Making a Wood-Fired Oven
From a "Spare" Ceramic Base


This page documents an experiment we conducted with turning a large Big Green Egg base into a wood-fired oven. As you will see, the results weren't all that great and we finally lost interest in the project. However, we recently posted on Facebook one of the photos of the Egg after the chop job. That post received more views than any other in our history. As a result, we thought we'd go ahead and post what we did accomplish for those that were curious and perhaps might want to continue the experiment. So, for what it's worth.....


Introduction
We recently found ourselves is possesion of a "spare" base to a Big Green Egg ceramic charcoal cooker. It had a crack in it so it couldn't be used as a cooker anymore. We didn't want to just trash it, so we tossed around the idea of using it as a tomato planter for a while. Then we decided it would be fun to see if we could make a wood-fired oven from it. We had recently read "Build Your Own Earth Oven" by Kiko Denzer and were somewhat inspired to make something of our own.

We had a few guidelines and goals for this project. First, we wanted to see how much we could do with as little as possible. We didn't want to incur too much expense. We also wanted the ceramic cooker to play a major role in the production of a wood-fired oven. Otherwise, we might as well build an earth oven as shown in the afore-mentioned book. So, we tried first with the absolute minimum of effort and materials. Then we tried to make improvements, testing along the way. We'll provide you some test data at each step so you can decide for yourself if you want to undertake a project like this and how far you want to take it.

Disclaimer: We did what we did and you can see what we did. Is this safe? We don't know. You do what you do at your own risk. We sincerely doubt that Big Green Egg would approve or condone what we did with our spare base. Please use care if you decide to undertake a project like this.


The First Experiment: Preparing the Cooker For Cutting
Our first thought was to place the cooker on the ground, upside down, and cut a piece out of the side of the cooker for the opening where the food would be placed in the cooker and removed. Also, we decided that the vent might make a good chimney to allow air to flow through the cooker to get the wood really burning hot. So we placed the cooker as you can see below, with the vent to the back, so that we could cut a segment out of the front:


This is our slightly cracked base from a large Big Green Egg.

We have placed it so that the vent is toward the back. We'll be cutting on the front.


We decided that our opening should be 15" wide and 6" high. We figured this would allow us to get a decent sized pizza in and out of the oven. Also, we thought this would allow us to get small loaves of bread in and out. If you have done some research on wood fired ovens, you may be aware that the recommendation for the height of the opening is two-thirds the height of the interior of the oven. This allows fresh air to enter the bottom of the opening and smoke to exit at the top of the opening. However, since we will have a small vent at the top, we decided to keep the opening small to conserve heat and increase the mass of ceramic available for holding heat.

So, the first task was to mark the side of the cooker to guide us in cutting out a section. Using a ruler, we marked two spots 15" apart on the front edge of the cooker:


Use the ruler to measure two spots 15" apart on the front edge.

We used a Sharpie to mark each side of the piece we were going to cut out.


The next step was to draw the outline of the area to be cut out. Using a square we drew two 6-inch vertical lines down from the marks we made for the sides of the cutout. Then, using the square as a guide, we roughly drew a line across the cooker between the two vertical lines:


Use the square to mark a 6-inch vertical line down from each of the side marks.

Mark 6 inches for the horizontal line.

Use the square to make marks 6 inches down from the top so you can then free hand a line between the two vertical lines.



The First Experiment: Drilling and Cutting the Cooker
In order to cut the cooker, you will need a few tools. We decided to drill holes at the two corners of the cutout in order to prevent any cracking caused by a square corner. We also decided to use a sabre saw to make the cut. Finally, we thought about using a Dremel tool to score the ceramic to prevent chipping of the glaze, but more on that in a bit. Here are the supplies we bought:


A ceramic/tile drill bit, a carbide saber saw blade, and Dremel cutting disks.


First, about scoring the glaze. Don't bother. It turned out that the sabre saw didn't really do much damage or chipping to the edges of the cutout. What little damage was done just wasn't worth any bother. Also, that glaze is quite tough! It took a whole disk to do about 1" of scoring, so you'll need a lot of disks. And to be honest, our Dremel tool broke on the first attempt at scoring, so we just gave up. Bottom line, don't bother!

Now, on to drilling the holes at the corners of the cutout. So as to avoid having the drill bit wander all over the side of the cooker when you start drilling, press the drill bit against the spot where you want the hole. Then manually rotate the whole drill left and right as you apply firm pressure. This will get a small indentation started so that when you turn on the drill, the bit will already be seated into the start of the hole. Start drilling slowly until you get your hole going, then you can pick up some speed. You will find that the drilling goes quite fast and that it is easy to do. Don't worry about cracking the cooker.


Getting the hole started by manually rotating the drill back and forth.

The finished hole. Perfect!

Don't bother trying to score the glaze. It isn't necessary or worth it.


Now, to make the actual cuts! But first a safety warning. Wear a mask! This is going to make a lot of powder and dust floating in the air and you don't want to breath it is. So do yourself a favor; do the smart thing. Wear a mask!

As you cut, just go slow and steady. It will take a little elbow grease and take some time, but you will make it through the whole cut. Just be patient and perservere. You may also find it helps a bit to use a sort of rocking motion as you cut.

We decided to make the horizontal cut first, followed by the two vertical cuts. This was to prevent having a big old piece of ceramic hanging by one side while we made cuts that might cause a crack. We then made the vertical cuts up from the holes to the edge. We made the first cut and stopped about an inch short of the edge. We then made the second vertical cut all the way up to the edge. To cut over the lip around the edge of the cooker (at least, using a sabre saw), you will need to cut from the inside of the cooker for about 1/2 inch and then you get resume cutting from the outside if you like. The whole piece then just broke free without having to go back and finish that first vertical cut.


We started with the horizontal cut first.

The horizontal cut is done! As you can see the edges didn't turn out too badly.

We stopped the first vertical cut about an inch from the edge.

When we completed the second vertical cut, the cutout just broke free. As you can see it was a very clean break.


So, there we have it. Our very own wood-fired oven. Well, sort of. Here's how the cutting turned out:


The cut is complete.

Save the cutout! It will be the door!

Here's what it looks like upside down, or right side up, depending on your point of view.



The First Experiment: Testing the Wood-fired Oven
Time to test our prototype! As you can see below, we placed two sheets of slate on the ground to be our base. We then put a 15" pizza stone on top of the slate to serve as the floor of our oven. We had a bunch of wood biscuits lying around so we thought we'd use them for fuel. As you can see in the second photo, we hosed everything down around our little oven to lessen the chance of unwanted fire. We then put the oven over the top of the pizza stone and wood biscuits.


Wood biscuites on a pizza stone on sheets of slate.

The oven has been placed over the stone and wood.


We lit the fire and let it rip. We placed the cutout in front of the opening of the oven to help keep heat in the oven, but still allow air in for the fire.


The cutout is the door.

You can see the fire down in the oven through the "chimney".

We had quite a fire going, flames coming out the chimney.



The First Experiment: Results
So, how did our oven perform? We measured the surface temperature of the oven's walls and ceiling as the fire heated it up. We also measured the air temperature inside the oven, a few inches above the pizza stone, once we scraped the coals to the sides of the oven and as the oven cooled off.

After about 30 minutes of fire, the surface temperature of the top was between 350 and 500 degrees, depending on where you measured. The front was cooler than the back where the got gases were exiting the cooker. The sides were between 250 and 300 degrees.

After about a total of 45 minutes, things seemed to be stabilized. The top temperature was between 430 and 540 degrees, while the sides were between 300 and 410 degrees. The air exiting the chimney was about 700 degrees.

We then scraped the coals to the edges of the pizza stone and placed a thermocouple so it was about 2 inches above the pizza stone. We then monitored the air temperature as the cooker cooled:


So, not too impressive, is it? The oven only got up to 400 degrees and it stayed above 300 degrees for only 13 minutes. You will be hard pressed to bake much of anything like that. Thus we shall move on to Experiment 2!


The Second Experiment: Keeping The Heat In
It suddenly struck a few days later that by leaving our "chimney" at the top of the dome open, we were allowing heat to escape. Also, we were allowing cool air to enter the bottom of the oven which helped to cool the sides of the oven. The air could then exit through the chimney. Since we still had the lower vent for this shell, we decided to put it back on the shell:


The "chimney" now has a cover.


We then repeated the process of firing the oven and then measuring the temperature inside the oven. This time, we could close the chimney. Here are the results of this second experiment:


Ok, it's getting better. We had a higher initial temperature and we stayed above 300 degrees for 41 minutes. The oven stayed above 450 degrees for 8 minutes, which might be time to do a quick pizza. Still not really worth the effort, though, is it? Well, we are doing better, so we feel it is worth the effort to move on to the third experiment, building a proper base for the oven.


The Third Experiment: Building a Proper Base For The Oven

Ok, we decided to take it to the next level. Let's build a proper base for our oven to get it up off the ground. Also, maybe placing fire bricks underneath the oven will allow them to hold more heat. Let's see.

First off we build a base out of treated lumber. We aren't going into all the techniques and tricks for building this base. You should be able to figure this out on your own. Just make it good and strong so that it will hold a lot of weight. (Just in case we take this experiment to the limit!) We'll give you some dimensions to help you out, though. We built ours 24" high and wish we had built it 30" high. Do what you think will suit you. As for the dimensions of the top, well the firebricks we bought at a local brickyard were 4" by 9". Based on the thought that there might be a fourth experiment, we decided on a top that is 32" by 45". This equates to 8 bricks by five bricks. The photos below should make this clear:


The table we built for the base. Notice that is has lots of bracing.

Level your table before you place any fire bricks on it!

We used backer board (for use as an under layer for tile in bathroms) as a safety barrier so embers don't fall down between gaps in the bricks.


As you can see in the third photo above, we used a backer board that is intended for use as an under layer for tiling in bathroooms as a safety layer. The idea is that if any embers slip down between a gap in the bricks, this will stop them from falling to the ground. Even so, when dealing with fire, it might be a good idea to give the ground a bit of a soaking before you start to prevent any embers from accidentally starting a fire.

Now on to fire bricks. First of all, you can buy them from any brickyard. There appear to be two kinds, wire cut and pressed. At the time of this writing, the wire cut bricks were $1.10 each. They are slightly irregular in size and a rather open brick, but we decided we were going for cheap! The pressed bricks should be more uniform in dimensions and shape, and they were $1.95 each. The choice is up to you. You may wish to wear gloves when handling these bricks as they are quite rough on the skin.


Our 45 bricks waiting to be placed on the new platform.

As you can see, there will be gaps between bricks. Thus the use of a backer board.

This is a somewhat large animated GIF showing the bricks being placed.


So how did we do? Well, we built a fire inside the oven using oak and poplar split into kindling size pieces. We kept the fire going until the surface temperature of the top of the oven reached a relatively steady temperature in the low 600's. We then closed the top vent, inserted our thermocouple a couple of inches off the surface of the oven floor, and pushed the oven door shut. Here are the results:


Hmmm, it looks like we may have taken a small step backwards? Hard to say. The results in experiment 2 were that the oven went from 550 to 300 degrees in about 40 minutes. Here we went from 600 to 300 in less than 30 minutes. We suspect that the ambient temperature may have played a part in this. We wouldn't expect this latest setup to do any worse than our setup on the ground. Well, when we did experiment 2, the temperature was about 90 degrees (August) while this experiment was done in 50 degree weather (November).

But now the question is, do we take it to the next level? We think it is pretty clear that we need more mass around the oven to hold more heat.


The Conclusion

Well, we finally lost interest and took our experiments no further. What we had planned to do was perhaps build up an outer layer by stacking bricks around the Egg's shell. We thought about packing the gaps between bricks and the shell with sand or vermiculite in order to better insulate the "wood fired oven". But like we said, we lost interest and never went any further. We post this just in case any of you want to try something with a discarded kamado-style cooker base. If so, we'd love to hear from you!


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