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Between Hearth SLAB and Hearth BRICKS, slip plane.

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  • Between Hearth SLAB and Hearth BRICKS, slip plane.


    (M) On

    I read:

    (JB) "Do not mortar the floor in place, but rather spread a thin layer of sand and fire clay to provide a slip plan between the oven and hearth. That will allow them to expand thermally at different rates."

    ================================================== ======

    (M) This will be a QUESTION, not a suggestion!

    (M) If I understand the purpose of the thin layer of non-adhesive sand and fire clay correctly, it acts as insurance against different rates of expansion of the Hearth SLAB and the Hearth BRICKS to prevent cracking. But since neither has adhsive to lock them together,the more important reason seems to be:

    (JB) " Tap the floor until it is smooth and level (photo 5). Check again for level, as this is your true cooking surface."

    1- (M) The mixture recommended seems to be designed to have thermal properties similar to that of the fire bricks. If so, then I would expect there to be heat of conduction transmitted through the BRICKS to the Hearth SLAB? ____

    2- (M) Please correct this next assumption but if a builder were to use e.g., Perlite, rather than the clay and sand mixture, wouldn't the slip plane be somwhat of a very thin insulator? _____

    3- (M) If "yes", then could that be an advantage in forcing the accumulated heat up and outward to the food, rather than downward into the far corners of the Hearth SLAB? ____

    4- (M) Is there a concern that providing a thin insulating layer between the bricks and the slab might result in too high a surface temperature? _______

    Thanks for your input(s)


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

  • #2
    Good questions

    I got the feeling that the un-mortared floor bricks were to make the replacement of a damaged floor brick possible, short of attacking it with a sledge hammer. Does the floor get damaged? Let's hear from those who know. The fire clay level has the same thermal properties as the fire brick floor, and the thermal layer below it.

    Is there too much thermal mass below the floor? I think so but I don't know. James new instalation has half thickness firebricks layed into the perlite concrete as a variation of the "island" hearth. We all look forward to hearing how that works. For myself, I'm not convinced that we need even that much, that the fire brick floor could profitably be put directly on top of the insulating layer.

    I don't think this fire clay layer functions as a slip plane at all. Everything above and below it should expand at the same rate. There was earlier discussion of whether even the flashing below the main slab was necessary.

    My geodesic oven project: part 1, part 2


    • #3
      Yep. The layer between the cooking floor and insulating hearth is not a slip plane, but more that you are looking for a conductive and easy way of making the connection. The Forno Bravo prefabricated ovens have you set the cooking floor pie shaped pieces to the hearth using refractory mortar and a knotched trowel -- like a big floor tile.

      It is true that the clay and sand method let's you remove a broken or chipped floor piece or brick more easily, but that has never happened to me. I can't imagine wearing out a floor -- though pizzeria ovens change the floor every 10 years or so (after 24 hours per day use).

      Your choice. :-)

      Is the 3 1/2" concrete layer set on the 2" vermiculite layer too much mass? It depends on what you are wanting to do with your oven, but I think the answer is "probably." I set my Casa90 directly on the vermiculite layer at our rental house last year, and the oven was great. We set our Casa90 in the demo kitchen here on firebrick splits (1 1/4") to increase the mass a little, without impacting the floor's ability to really hold heat, and cook pizza all night.

      We're making good progress and will have more to report on how it is cooking soon.

      Pizza Ovens
      Outdoor Fireplaces