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Fire brick advice please high density or medium..

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  • Fire brick advice please high density or medium..

    Hi All,

    I have started my base for a 105cm brick oven and am starting to get my materials together for the oven. I am looking for some advice on fire bricks. I am able to get some high duty fire bricks, but am not sure if they are suitable or possibly too high a grade. I am sure I have read somewhere that medium grade bricks are better than high? The bricks are 42% alumna and are heavy, These are designed for use in large Furness’s and heavy use, they are 230x114x76mm.
    would these be suitable or am I better getting a medium density brick.

    any advice gratefully received…

  • #2
    Hi Stalbot,
    Welcome to the forum. There is a lot of confusion in the terms low, medium, high, and super as they are used to relate to firebricks. Regardless, even if the terms "duty" or "density" are used along with those terms. All those terms can vary from manufacturer or the end user. Thankfully, the alumina content can be used for our purposes. Minimum duty firebrick for residential fireplaces will usually be found in the 23-26% alumina content range. They are just fine for both the floor and dome of a wood fired oven. Medium duty can range around 30+ percent alumina. I've seen many who call any firebrick above 48% high duty. The super duty firebrick that I am familiar with are in the 70+ alumina range. "Duty" "Density", and "Alumina content" all relate to hardness.

    Forno Bravo recommends medium duty. for the dome and minimum duty for the floor. However, most ovens in the US are built completely by the firebrick that can be sourced. That is is usually the fireplace grade. They have held up just fine for our purposes. 42% Alumina is at the upper end of medium duty in many folks terms. They would make a fine oven if the price is right. Just remember that the higher the alumina content the harder they are to cut. That can result in more or more expensive diamond blades.
    Joe Watson " A year from now, you will wish that you had started today" My Build Album / My Build


    • #3
      Thanks for that excellent explanation.
      Just to add a couple of extra points, the temperature of an oven is only around 1/2 that which a kiln would see, so the high content of silica and associated fluxes in a brick, that can turn to glass at high temperatures, is not an issue. Some brick specs will give iron content as well as alumina content as iron is a strong flux (over1200C).Therefore the use of a high duty brick may be simply a waste of money, for the more expensive bricks. Likewise the temperature the brick has been fired to is less important than the materials it contains. Sometimes the reverse can occur as in the Sth. American pottery that has high thermal shock resistance, but is made from a very open clay body, high in iron, but fired to a low temperature (600 C)
      The other point is that it is the floor that gets a greater beating because of the greater temperature rise rate as well as greater difference in temperature throughout individual bricks.So, use fire brick there, whilst most domes will cope ok with standard solid reds.
      Lastly, to be kind to your bricks, heat them slowly and avoid forced air induction.
      Last edited by david s; 03-15-2023, 09:14 PM.
      Kindled with zeal and fired with passion.


      • #4
        Thank you both for your reply’s …. I guess my dilemma is I can purchase the 42% alumina content bricks for less than the cost of lower duty bricks. This is due to someone wishing to clear a large quantity at less than cost.

        So from what I read, the only disadvantages appear to be increased weight and hardness meaning they are going to be be a lot harder to cut and work with on the build. Also the weight of the finished oven is going to be significantly heavier - My base is 10” of concrete reinforced with mesh, so assume will be ok.

        The specification on the bricks say they can withstand temperatures up to approximately 1600 degrees, so way over the top for what I need clearly.

        I guess the density and mass is going to impact fire up times as well?, with I assume a denser brick requiring more time and energy to reach 500 degrees? I guess also that will reflect in the residual cool down meaning it will be able to be used for longer as the cool down is slower?

        I guess these bricks are tempting given the cost, but I don’t want to be in a situation when I have to be waiting 5 hours to fire the oven up to temperature and have an inefficient oven.


        • #5
          UtahBeehiver used 70+ alumina brick on his oven. I don’t remember any comments about extremely long heat up times. I’m sure that he will comment here about that.
          Joe Watson " A year from now, you will wish that you had started today" My Build Album / My Build


          • #6
            As Gulf said, my build was done with surplus "super" duty bricks that were surplus from an old steel mill. Super duty are tough on diamond wet saw blades, I went through a dozen but I also did full bevel and tapers on my dome bricks so a lot of cutting ( I wanted 1/6" joints and to show it could be done, but is is not worth the time and effort). Look at JR Pizza's build, he minimized the amount of brick cutting by only beveling the first 25% of the inside of the dome brick, and did not taper the bricks and used mortar to back fill the joints. I would do this on my next build as well. My oven has a 3" floor, 4.5" walls, and 42" ID and it takes about 2-3 hours to fully saturate with heat. David S ovens heat up much faster due to the smaller ID.
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