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Hearth design

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  • Hearth design

    Hi everyone.

    I am starting to build a WFO.
    I've seen many plans and got an general idea of what I want.
    Materials here in Portugal are cheap but sometimes it's difficult for me to 'translate' the exact material.

    The concrete slab is already build and I will use a 30mm calcium silicate board as a bottom hearth insulation.
    My question concerns the layer between the CS board and the oven's floor tiles: since I will be using 'special' 30mm tiles for the oven floor (25x25x3) I guess I will be needing a layer to add some thermal mass under those tiles. I was thinking in making a 25mm-30mm additional refractory mortar layer for that purpose.

    What do you think?

    Thanks in advance.


  • #2
    I have a similar question - I've heard of people putting a thin layer of sand on the CalSil board (mainly to level) and then putting the firebricks on the sand for the floor. I have 2" 12x12 squares for the floor. Does anyone see a problem with this design? Small backyard operation mainly looking to cook pizzas, veggies, and some roasting.


    • #3
      Sorry for the late reply. I'm glad that Jon bumped this post with his question. I'm not sure where you are on the build, but a secondary insulation layer under the CalSil will help a lot with this design. You said that "material is cheap". Another layer of the CalSil would be great. You mentioned "refractory mortar" for extra thermal mass. If, you could add some crushed firebrick to that mix you should be ok on the shrinkage.

      Jon, If your CalSil layer is level and your tiles are uniform, you won't need any thing betwenn them imo.
      Joe Watson " A year from now, you will wish that you had started today" My Build Album / My Build


      • #4

        Thanks for the reply.
        However when I said "material is cheap" I was thinking of the 'general' materials: brick, mortar, etc. The only ones that are actually not cheap are the calcium silicate board and the fiber ceramics blanket. These 2 are not commonly available here so they cost a little more.

        Also, the silicate board I got here is not exactly like the ones you use there. The only ones available here seem to be commercial high-end products and judging by the technical data the 30mm ones insulation capacity are similar to the 50-60mm regular ones.

        Since my floor bricks are only 3cm high (about 1.18") I was thinking in adding a layer between the silcal board and the floor bricks.

        One question that I have: what exactly is the difference between refractory mortar and refractory cement?
        Here we have both: one just needs water and the other needs sand and water. What is the best to use for this layer and for the brick dome?



        • #5
          The floor tiles being 30mm or about 1.18" are a little light on the thickness side for thermal mass. It will work but the floor will not hold heat as well as a thicker floor. If your budget can afford it, a second layer of 30mm floor tiles and 30mm silcal board would be ideal. Without knowing the specs, refractory mortar vs cement, you answered you own question by adding sand vs not. Do look at maximum joint size on refractory mortar though, sometimes it can be very small, ie 1/8".
          Google Photo Album []


          • #6

            Yes, I know 30mm on the floor is not ideal. That is why I want to add another 30mm or so for thermal mass. Laying another layer of brick tiles is not an option because these particular tiles have a special coating on top (kind of a vitrified skin).

            I was thinking in adding an extra refractory cement layer between the floor tiles and the calsil board.

            Will it be ok?


            • #7
              “Special coating, kind of vitrified skin” sounds very much like glazed tiles, in which case they may be insuitable. Rather like cooking a pizza on a tray when the moisture gets trapped under the base resulting in a soggy base. Unglazed firebrick is porous and that moisture is drawn away. If this is the situation you might be able to turn the tiles upside down so you can cook on the unglazed surface provided it is flat and not ribbed.
              Kindled with zeal and fired with passion.


              • #8
                Ok... so I have my dome ready. The brick tiles where not glazed. They were regular ones.

                Now I have another question:

                I have 2 layers of ceramic fire blanket to cover the dome but I am worried that the dome might get some cracks eventually (they always do) and that the small pieces of the blanket might get into the food.
                I have read somewhere that you could use aluminum foil to cover the dome before you lay the ceramic blanket but it seems a bit odd.

                What can I do to 'seal' the blanket so that it never gets in the food?



                • #9
                  I did the expensive option and covered the entire dome with refractory cement before putting on the blanket... this totally sealed the dome and prevents any smoke from leaking out. It also adds to the thermal mass. I did a bunch of test fires once everything had cured and dried. Worked without cracking so far.


                  • #10
                    Will that refractory mortar layer be enough to seal the dome from ceramic fibre particles?


                    • #11
                      That’s my understanding. From the research I did, the divers from the blanket and actually stable except at the cut edges. Make sure you wear a resperator and use coveralls when you are working with the blanket. !


                      • #12

                        So... I have my WFO almost finished. I only need to finish the area around the vent arch and it is done.

                        I am already starting my curing fires but I have two questions:

                        1 - When can I paint the dome? Can it be immediately after the curing fires or do I have to wait some more time?
                        2 - Do I need high temperature paint for the vent arch area?