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Using Porland/Fireclay cement for a Chiminea?

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  • Using Porland/Fireclay cement for a Chiminea?


    I'm am new to this site and after reading about a lot of the projects I am tempted to chance my current project into to pizza oven project ;-)

    I live in Wellington, New zealand, summer is coming and I want to make an outdoor fireplace.

    I want to build a small Chiminea (the mexican clay type round heater, Chimenea - Wikipedia, the free encyclopedia) and was thinking of using ferrocement with the Portland/clay mixture that is mentioned in the Primer on this site (High Heat Mortar Primer)

    Plan is to wrap a large beach ball with 5 layers of chicken wire and apply the cement in layers, perhaps letting it cure and applying another layer of chicken wire and cement to gain mass.

    I would construct the chimney in the same way.

    This would be my first attempt so my main focus is on costs and getting moving as soon as possible. I fully expect the first version to only half perfect ;-) but building it is half the fun.

    Does anybody have experience with making larger structures with Portland/fireclay cement? Any thoughts?

    Thanks a million!



  • #2
    Re: Using Porland/Fireclay cement for a Chiminea?

    That mixture is specifically a mortar: meant to put firebricks together. I don't think it has big enough aggregate to work in a thick layer.

    You might want to look into building it with cobb. At least if it failed you could hose it into the lawn.

    As a side note, at least around here, chimineas, the mexican terra cotta ones, are close to free. No one ever uses them, and they are on craigslist all the time for anyone willing to haul them off.
    My geodesic oven project: part 1, part 2


    • #3
      Re: Using Porland/Fireclay cement for a Chiminea?

      Thanks for the insights.

      So the structure is to brittle for larger spans? Even with 5 layers of chicken wire in there? They claim ferrocement is very strong (they make boat hulls and domes out it)

      I was inspired by this post, but dont know what mixture he used:

      the reason I would not think of a normal concrete mixture is because fire destoys concrete, doesn't it? and an fire pit like that could get pretty hot.

      I would buy one but unfortunately the chiminea fad has not hit New Zealand yet (this is BBQ country ;-).

      I understand that many in the us are of poor quality and cheap, but other than that, why wouldn't you use one? Is sounds like the ideal fireplace for a small garden? (have used it once or twice as friend's houses)

      Would love to hear your thoughts.



      • #4
        Re: Using Porland/Fireclay cement for a Chiminea?

        Hi mourits
        personally, although over here in Aus, chimineas are not so inexpensive, BUT I would rather spend the dollars and get one that is manufactured to meet the needs rather than try to build one, a little risky with all your time, expense and effort!
        Check up on ebay or similar and look for a brazier which might also meet your needs, although built from steel strips for heating a patio or outdoor living area.
        Prevention is better than cure, - do it right the first time!

        The more I learn, the more I realise how little I know

        Neillís Pompeiii #1
        Neillís kitchen underway


        • #5
          Re: Using Porland/Fireclay cement for a Chiminea?

          Hi mourits, around here many towns have banned chimineas cuz people put them too close to flammable structures , melt the vinyl siding off their house or start the nieghbour's fence on fire , as a ceramics guy I would opine that most single wall ovens /chimeneas are going to crack .. too much expansion on the inside too cool [less expansion] on the outside = stress+ cracksl


          • #6
            Re: Using Porland/Fireclay cement for a Chiminea?

            Hi mourits,
            I guess I?m the only one firing up in ferro cement so I?ll try to answer your questions. The pizza oven has been in operation for 2 years now and has been a good performer. Lately, we have slowed down on how often we fire up (back to building boats instead), but I guess its been up to full temp 20-25 times since it was built. I have a friend (former professional chef) that built a brick oven in his yard several years ago and hasn?t fired it up for the past 4 years ?every time he cooks in mine, he is just astonished on how much bettor mine has held up. The shell has no cracks or shows any signs of deterioration. I do have a few cracks in the floor (not ferro reinforced) and a problem with where I joined the shell to the chimney that I need to seal better, but all in all its been great and well worth the minimal investment I put into it.

            I assume you have read my other posts on the massive ferro-cement firebeast that is more like the project you are describing:


            Basically, I started off by making a wire armature from discarded hog wire (about 10 gauge 2? x $4? grid wire) picked up off a neighbors trash heap. I have made ferro cement structures before without the thicker wire, but I find its helpful to keep big things in shape. Next I covered the armature with about 3 layers of 1?hexagonal chicken wire?2 layers on the outside and 1 on the inside. A few places ended up with extra layers where new pieces of chicken wire got added in or where it was necessary to cut and fold it to form it to the shape.

            A key thing here is to get the layers of wire offset from each other so the hexagonal cells don?t line up right over each other. The other thing you need to do is run wires (or string even) through the layers to pull everything together. Pull the chicken wire from the inner layer through the outer layers and vice versa. Once its all wired up, the thickness of the layers will only be about ?? to ??.

            I just use bagged pre-mix mortar mix ? the kind used for regular brickwork, or what they sell here as ?topping? mix ? without adding any fireclay, alumina etc.

            Another key thing with ferro cement is that the mortar has to be applied in one sitting. Start at one end of your piece and keep packing the mortar in until you get to the other end?You always have to work to a wet edge. (As you noted, boats are built with this stuff?the people that build ferro cement yachts line up lots of help so they can mortar the entire thing in a day, which is supposed to be quite a sight to watch). Your plan to do a couple of layers, add more chicken wire, and then more mortar is likely to fail as the second layer will not adhere well to the first (now if you wanted to insulate between the 2 layers, that could work?but then you lose the radiant qualities of a chimenea). The pizza oven shell wasn?t too bad to do because it was small but the firebeast was pushing the limit for just me alone. Luckily the boy was home from college, so I enlisted his aid?I put him inside the firebeast so he could brace up the inside with his hands while I packed in the mortar from the outside. It took some real coordination once the mortar got higher than his head because he couldn?t see where I was packing mortar and it was little tight up in the chimney area. I finished the firebeast off with a chimney piece that I made separately. I figure the area where the two pieces join is the weak spot in the thing, but so far its held up okay.

            A long slow moist cure is the next key thing with any ferro cement, but especially ferro that?s going to see fire. 28 days is really optimal but definitely nothing less than a week. Spray it down good and wrap it completely in wet burlap and then plastic if you can. I didn?t have enough plastic on hand for the firebeast and I had a few spots on the inside that dried too fast. Your plan to use a beachball on the inside will work to your advantage (if the chicken wire doesn?t puncture it)

            After the mortar fully cured, I then coated the inside of the firebeast and as far up the chimney as I could reach with a mixture of Portland, sand, fireclay and kaolin clay. The pizza oven had a few other layers of pottery clay, etc, so read that post again if you want to follow that lead. I coated the outside of the beast with a stucco of fiberglass re-inforcing cement (sure-wall). You really don?t need this but I add it because everything else is covered with it and it ties it all together.

            I guess the firebeast has had a full roaring fire 15 times now. At full roar, the outside of the chimney just above the firebox area has measured out over 900 degrees before, but I try to avoid getting it that hot. That time several paraffin coated boxes were thrown in the fire?there were flames shooting out at least 10 feet HORIZONTALLY From the top of the chimney (because of how I finished the top with a rain cap) and it didn?t seem safe. Usually, it cranks along at about 650 degrees, which is plenty hot for a chimney. The fireclay lining has spalled off in two small spots on the inside that just happens to correspond to the areas that cured too fast. And the stucco has a few heat expansion cracks on the outside that I need to patch. Not bad considering the total construction cost was less than $300 including the tilework.
            Paradise is where you make it.


            • #7
              Re: Using Porland/Fireclay cement for a Chiminea?


              thanks for your excellent story, I wish I could see this beast ;-)

              Sorry i did not reply sooner, the notification when replied did not work for some reason.

              Now I am definitely going through with it, nay-sayers be... said nay to.

              Just one question, what is your theory on way the cement does not crumble
              after being in contact with so much heat. You are using just portland cement in your mix so regular theory says that fire ruins concrete.
              Is the added ferro such a contributor to the overal strengt that although the cement might become a little brittle the ferro keeps it together?

              Also, you mention you use a finish on the in and outside. Is this purely for cosmetic reasons or do you think it help for fire proofing as well?

              Thanks again, I will put my project up in the fireplace forum when it's done.



              • #8
                Re: Using Porland/Fireclay cement for a Chiminea?

                Originally posted by mourits View Post
                Just one question, what is your theory on way the cement does not crumble
                after being in contact with so much heat. You are using just portland cement in your mix so regular theory says that fire ruins concrete.
                Is the added ferro such a contributor to the overal strengt that although the cement might become a little brittle the ferro keeps it together?

                Also, you mention you use a finish on the in and outside. Is this purely for cosmetic reasons or do you think it help for fire proofing as well?

                Theory as you put it is probably correct. Also, really attribute strength to the long wet cure.

                Finish on the outside is purely cosmetic. Started with it because my brickwork for the oven complex was so lousy. Once I started covering with a stucco, everything had to be the same.

                The inside may have some heat proofing properties. Possibly due to the high alumina content of the kaolin clay I add to the mix. Maybe somewhat to the fireclay. I don't really know. It works, so I continue to do it (but then again, the sun always returned in the springtime after the ancients sacrificed a virgin on the winter solstice...and that wasn't necessarily cause-and-effect, was it?). I suppose one of us could make two identical fireboxes and test one with, and one without the inside coating.

                I keep trying to find somebody around here that will make an oven with my method to make sure mine just wasn't a fluke...but once they come here, they don't want to go back to cooking in their own yards!
                Paradise is where you make it.


                • #9
                  Re: Using Porland/Fireclay cement for a Chiminea?

                  Here is a quick update. (a complete guide with pictures will follow).

                  I finished applying the chicken wire yesterday and hope to start slapping on the cement tomorrow. The mesh consists of 4 layers of chicken wire (with ridges pressed in to them to get more thinkness and (hopefully) get the cement to stick better.

                  i will re-inflate the bunny ball inside before starting with the cement.
                  My only fear is that the cement won't hold enough on the underside of the sphere before it cures. Perhaps I will have some sheets of card board that I can stretch underneath the ball immediatly after applying the cement.

                  Also am curiuous how heavy the cimney will be. The cage is pretty strong now as it is but I would hate to see the chimney sack to one side. Will probably have to support the chimney horizontaly with some ropes.

                  Anyone with tips about applying the cement would be very helpful.

                  Well, that's what we in New Zealand will be doing around christmas: BBQ-ing and playing in our Shed. ;-)



                  • #10
                    Re: Using Porland/Fireclay cement for a Chiminea?

                    about the only tips I can offer at this point is to take a handful of mortar and just squish it in. Don't mix it too wet.
                    Put lots of plastic down around your work area.
                    If you can do it with gloves on, so much the better. I can't do anything wearing gloves, so its always bare-handed. If you go that route, rub in plenty of lotion ahead of time...even then, your hands will be so dry at the end of the project.

                    Supporting underneath .... i've always been able to pack in from both sides of the wire, so this hasn't been a problem. Some mortar falls through but I just scrape it back up and repack that area again. Eventually it all sticks. Might not be as easy with the inflated ball in place...or it may be easier!

                    Good luck with it. May be you are a starting a new Kiwi Holiday tradition.
                    Paradise is where you make it.


                    • #11
                      Re: Using Porland/Fireclay cement for a Chiminea?

                      There are times when you just have to get your hands into the mortar i have been using a product called gloves in a bottle , hand blocker it really helps and a trick I learned about 1/2 way through my oven , when you stop work , rinse your hands in plain vinegar.. it seems counter-intuitive but the vinegar seeks out all the bits of lime in /on your skin and neutralizes them .. it works!


                      • #12
                        Re: Using Porland/Fireclay cement for a Chiminea?

                        Another update.

                        I am writing a full how-to for instructables.com so just a few quick notes here.

                        I just applied the second batch of cement today and finished the second half of the sphere and the chimney.
                        I just have to wait for it to cure and attach a spark-catching hood on top of the chimney and we can do a first burn. (see pictures)
                        • I decided to take cvdukes's and the guy at Wellington's Ablaze firewood emporium's advise and went for normal Portland cement. No, fireclay or other additives.
                        • I added iron oxide to color the concrete black. The end result is more dark gray but it's fine for this version 1.0
                        • Don't get 'concrete' ready mix. It contains gravel (duh) which makes it impossible to push the mix in the mesh. I had to sieve it out.
                        • I made the mesh way to thick. I made ridges in the chicken wire to give it more volume but that was not necessary and made it harder to get a smooth finish.
                        • When the chicken wire is flat and straight it is actually very easy to apply the mix (I tried both a wet and a drier mix, both were fine) and the result is incredibly strong for such a thin layer.
                        • Unfortunately the concrete will only stick where the chicken wire forms a nice even surface. Just pushing some scrunched-up wire in an opening and hoping the mix will stick to it will not work.
                        • Working with chicken wire is very painful! Every bit you cut of and every piece of metal wire will try to cut you and for some reason the cement mix creates nasty infections.
                        • The skippy ball had to go straight away, you really need two hand (in gloves) pushing fromt both sides.
                        • Of course you need 2,5 times more cement than you thought. I will be making the chimney lower because of this.

                        So, next post will be the complete story with all considerations included.

                        As of now, assuming the chimenea will not explode or fall apart after the first burn, I think ferro cement is good technique for such a project with two reservations:
                        - Preparing the chicken wire mesh should be done with a lot of care to get the surface as smooth and even as possible.
                        - Prevent having to cut the chicken wire. It is like working with a pin cushion. (with the sharp end pointed outwards)
                        - Do a trial pour to see if you like the texture and color of the concrete (many colors are available). Rough concrete by itself is butt ugly.



                        • #13
                          Re: Using Porland/Fireclay cement for a Chiminea?

                          The mixture cvdukes suggests for the inside can be improved with the addition of lime. I use equal parts of sand, cement, lime and clay. If you want the mixture to be white, say for coating the inside of a dome, then use white ingredients ie white or silica sand, white portland cement, lime, kaolin.
                          Kindled with zeal and fired with passion.


                          • #14
                            Re: Using Porland/Fireclay cement for a Chiminea?

                            Thanks David,

                            I agree that the finish needs work.

                            I would recommend to anyone who makes his first attempt and building such a thing (from either cement or bricks) to make a cheap, trail version first to see what works and what looks good.

                            I am very happy with my slightly rough and cheeky looking result as it was cheap to do and very educational. For a next version (that would probably stand in our, yet to aquire, garden in Amsterdam) I will be paying much more attention on the innner and outer coating and color.

                            One question: How thick would such a coating be and how should the main wall off the room be prepared for the coating to stick properly?



                            • #15
                              Re: Using Porland/Fireclay cement for a Chiminea?

                              The coating for the inside, as suggested, just apply by painting on a good thick paint-like layer.
                              Kindled with zeal and fired with passion.