I wanted to have a fireplace in the garden that would allow our guests to stay comfortable,
so I made a Chimenea, the Mexican wood burner, not from clay or adobe but from concrete!
I was told imported Mexican chimeneas were popular here for some time a couple of years ago but the clay is not really resistant to this climate. Water would seep into the cracks, the bottom would fall out at best and at worst they would explode. (Not a pleasant thought.)
I would buy a proper one but they are not available cheaply here and besides, where is the fun in that?
Large amounts of clay were not easy to find cheaply and while searching the web I read about ferrocement: plastering ordinary Portland cement on a mesh of chicken wire. With this interesting technique one can create all kinds of forms that are very strong, cheap to make and relative to its size reasonably light. Because is is very labour intensive it is not widely used but during both world wars the shortage of steel they constructed ship's hulls out of the stuff, the largest being the SS Selma that weight 7500 tons. (!)
This inspired me to try create my own chimenea from chicken wire and cement.
For the design I was inspired by this Instructable by Tim Anderson of his chimenea made from an old propane tank . I wanted it to resemble a Maori tiki face,
complete with paua shells for eyes. The pupils are formed by holes in the shells and the concrete wall so they will flicker with the light of the fire, just like Tim's chimenea.
Step 1: Get the components
My favourite is The Wellington City Landfill's Second Treasure Shop, a wonderful source of materials and inspiration.
Price list (new & used in $NZ)
from the Landfill's Second Treasure Shop
1 bar stool $3
1 meter black stove pipe $5
1 black cast-iron pan lid $1
1m x 20cm black stove pipe $3
15m 2.1mm steel wire $2.99
10m x 90cm galvanized chicken wire mesh $44
(Alternatively, use Expanded metal, might be cheaper)
60kg ready mix cement mortar $36
1kg iron oxide cement coloring $11.50
from the $2 Shop
1 Pink Skippy ball (or 'space hopper') $2
2 black wigs $4
big and small buckets
from Muritai beach
2 small Paua shells for the eyes
15 or so Paua shell fragments for decoration
Step 2: Choose the cement mix
First of, I have used ordinary ready-made mortar mix – the kind used for regular brickwork, that consists of just Portland cement and sand. I recommend using the ready made mix, for a small batch it is not worth the hassle to mix your own from cement and sand. The ratio of the ingredients is very important in making concrete so you don't want to mess that up. Ready made mix also contains some additives that make it easier to work with.
Tip: Do not get ready-made 'concrete' mix! (as I did by accident). Concrete mix contains gravel (duh) and that makes it impossible to smear the cement between the chicken wire mesh or to end up with a smooth finish. Because I wanted to get started on Christmas day and therefor could not get any other mix I ended up sieving the gravel out of my dry cement with an artifact I had left over from making a WiFi antenna. ;-)
There is a wide range of colours available for concrete, based on iron oxide, and it's very cheap.
Originally I wanted to colour the chimenea in a terra cotta orange-reddish-brown like its Mexican brethren but Ablaze only had black available and as I was impatient I went with that. I would go for terra cotta the next time though. Portland cement is gray and will influence the final colour. Use white cement and sand if you want the colour to come out better.
Concrete and heat
I did not use any 'fire resistant' cement mix mainly because cvdukes over at the Forno Bravo Forum for Wood-fired-pizza-oven geeks told me it would be ok. The guy that works at Ablaze, a huge firewood and fireplace supplier nearby also thought that for this particular project I would probably be fine with regular cement mix.
However: Fire can destroy concrete. When concrete hardens ('cures') it does not actually 'dry'. The water in the mixture does not evaporate but reacts with the solids in Portland cement and remains locked inside the concrete and is a vital element to give strength to the structure. This is why you need to keep concrete wet while it 'dries' and cover it with plastic to prevent evaporation.
A hot fire removes the water and can destroy the integrity of the concrete structure.
Air pockets and cheap wigs
Besides that there may be air pockets left trapped inside the cured concrete. When heated, the air will expand and can cause many 'micro explosions' that weaken the concrete from the inside.
I read somewhere that to fire-proof concrete buildings they add plastic fibers to the mix that will melt in case of a fire and create little channels to let the expanding air escape. I decided to go with this idea and bought two black wigs at the $2 Shop which I will cut in small pieces and add to the mix.
An alternative mix
For future reference sake i will give you the recipe for an alternative and more 'fire resistant' concrete mix although cvdukes warned that it might be to brittle to hold up a structure on it's own. I think that since the chicken wire adds so much strength to the structure I would be interested to try it next time. On the other hand, if my chimenea with regular cement mix holds up after repeated thermal abuse there seems to be little reason to spend the extra cash and effort.
Fire-resistant cement, called 'refractory mortar' (NOTE: not used in this project!) is sold as "RefMix" or to make it yourself use:
- 1 part Portland cement
- 3 parts sand
- 1 part lime
- 1 part fireclay
Step 3: Prepare the decorations
To cut the mouth from the wire mesh and determine the position of the eyes and eye brows I drew them on the skippy ball before I started.
Soak the shells in water
Concrete uses water to cure and anything extracting water from it while curing will weaken the bond.
If you want to attach wet cement to bricks, ornaments or an existing concrete structure it is a good idea to make those parts wet before you start.
I submerged the paua shells in water to ensure a good bond with the wet cement.
Drill holes in the shells
I was inspired by an other excellent Instructable of Instructables top tinkerer Tim Anderson to have the eyes burn with fire, so I made holes as pupils. Next time I will make them a little bigger and maybe change the angle so the light of the fire might reflex the awesome colours inside the shells. Keith suggested placing clear class marbles inside the eye sockets so the light might reflect back into the shell.
Break up some more shells for other decorations
Paua fragments will form a line around the base of the chimney.
WARNING! Paua dust is possibly harmful to your lungs!!
Do not sand or saw paua shells without wearing a mouth mask. The dust has the same characteristics as asbestos and might give you lung cancer.
(Only in fine dust form. Normal handling is completely safe of course)
Plan where to use the decorations
Normally the layer of cement will not be thick enough to press any decoration into it because the chicken wire will be to close to the surface. You will have to make special arrangements on the place where you want to embed something into the cement.
Step 4: Cut the frame with an angle grinder
I simply cut of the bar stool's legs above the first tier at an angle that more or less matched the curve of the skippy ball. It does not really matter as this is going to be covered in cement anyway.
Strip the paint
The paint on the frame might smolder and stink inside the concrete shell of the chimenea, something that would really be a bummer. To avoid this I sanded the paint of the frame where it was going to be covered with cement.
Step 5: Fold chicken wire into mats
Tip: Try to avoid cutting the chicken wire if you can. The edges are razor sharp and you will have to do a lot of pulling and pushing to get the shape right. Also you will have to apply the cement mix with your hands and you need thin gloves to work effectively. My hands were covered with small cuts and for some reason the cement made the wounds infect in a rather nasty way. You have been warned. (My wife suggested finding thin leather gloves that would withstand the sharp points. Might try that next time)
To protect myself from the sharp edges I taped the edges with duct tape. Be aware that getting the duct tape of again is nearly impossible so make sure you have enough extra length to cut the ends of with your angle grinder
Cut strips and fold them
I recommend used an angle grinder, although a good pair of pliers could do it, to cut the 90cm wide mats long enough to go from the base of the chimney, all the way around the skippy ball and up the other side to the base of the chimney again. Next time I would make the mats long enough to reach to the top of the chimney so as to get one smooth surface and reduce cutting and prevent connecting separate pieces of chicken wire.
The documentation about ferrocement I found on the web suggested to use at least 5 layers of chicken wire but the distance between the legs of the frame was exactly 22,5cm so I folded the 90cm wide chicken wire 4 times.
Mind the gap
It is very important to overlap the pattern of one layer with the next so you can see as much wire as possible. This way the space between metal wires is as small as possible and this will increase the strength of the final construction. See picture for a better explanation
Another important thing to realize is that the cement mix will only stick where there is chicken wire. So with 4 layers tightly bound together the shell of the chimenea will be about 1.5cm thick (half an inch). Now, 1.5cm is probably enough. The resulting concrete is incredibly strong and will have little load the carry.
On the other hand the more mass the chimenea has the better it will retain and radiate heat and it will also look more bad-ass. I had decided I wanted to go a little thicker.
Fluffing the chicken (Optional)
To gain extra thickness, I went through a procedure that I would like to call "Fluffing the Chicken Wire" (patent pending). I put the chicken wire down on our wooden deck that has narrow gaps between the boards and I ran a piece of wood over the chicken wire, pressing down into the gaps. I then folded the chicken wire backwards twice so that the top of the ridges met. This gave me a nice thickness.
Note. Again, the cement will only stick where there is plenty of chicken wire so it might be a good idea to keep the layer that faces out free from ridges and be nice and smooth.
I zigzagged a thick piece of wire through the side of the mat to connect all 4 layers and to give the whole construction some more substantial skeleton. In hindsight I don't think it matters for a small object like this. If I had connected the chicken wire mats with thin wire the strength of the cured concrete construction would probably have been the same.
Step 6: Create the sphere
With or without frame?
I decided to included the frame into the whole wire structure that is going to be covered in cement as it will be much easier to work on the project from a stable platform. The reason for this was that I thought that during the plastering phase it would be difficult to handle a ball that has a 90cm pipe stuck to one end and was completely covered in wet cement. The chimney end would be top-heavy and if I would let the ball rest on the frame I would be unable to apply cement at the point where the ball was pressing against the frame.
This worked out well for me but it was hard to achieve a pure spherical shape.
You might want to create your sphere (or cylinder) separately and rest it on a frame when the cement is cured. I think I might go for a cylinder next time as filling the gaps and trying to get a true round shape was challenging with the frame in place.
The other pieces
Everyone knows you can't make a sphere out of two rectangular strips so you will have to make some extra, smaller mats to cover the gaps. The only thing I can say about that is make sure the surface is nice and smooth and the wire mesh is even, uninterpreted and without loose wires sticking out. The cement mix will not stick otherwise. More on this in the "Apply the cement mix" step.
Step 7: Create mouth,eyes and and eyebrows
Again, make sure the surface is smooth and no loose ends are hanging out.
To let some light shine in to his eyes I lodged two rolled up pieces of cardboard in the wire mesh directly behind the holes in the shells. This way I can pack in the cement and keep an opening that will go all the way to the fire.
Step 8: Attach chimney
I created a separate chimney by folding another 90cm x 90cm piece of chicken wire 4 times and rolling it round a cylinder I'd made from cardboard.
This I then place on top of the sphere, over the 20cm diameter hole in the mesh I had saved.
For next time...
It was so easy to make the cylinder and the resulting wire mesh was so smooth and even (important!) that I wished I had created the main body of the chimenea as a cylinder as well. It would have required a lot less cutting of chicken wire and a lot less fiddling to get the gaps closed and still keep the spherical shape. On the other hand the round design of this one appeals much more to me so I'm happy I went through with it and got an awesome result. Maybe next time...
The thicker pieces of wire that run through the sphere run up the chimney as well to give it some more strength. I am writing this after the whole construction is finished and I am not convinced it makes much difference on a small, non load bearing structure like this.
See later steps to find out what happened to the chimney...
Step 9: Apply the cement mix
Will it stick?
The cement is pretty sticky (one batch was wetter than the previous and both were workable) but it will absolutely not stay if there is no wire mesh to hold it! So don't expect to bridge any gaps, specially if they are on a tilted or horizontal plane.
Originally I thought I would leave the skippy ball inside and pack the cement against it, but you really need two hands to push it in.
You might need some support on the underside to prevent the cement from falling out. Even if it sticks initially it might fall out after half an hour while you're not looking. I folded some cardboard around the bottom and then wrapped the whole thing in plastic.
You should wrap the whole structure in plastic anyway when you're finished to prevent water evaporation. I used cling foil to cover it all the way to the chimney so no water vapor can escape from the top.
- Be careful with adding water to the mix. You will be surprise how little water is needed to go from 'to dry' to 'runny as snot'.
- Leave some cement in the bag to add to the mix if you accidently find yourself in Snotty-ville
- Wear (pref. pink) rubber gloves at all times . The cement will dehydrate your skin. Trust me, it is very painful, even if you're a get-tough-I'm-a-real-man-sort-of-kiwi-bloke. (which I am not)
- A regular (plastic) garbage can be made into quite a handy manual cement mixer. Just hold it at a 30 degree angle and roll the edge of the bottom along the ground to mix your mix. (I'd use a cement mixer and a power drill any day of the week though, if I'd had them available)
- You will (always) need about twice as much cement as you originally calculated ;-)
Step 10: Make holes for eyes in shells and in the mesh
You can not press anything into the wire mesh as there is simply not enough 'free' cement between the surface and the chicken wire.
I made special sockets for the eyes and around the chimney base I plastered a thick layer of cement in which I pressed a row of shell fragments.
Because there is no support for this think layer of cement is started sacking immediately, which luckily in my case turned out all right but you have been warned: you really can't sculpt with cement without support.
Step 11: Let the cement cure for at least 7 days
Make sure your project is completely wrapped in plastic, it's not in direct sunlight and just let it sit there for at least a week. The cement will continue to cure for 28 days (!) and only then will have reached it's full strength.
Hose it down with water every other day if you can. Someone recommended wrapping the thing in a burlap sack or old blanket before wrapping the plastic and keeping that blanket wet all the time.
Now, I am going to be honest with you and admit that after 6 days I could not wait any longer and did the first test fire.
Step 12: Add a stove pipe as an alternative chimney
Because I was out of time, out of cement and out of black iron oxide, I decided not make the whole chimney with ferrocement but to insert an iron stove pipe. I am very happy how my design worked out and how the stove pipe nicely 'breaks' the surface that otherwise would be completely covered in the rather rough and bumpy concrete. When my techniques for finishing,colouring and smoothing the surface will improve for version 2.0 I think I would recommend going with cement all the way to the top.
Why you really need a chimney
Yes, you really need one because:
1. the fire will burn much easier and smoother because of the constant air draft the chimney causes
2. the smoke will blow over your head when you are seated around the fire
3. it just looks better
Any chimney can look good if you take some time to make it clean and proportional.
Cut the chicken wire and insert a stove pipe
So in this case I cut off the remaining chicken wire with the stone-cutting disc on an angle grinder. (Note how I used the Petone shoreline on the other side of Wellington harbor to get a level cut)
I found a black stove pipe at the landfill treasure store and the diameter fitted perfectly into the short chimney base. It was made about 40 cm shorter so the height of the chimney is in proportion to the size of the body. I just filled the gap with some more liquid cement and use slightly ticker cement to make a nice transition between the concrete base and the metal pipe. I then pressed paua pieces into the mortar for a little decoration.
Add a spark catcher
The landfill also provided me with a black cast iron pan lid because the top of the chimney really needs a spark catcher. It can get pretty dry around these parts and the newspaper already reported a bush fire less than 500 meter from our house in the week I was finishing my (potentiality forest-fire-hazardous) project.
An angle grinder was used to cut three thin supports which I bent and attached to the pan lid with small bolts.
Step 13: Test fire and launch
Short version (just the awesome fire-shooting-out-the-top and eyes-glowing)
Step 14: Final thoughts
- rimar2000 suggests using "expanded metal" sheets instead of chicken wire. It is apparently cheaper and from the looks of it it is ticker and the gaps are smaller so you might need less layers.
- Try to make the design look more like an actual tiki (by putting the fire in it's belly)
- Make the walls thicker to get better heat retention and radiation
- Use white cement instead of gray to get better colours
- Make the chimney wider, it will pull better
- Maybe use normal, cheap (gray) cement for the inner structure and finish with (coloured) refractory cement or clay.
- Make a tray to insert a BBQ grill rack
- Try to make the insides of the eyes glow with fire when all surroundings are dark
- Try not to cut my hands a hundred times on the chicken wire
I've had great fun designing and building this small devil and have enjoyed it's warmth a couple of time already. If you are thinking of creating a free-form structure that needs to be cheap, strong and durable I can highly recommend ferrocement although you might want to do a small proof of concept first to get used to the technique.