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Refractory - Forno Bravo Forum: The Wood-Fired Oven Community


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  • Refractory

    Is there any reason I should not build my oven using just refractory?

    I presume that's what the Forno Bravo ovens are made of.....and I might have a line on two types of refractory.. one is Sairset: it's wet and in buckets. The other is sacks of dry mix labaled "refractory" and is from Harbison-Walker.

    Any reasons I couldn't build a hearth or a dome or both from just these products?

  • #2

    Nope, no reason at all, but it's tricky stuff to handle. We can discuss further, if you like.

    "Made are tools, and born are hands"--William Blake, 1757-1827


    • #3
      I sure would like to discuss it further!

      I'm going to guess that uniform thickness is one thing to be aware of and strive for.

      I have worked with concrete before but that's about the extent of my experience with masonry. I'm about ready to pour the foundation and begin building the base for the oven; it's going to be made of stone which is in abundance on my place. I've been gathering, sorting and practice-stacking all winter. Now, Spring is here and it's time to get busy.

      Grab a homebrew, Jim, and let's talk.


      • #4

        First off, I'm not familiar with the brands of refractory you mention, but that should not matter. I used LaFarge Fondue. The bags list two formulas (not the same ratios as ordinary cement or mortar) by weight. It's important to remember that the hydration level is only a guideline, but not so sand and aggregate. For the slab and cladding, be careful with your aggregate. I used washed P-gravel. Terms vary across the continent, but these are round white stones about the size of a green pea; same stuff sometimes spread dry for walkways. For the mortar, I used the finest, cleanest brick sand I could find. There are others, but these were what was available to me at the time. Clean is an absolute: no mud, debris, beer caps. Strength is what you're after; dirt weakens the set concrete or mortar.

        I've handled tons and tons of mortar and cement in my time, both professionally and personally, and refractory has a different set of rules. I had trouble with it at first, and I'm very experienced. Prime among them is that it sets very, very quickly but cures slowly. Second, it will stain absolutely everything, including you. Soak your tools as you work, otherwise it's there for life. The shiny interior of your wheelbarrow is just a fond memory. The upside is that it's nowhere near as caustic as, say, Portland. One provisio, if you're tempted to use Portland for block work or stone laying, don't. It does not have the stickiness you need and has very little flex to allow for expansion/contraction (cracking will happen). Use Type S. It has a bit of Sealbond in it for stickiness, it's more resistant to weather and has enough flex. For a foundation pour, Portland is fine.

        However, like all mortars, refractory is dependant on humidity and temp. Dry and hot equals more water. For brick work, you must soak each brick in water until it stops hissing, or else the mortar will not stick to it, because the porous firebrick will instantly suck the moisture out of it. Mortar should be mixed in small batches (say enough for about ten bricks at first, until you find your personal speed) and kept out of direct sunlight. BEFORE you start laying brick in earnest, build a small practice wall to get the hang of refractory. I'm talking here about the hydration level of the mortar and the wetness of the bricks. Have an old paint brush in a pail of water nearby to wet mating surfaces like the hearth bricks to the wall/dome bricks.

        Pay attention to the formula on the bag, sure, but be prepared to add more water; you want it to set up on the trowel, not run off the edges, turn the trowel sideways and you should have to shake it once to get the mortar off. Try to get the right hydration level from the very beginning, because, unlike conventional mortar, you can't rehydrate refractory more than about two times before you get stone. This is a chemical reaction that cannot be stopped. My experience is that you'll find it looks just right, then five minutes later it's too dry; add a bit more water to adjust for this. The mortar should be shiny when it's right and hold the mark you make in it with the point of your trowel. Dang, this is much more difficult to describe than show.

        Do yourself a very large favour and buy a very good brick trowel (Marshalltown [top of the line] or Rose), because the way it's made will save your forearm and wrist. Each brand has a different handle diameter and a different handle to blade angle. If it feels good in your hand at the store; it will feel good when you use it--a lot. Same goes for other masonry tools.

        Because refractory sets so fast, you will have to point your joints much earlier than normal. Buy a set of pointers from Home Depot. There's a pic on this forum of my masonry tools. Pointing has to do with strength and making sure all joints are full. The joint should shine slightly if you do it properly, because the remaining water has been pulled to the surface by the action of the tool. Keep your brick joints at 1/8" because the mortar joint is the weakest point. I'll walk you through pointing later if needed.

        For the slab and cladding, make your mix pourable but not sloppy, no pools of water on top. But, you don't want it so dry that you'll be poking at it to fill gaps. Do yourself another large favour and rent a mixer on the days you pour. Just be sure to clean it out right away. For the slab, bear in mind that refractory shrinks more than ordinary concrete. Concrete gets warm as it sets and cures; refractory actually gets hot. Once you've levelled and smoothed your slab, cover it with the burlap you bought from Home Depot and keep it wet for about three days (depending on weather), out of direct sunlight (tarp), but do allow for air circulation. This will minimize shrinkage but more important it will maximize strength. Same goes for your cladding. Once it has cured to gray (it's black when wet), off comes the burlap, but now you must keep it dry, dry, dry. Refractory cement is used to make things like steel mill floors, and it's very resistant to impact when cured. Any sloppy bits that might be in the way later will be in the way later, so chip them off soon as you can; otherwise dynamite.

        All this wetting will affect curing and drying times, but there's no way around it.

        The one major thing to keep in the front of your mind when working with refractory is not to get frustrated. It's not a race, work quickly, but take your time. If it doesn't look right, knock it down and do it again. You'll only be doing this once, for a lifetime. Plan the positions of your thermocouples carefully if you'll be using them. Suggest you use about four.

        "Made are tools, and born are hands"--William Blake, 1757-1827


        • #5
          tools shots

          that Jim is talking about can be found on this thread



          • #6

            Thanks for this lesson in refractory mortar. I have purchased Alsey flueset refractory mortar and I am going to start laying up the acutal oven part this weekend, so your timing is perfect! (I have also really appreciated all of your posts on bread making, those baguettes looked amazing!)


            Are you asking about casting your own oven from refractory concrete instead of using bricks and refractory mortar? That is how I interpreted your post...
            I am no expert, but I did purchase some castable refractory concrete in order to make a vent like kiwipete did in his oven. The castable refractory concrete appears to come in a few "flavors". Some are insulating and some will add mass to the oven. I believe all of them gain strength through heat curing. For the vent, I am planning on "Baking" mine in my regular electric oven. I am not sure you could properly cure an oven that you cast yourself...

            If I totally misinterpreted your post, please ignore me

            My Oven Thread:


            • #7

              You can also look at Refrax. There is a posting on it here. It's an air drying refractory mortar made specifically for pizza ovens (and fireplaces). Hard, heat resistant and fast drying.

              Basically everything in your oven should either be thermal, in that it is dense and does a good job of absorbing, holding and withstanding heat, or an insulator. It's interesting that high heat refractory products and insulators are made from the same minerals, because they are heat resistant. The insulators have lots of little air holes, which is why they insulate, and the thermal products (like a Forno Bravo oven) are vibrated to get the air holes out.

              Refrax is very dense, where Insulfrax and Vermiculite are very light.


              Pizza Ovens
              Outdoor Fireplaces


              • #8
                Go away to Chicago for a couple weeks, and see what happens. Well, you've given me plenty to think about.

                The plan at this point is to do the foundation slab with premix, using rebar or mesh.

                Hearth slab (4") will be made using vermiculite.

                You are right, Drake, I was thinking about casting the dome from this refractory. I have some buckets of Sairset, which is wet and in buckets. I also have some bags of dry mix simply labeled "Refractory".

                I guess I will experiment some with the refractory. I had thought that since I had a bunch of it given to me I might try forming the oven with that, using a sand dome as a mold. I suppose I can build a really small version and just see what happens. I also have a bunch of firebrick so I might just do it that way too. Decisions, decisions. And all this has to happen in between work, and I'm in my busy time of year. Too bad I can't do the construction during winter, when I actually have time.


                • #9
                  "Hearth slab (4&quot will be made using vermiculite." ???

                  Originally posted by Kinsman

                  (M) <snip>

                  (K) "Hearth slab (4") will be made using vermiculite."
                  (M) I just want to be sure that this wasn't a "typo". The Hearth slab is typically a 2 layer project: Insulating mortar, made with Vermiculite or Perlite, and the other layer is a refractory mortar. Which should be on the bottom is a subject of much debate.

                  (M) You really need a layer of strong, steel reinforced refractory mortar, typically about 3" thick. Your heavy dome needs a strong support. If you use *only* "perlcrete" or "vermicrete" you take the chance that your dome will not be supported!


                  "Everything should be made as simple as possible, ...
                  but no simpler!" (Albert Einstein)


                  • #10
                    Whoops, sorry......

                    Stand made of stone, mortared together in the usual way.
                    On top of that, 3" or so of reinforced concrete, as a base for the hearth which will be:
                    3" or so of insulating concrete, topped with firebrick for the actual hearth.

                    I'm still debating whether to use pure refractory for the dome.

                    Cladding of stone and brick.


                    • #11
                      Back to the hearth

                      I think of the hearth and cooking floor assembly as serving three functions:

                      1. Structural support -- holding the oven up and not sagging. Nothing thermal here, which is why you can use standard concrete (cheap) with rebar.

                      2. Insulation -- keeping heat in the oven and floor. That's the insulating concrete layer, with vermiculite held together with portland cement.

                      3. The cooking floor -- which is thermal. You want it to heat up, hold heat, and efficiently re-charge the heat in the floor from your live fire. The firebrick on its flat side gives you enough mass for typical backyard cooking and baking, and doesn't have too many seams. The Forno Bravo ovens have 2" floors that come in pie-shaped pieces. If you really want a little more mass under your floor (don't forget that commerical pizza ovens only have 3"-4" floors), you can add more mass under the floor, either with a second layer of bricks, or a poured disk of refractory mortar. That's the Island hearth (the extra mass under the cooking floor is surrounded by vermiculite concrete).

                      That's why we have formally changed the recommended hearth design. For most installations it is good to put the cooking floor directly on the insulating layer, and if you want more mass you can add it, while taking advantage of a more efficient design.
                      Pizza Ovens
                      Outdoor Fireplaces


                      • #12
                        High alumina or refractory cements are ok for casting parts for an oven or furnace, Sairset isn?t, it?s a different animal and should only be used for thin bed firebrick work. As with all concretes it?s a combination of the right type and quantity of cement and aggregate. The Forno Bravo ovens are cast using an aggregate that is probably volcanic and therefore very good at holding heat. When you get the chance to look at a broken piece of an FB oven lying on the floor after having been dropped one is amazed at the different colours of aggregate in the concrete.

                        We use an aggregate called ?grog? that is derived from crushed pottery waste i.e. material that has been fired to a high temperature for castings, however, good quality washed sand and gravel should work ok Interestingly the international company Harbison-Walker who have a plant in Liverpool and specialize in refractory ready mix cements and coatings get aggregate from the sane guy I use.



                        • #13
                          Re: Refractory

                          Any progress on your "cast" oven? I'm getting ready to do something similar and would like to take advantage of any experience you've gained?


                          • #14
                            Re: Refractory

                            I have been told the refractory concrete I can get has a set temperate of 1250C does that mean I can't use it as the oven will never reach that.

                            Also how thick can I trowell the refractory concrete on the bricks?

                            Thanks in advance for any help.


                            • #15
                              Re: Refractory

                              When you say concrete I assume you mean mortar. If the mortar truly will not set until it reaches 1250C you may not want to use it because your oven will not get that hot. It is my understanding that some refractory mortar will not fully set until you fire them, the most basic of which is fireclay. Are you able to get portland cement...hydrated lime...fireclay? Or better yet calcium aluminate cement instead of portland. If so you may want to go the route of mixing your own.
                              Just a thought!
                              As for the thickness...you will want to strive for minimal to no mortar joint on the inside face and about one eighth of an inch on the outside face. That should give you an idea of how thin rather than how thick.
                              Hope this helps!
                              "Making the simple complicated is commonplace; making the complicated simple, awesomely simple, that's creativity. " Charles Mingus
                              "Build at least two brick ovens...one to make all the mistakes on and the other to be just like you dreamed of!" Dutch