’ of the firing, as the kiln should spend a considerable amount of time with this atmosphere. Needless to say, since there is less oxygen in the atmosphere, the firing speed will slow down a bit.
This is where the magic happens. This kind of atmosphere gets it name from the fact that we reduce the amount of oxygen that is available in the chamber for combustion. There is so much energy in the atmosphere though, that the The flame will take on a yellow or orangeish cast, and will look rather ‘lazy’ or ‘soft’ (#1 above). During very HARD reduction, black soot can sometimes be seen coming off of the ends of the flame. The atmosphere in the kiln chamber will look significantly hazy. There will be back pressure at the spy holes (more than 2″ typically), and flame can be seen at the exit flue of the kiln. This is when soot deposits will collect on the outside of the kiln, or on the kiln’s hood. (as a side note, if you note where the kiln is sooting, you will know where the kiln is leaking heat/air. Ceramic fiber can be used to seal-up these openings). When the kiln is in reduction, the temperature climb will slow a LOT and can even stall. If it stalls, taking the kiln into a lighter level of reduction will usually get the temperature climbing again. Typically the kiln will spend the least amount of time in the reduction atmosphere, to avoid burning-off the coloring oxides, making the glazes ‘muddy’, or even carbon-coring the clay body (which makes it significantly weaker and susceptible to breakage after the firing)
Cool, now what order do I do this in?
A typical reduction firing will go in stages, and you will be changing the atmosphere during the firing. There are many different schools of thought on the order, but the basic order I find to be the most efficient is the following:
Step 1) SLOW speed, oxidation atmosphere until about ^010
Step 2) Slow speed, moderate reduction atmosphere until ^08-06. This will take anywhere from 20-40 minutes depending on a number of different factors. This is called ‘body reduction’, and is what gives you the wonderful toasty clay bodies.
Step 3) Moderate speed, NEUTRAL atmosphere until 1 cone below your target shutoff cone. Be careful to NOT go back into an oxidation atmosphere, or you will un-do the body reduction that you achieved in step 2.
Step 4) Best speed possible, moderate to heavy Reduction Atmosphere until target cone is at 3 o’clock. (watch for stalling, and back off the level of reduction if you stall or drop in temp)
Step 4) (Do NOT skip this step!!!!), OXIDATION atmosphere until the atmosphere in the kiln clears up. This won’t take long (5 min or so), and the target cone will finish dropping to the 4’oclock position during this stage. This ‘clearing of the kiln’ will produce brighter, less muddy glazes in the finished product.
Step 6) Shutoff the kiln, close the damper completely and block the burner entry-holes.
Why would I deviate from this order?
As I mentioned, there are MANY different schools of thought when it comes to firing. There are types of glazes that benefit from changing this order. Copper Reds may benefit from reducing harder, earlier, and finishing off neutral (or simply reducing during the cooling instead). Shinos and carbon-trap glazes want HEAVY reduction to come out properly, and that kind of atmosphere may be detrimental to the other glazes you may be using.
Are there any special tools or indicators that I need to know, have, or understand before firing?
Cone packs are ESSENTIAL during a firing so you know when to change the kiln’s atmosphere and rate of climb. Make sure you position the cone packs so you can see them during the firing. More information on how to make cone packs may be found here, and what you can expect a cone pack to look like during a firing may be seen here.
A Pyrometer is a fantastic tool to have during a firing as it will let you monitor the firing rate more accurately, and can let you know if your kiln has stalled out quickly. Make sure you get one that is accurate to the temperature range you are firing at. (2300 for ^10, or 2200 for ^6. More is better, but less won’t work)
Oxyprobes are VERY nice tools that can measure the actual amount of oxygen in the atmosphere of a kiln. They are also EXTREMELY expensive, ranging in the $800-$1000 range for a good one. Best practice learns how to fire without them (as humans have done for centuries), and only adds them later if you find that you really need one. If you have any questions about Pyrometers or Oxyprobes, or would like to order one, feel free to give us a call at Bracker’s, or drop us an email. We will be more than happy to help.
Ok. So how do I actually ‘do’ a reduction firing?
First off – when learning how to fire reduction, I would focus on the FIRING, and not the finish product. Start by using glazes that do not need a special kind of reduction firing, and learn how to make the kiln go into reduction, come out of reduction, go into neutral, etc. PLAY with your first couple of firings to get the handle of how your kiln reacts. It is important to note that you don’t want to do this with an empty kiln. If you don’t want to sacrifice pieces to the learning process, load the kiln with soft or hard firebrick to simulate a kiln load, and fire away.
During the firing, there are 3 things that you will need to adjust. 1) The amount of gas entering the burner, 2) The amount of air that mixes with the gas at the burner point, and 3) the damper – which controls the speed of the flue draft.
Adjusting the amount of gas
This is probably the simplest adjustment. There is a main gas valve on the burner. Opening it up will let more gas into the burner, and closing it will let less gas through. You will start with a SMALL amount of gas, and will gradually increase the amount during the firing to achieve the rate of climb you are looking for. Remember if you are using Propane cylinders, you need to have the cylinder valve all the way open (to ensure proper pressure), and do the gas adjustments at the burner’s main valve.
Adjusting the amount of air
Air is adjusted to fine-tune the kind of flame that is leaving the burner. Look at the flame itself, and adjust the air inlet until you get the kind of flame you are looking for (see above)
There are two kinds of air inlet for burners, Active and Passive. If you have a burner with an electric blower, you will be able to adjust both. If you only have a venturi or weed-style burner, you can only adjust the Passive air inlet.
The Active air inlet for a burner with an electric blower its the blower’s speed control. This works a lot like a dimmer switch for a room light. The lower you turn it, the slower the blower motor spins, and the less oxygen is added to the mix. The Passive air inlet for this kind of burner is an adjustable flap that can cover the opening on the side of the blower motor fan. By closing this off a little, you can retard how much oxygen enters the blower to begin with, but maintain a higher flame velocity. This can lengthen the flame, and can get more even firings if your kiln is updraft OR has cold spots.
A venturi-style burner’s passive air inlet is the small disc on the back of the burner. You turn it clockwise to close it down for less oxygen, and counter-clockwise to open it up and let more air in.
A weed burner’s passive air inlet is adjusted by moving the burner tip closer to, and further away from the burner entry hole in the kiln. Further away allows more oxygen, and closer allows less. You have to be careful that the flame is actually going INTO the kiln (especially during reduction), and not licking up the outside of the kiln wall. Wind blocks may also need to be used to keep the wind from blowing the flame out of the kiln, or blowing the burner out. Firebrick is the best thing to use for a wind block.
Adjusting the damper
This is perhaps the trickiest part. The damper slab controls how fast the combustion gasses move through the kiln. When the damper is partially closed, the exploded gasses can’t leave the chamber quickly, and the amount of combustable oxygen that is in the kiln chamber goes down. The temperature also climbs as more heat is retained in the kiln instead of running out the chimney. NEVER close the damper completely until after you have turned off the gas. If you do, you run the risk of filling the chamber with unexploded gas, which will explode rather violently when it finds a source of flame (or a spark).
Putting it all together
It is sometimes hard to remember that the damper is not the whole story though, and reduction is not achieved by simply closing the damper. You have to adjust the air and gas as well to achieve a balance of desired atmospheric condition and rate of climb. False-reduction happens frequently, and is often indicated by a ‘sooty’ flame that is coming forcefully out of the spy holes or chimney flue, but the kiln atmosphere is clear-as-a-bell. This is caused by over-pressurizing the kiln, and can be fixed by backing off the blower speed (if you have one), the gas pressure, and opening up the flue slightly.
MOST IMPORTANTLY – kiln chambers take time to balance. Small adjustments will achieve LARGE results in a chamber. When opening or closing flues, work 1/4″ at a time. When adjusting gas, go in small amounts. If you have a pressure gauge, work in 1/2″ WC increments. When adjusting passive air inlets, work in 1/2″ increments. Make a single adjustment and wait 5-10 minutes to let the kiln balance and the changes take effect, then check again. See if the adjustment worked, or at least ‘went in the direction you were wanting it to go’. Then re-adjust.
A couple of years ago, I was filmed giving a basic overview of how to fire a Geil 24 cu ft gas kiln. This video may be useful to you, as I go over the damper and gas adjustment concepts during it. You may find that video here. The quality is a little rough, but the information is solid.
As always, if you have any more questions, feel free to pop me an email, or comment on this post, and I’ll be happy to get back with you.