How to fire the wood kiln

This is an overview of how to fire the wood kiln at IUS Ceramics. Please keep in mind that this is a general guide, not a specific “to do” list. The most important thing to do in a wood firing is listening to the kiln and responding to what it is telling you. The ability to listen and respond to the kiln does not come overnight, but if you pay attention, ask questions, and focus on learning – you’ll be able to do it before you realize it. This page is about how to fire this kiln, but there is more information about the kiln, posting, wadding, etc here: https://claybucket.com/ius-wood-kiln/

Shift 1 (0-4 hours in): During this shift, your main goal is keeping it from getting too hot. This is a preheating shift, so just keep a small campfire going in the firebox. Stoke small pieces of wood from the lower air ports. Usually we keep the middle holes open (bricks out) and the left and right ports closed. The damper is usually fairly closed down – often to about 3 or 4 inches open. Temperature by the end of this shift is usually between 200 – 400 degrees. Near the end of this shift, start increasing the fire slightly.

Shift 2 (4-8 hours in): We normally start increasing the temperature at the beginning of this shift. Do not raise the temperature too fast – it’s very easy to crack bisqued pieces and you can blow up green pieces and/or large wads. A steady 100 degrees an hour is good, just try and avoid big spikes in temp rise. Add a little more wood than what the previous shift was doing. Continue stoking through the bottom holes. During the second half of this shift, put 1 log on the grate bars and just leave it there until it completely burns away (while you are doing your normal stoking rhythm in the bottom holes). The kiln is not ready to stoke only through the top stoke hole – you are just starting the transition by putting 1 log in. Look for temperature gain around 100 degrees an hour during this shift. Be careful not to add wood too fast – when the fire is low, you can keep adding wood and then it all catches at the same time. This will make the fire too large. By the end of this shift, you should be around 650 – 800 degrees.

Shift 3 (8-12 hours in): This shift is an important shift and you need to focus on the kiln. This is usually the shift where we transition from stoking from the bottom holes to the top hole. The way you do that is to carefully set 1 piece of wood across the grate bars. Then, continue stoking through the bottom hole to increase temp. When the piece of wood on the grate bars burns away, add another one. In the early stages, this can take 20 minutes, but as the kiln slowly gets hotter, you will be able to put wood in the top stoke hole more often. Once 1 piece is burning on the grate easily, you can start adding two pieces at a time in the top hole. Eventually you can switch to only stoking through the top stoke hole. You will find that stoking through the bottom is not as effective as the top, but keep in mind that you will likely be stoking through the top and bottom for several hours. You probably can have the side primary (bottom) air closed and the two in the middle 1/2 to 3/4 open. During this shift, you should see the front row of work start to glow and the black soot should be burning off the work. Temp by the end of the shift should be around 900 – 1100 degrees.

Shift 4 (12-16 hours in): In this shift, you are normally slowly pushing the heat back through the kiln. The front row of the kiln should be glowing red/orange by now and you should see flame starting to flow through the middle of the kiln. The middle of the kiln should be starting to glow. Even though there is flame in the middle of the kiln, you can still crack pieces in the back as they go through quartz inversion. So, just keep a slow and steady climb until you see the back of the kiln glowing too. During this shift, you should be fully transitioned to stoking through the main stoke hole (no longer stoking through the bottom). This means you will also probably need to lessen the primary (bottom holes) air so there is less air going in than in previous shifts. If you see flame and dark smoke licking out of the top of the stoke hole when you are stoking, open the damper a little bit and you should see it subside. Temp by the end of this shift should be around 1400 – 1600. You may start to see some of the early cones like 010 bending in the front. If the temp starts to stall during this shift, even when increasing the wood, open the damper a little bit.

Shift 5 (16-20 hours in): During this shift, the front stoke hole will probably start to get uncomfortably hot to stoke. You may start to see some significant cones dropping (cone 6 and higher). At this point, you should try to keep a strong flame in the firebox nearly all of the time. Do not let it “clear” in the front. “Clearing” means that you can easily see the shelves and the pots. When you clear a kiln, that means it’s going into oxidation and in terms of the front, it can mean very rapid heat rise. At this point, we don’t want the front to get too hot. By now, you probably have strong flame going through to the base of the chimney and maybe even part way up the chimney. You want to be clearing the back of the kiln and deciding when to stoke off of that. When you can see through the flame easily (there will still be flame, but when it’s wispy and thin), then stoke the front. Rhythm is everything in a wood firing, so find a good one and stay focused on it. We occasionally start side stoking in this shift – if not this shift, then definitely next shift. Temp by the end of this shift is probably around 1800 – 1950.

Shift 6 (20-24 hours in): This is the shift where you will probably start side stoking if the previous shift didn’t start already. On this shift, you are only about half-way through the firing, so it’s important to pace yourself when working on the kiln. Remember that the kiln takes more work as it gets hotter, so we need the most work near the end and you aren’t there yet. If you have a good reserve of side stoke wood, you can start side stoking with a couple of sticks in each side first. Always stoke from the back to the front of the kiln. This means that when the back of the kiln has “cleared”, meaning you can see through the wispy flames (or there is no flame), then side stoke both holes at the same time. Communicate with the person at the other side stoke hole so your actions are coordinated. Once you side stoke, wait about 20-30 seconds until you stoke the front. This about the amount of time it takes to walk from the side stoke hole to the front, pick up a 3-5 pieces of wood to throw in the front, then wistfully look off into the distance and ponder the very real chance that we are not alone in the universe. We very often skip every other side stoke (do 2 front stokes for each side stoke) during this part. This helps maintain a more steady rhythm in the front and keeps the front coal bed from burning down too much. If you hear the pieces of work clunking on the metal grate bars, then there isn’t enough wood in there. Temp by the end of this shift is probably over 2000 and you may be getting early cones (010) dropping in the way back. The middle should be a strong glowing high bisque/mid-range color.

An important note about the front stoke door. One person’s job should be to open and hold the stoke door. DO NOT drag it across the bricks. It is fragile and the fiber and ceramic buttons that hold the fiber on can easily break off. If that happens, you are in for a world of hurt – fixing a hot door on a very hot kiln is not fun. Not fun at all. Plus, it’s really bad for the rhythm of the kiln and can damage work in the front. So, be gentle!

Shift 7 (24-28 hours in): During this shift, you will most likely just be continuing a similar type of rhythm – clear the back, side stoke 2-4 pieces on each side, then wait about 20 seconds or so, then stoke the front. A typical front stoke at this point is around 5-7 pieces of wood. Start to look at the coal bed developing. If you hear the wood clunking on the metal grates when you stoke the front, then there isn’t enough wood in the front. If that’s the case, then just increase the amount by a piece or two for a little bit and reassess. Also make sure and look at the coals developing beneath the grate bars. You should ALWAYS have a gap where air can flow across the top of the coal bed and underneath the grate bars. This space can sometimes be as small as a few inches, but it always needs to be there. If the coals build up from the bottom too much, the air flow will choke off and you’ll be losing temp. Temp by the end of this shift is probably around 2050 – 2150. Cone 6 may be falling in the middle of the kiln. The front cones are probably toast.

Notes about the coal bed. The coal bed is one of the most important parts of a wood firing. It is the engine that powers the kiln to higher temps. The flame alone won’t reach top temp, you need a good strong coal bed to achieve high temps. Some things to look for: 1) Always make sure you have a little space between the top of the coal bed (the part under the grate bars) and the bottom of the grate bars. If you can’t see it, pull out some of the bricks from the primary air holes (the large ones on the bottom of the door). If you don’t see any space where air can flow underneath the grate bars, you need to rake a little bit out (or spread it around) and make a space for air to flow. If the problem is chronic and the space keeps getting clogged, you need either: a) less wood or smaller pieces of wood, b) more air, or c) more time between stokes. All of these do what you need – increase air to help burn the coal bed so that it doesn’t build up too much. The coal bed is always a delicate balancing act. Too much fuel and it builds up too much and lowers the temp, not enough fuel and it burns down too much leaving you without a good reserve of heat to reach high temps. IDEALLY, your coal bed should have bright flickering near white spots on it. Some call this “sunshine”. The coal bed should not be dense and dusty. If you see lots of very black dust (without burning coals), then you don’t have enough air. You can open the damper a touch, or open a couple of primary air holes. Check the coal bed often so it doesn’t get away from you. It’s a like a small child – it can be beautiful at times, but it needs to be closely watched after.

Shift 8 (28-32 hours in): During this shift you may start to see some actual decent cones starting to drop in the middle of the kiln. Maybe 6 is done and maybe that 8 is starting to get soft. You’ll be wanting to just keep a slow and steady rise in the middle or back. The front may be getting hard to control at this point and you probably have cone 10/11/12 in a little puddle already. If you are finding that the kiln is starting to stall out, this is pretty normal. It’s not necessarily a bad thing to stall at a higher temp – you’ll be running more wood through the kiln and building ash deposits and color. If it’s stalling and you want to raise the temp, just inch the damper open a bit – maybe a 1/2 – 1 inch more open on each side. You may find that the rhythm becomes faster when you do this – the reason for that is that you just started letting more air come in the front, so you might be burning wood faster. This is why you always look for a signal like clearing in the back (rather than timing it at 5 minutes or something). When the rhythm becomes faster, you’ll be able to adapt because you’re looking for a signal from the kiln. So, for this shift, just keep rolling – the point right now is to be easing the kiln up, or holding temp, and giving yourself time to get more of that beautiful wood through the kiln. Just keep holding that high temp and let it soak through the work and the kiln. Temp during this shift is usually hanging around between 2100 – 2200.

Shift 9 (32-36 hours in): Going into these later shifts, it’s important to remind yourself that if you are off shift, your job is to get some rest. There’s no glory in staying up for 36 hours and being too tired to make important decisions when you get to the end of the kiln. Remember that at this point, you still have somewhere around 8-12 hours left, so eat meals and stay hydrated. During this shift, you will still be side stoking around 3-5 pieces on each side, then probably around 3-7 pieces in the front. You might have opened the damper a little more. You might start to see some serious cones moving slowly in the middle of the kiln. Maybe 8, 9, or even 10 is getting soft. That’s good. We don’t need to drop all the cones in the middle now, but we want to know that we are starting to get the kiln hot in the middle. The front is probably a runaway freight train right now. It’s probably so hot that when you stoke, you think to yourself, “You know, maybe I should do more lowfire work.” If the wood is really piling up and you’re having a hard time getting wood in, open some of the secondary air holes (small soap-sized holes in the door), or take the brick out and replace it with a smaller chunk. At this point, it’s a good idea to double check and make sure your end of the firing shifts are covered well. Keep a close eye on the coal bed during this shift. If you start seeing the temperature drop even though you are doing the same thing, it’s usually the coal bed getting choked off. Remember, always have an air space between the top of the coal bed and the bottom of the grate bars. Without that space, you’ll never get the kiln to peak temp. Temp during this shift is still probably hanging between 2150 – 2200-ish.

Shift 10 (36-40 hours in): During this shift, you will need to be paying close attention to the coal bed, usually raking it every 20 minutes or so. There are times when some of the coals have to be raked out of the primary airs, but only do that if they are really building up and you can’t get them to burn down. If you rake coals out of the primaries, scoop them up with a metal dustpan and put them in one of the empty metal raku cans. Do not put them in the dumpster under any circumstance. Your raking should be to make sure you have air flow over the coals and underneath the wood. Sometimes we rake through the top hole as well, usually to even out the pile of wood and assess the depth of the burning wood on top of the grates. In this shift, you should try to inch up the temp and start allowing those high temps to ease to the back of the kiln. The stoking rhythm is usually still being called off of the wispy flame in the back of the kiln, and often we look through the passive air dampers (the holes in the base of the chimney) to watch when the flame clears back into the chamber. Usually the pyro is reading the 2250 – 2275 range now. If the kiln is stalling lower than this, open the damper a little more and you should see some temperature rise. It’s normal for the damper to be open about 10-12″ by now.

Shift 11 (40-44 hours in): This shift is often focused on prepping the kiln to be fired off, which means bringing the kiln very close to peak temp and getting within striking distance of the final cones. When the last shift starts, you really want to be within a couple of cones of your final destination. So, that’s your goal here. Cones are all down in the front and usually the middle, or at least in the middle you have cone 10 down by now. This shift will focus on the back and getting into the cone 9, 10 soft range in the last stack of work.

Shift 12 (44-48 hours in): This shift is hopefully the last shift of the firing. It’s extremely important, so it helps to have a couple of fresh people on – someone that has eaten a meal recently and actually slept in a real bed. The goal of this shift is pretty simple – get 10 down in the back and end at the highest temp in the front and middle of the kiln. Sometimes during this shift (or Shift 11), we might “bundle” the side stokes. This means that when it’s time to stoke the sides, you toss 2-3 sticks into the kiln, then you fill the hole with sticks so that they burn off the tips of the wood into the kiln. The hole will basically be stuffed with wood and the kiln will suck air through the wood and the flame will jump right off the ends of the wood. When the flame starts to burn into the hole, then push the wood in a little more. Usually, the process is “bundle in”, “bundle half”, “bundle flush”, then “down bundle” is when you push all the wood in. This can be tricky as it can throw off the rhythm in the front, so pay close attention to the color in the front. You do not want to see very clear flame in the front. If you get to the end of your bundle cycle and when you stoke the front you can see very clearly all the work and there is very flame, then you are waiting too long. You should also have a good amount of flame in the front. If not, you will over oxidize the front, burn down the coal bed, lose temp, and risk getting melon skin on the work. The temp on the pyro should be in the 2315 – 2350 range. We usually fire this kiln so that we have 12/13 down in front, 11 down in the middle, and 10 down in the back.

Shutting the kiln down: The process for shutting the kiln is extensive and takes well over an hour, so when you reach temperature, the firing isn’t over. When all the cones are down and you are satisfied with the firing, we usually stoke for about 20 minutes or so in the front with side stoke wood – lots of it. Several armloads per stoke. This will super heat the front and make sure you are ending on a high point. While 2 people are doing this, others should be making slip (if it hasn’t already been made) to use to mud up the holes in the kiln. The slip is about 50/50 sand and reclaim, blunged to a relatively smooth consistency. Mud around the side stoke holes, air holes, etc by dipping newspaper strips into the slip, then carefully pressing it around the bricks (using gloves). After the last front stoke, plug the main stoke hole with bricks. It usually takes about 6 bricks. Mud those bricks BEFORE you close the damper. If you close the damper and then mud the holes, the hot air shoots out of all the holes and it is quite the uncomfortable experience. When all the holes are mudded, close both dampers. Remove the soap bricks from the slots above the damper. Remove the passive air damper brick from high up in the chimney (facing the firebox). This will help cool the top part of the chimney. Once the kiln is mudded and closed, you must clean up the area around the kiln. There’s an old saying, “The kiln shed always burns down in the cooling.” The point is, things in the shed catch fire when nobody is around to put them out. All wood needs to go back on the racks, all paper and debris need to be swept away. Clean up all food, coffee makers, water, etc. Just clean up the space so that when others walk by after you leave, it’s presentable. If there are others on the schedule after you, text them and let them know the kiln is finished. Leave any coals from the firebox in metal trash cans, but put the cans away from any flammables. After you have adequately cleaned up and the kiln pad is in a position to be left unattended, you are now finished! High five! Woohoo!! Now, be VERY careful driving home if you have been up all night or are completely exhausted. When you get home, go to sleep!