K-23 Insulating Fire Brick.

[BenKeith]

I've just picked up 60, K-23 IFB between my castable refractory hearth/floor of my oven and the concrete slab.
My friend that installs and maintains industrial refractories assures me, one layer of these 2,5" bricks laid flat will be more than enough insulation between the 2.5" of denscrete castable I'm using on the floor and the concrete slab the oven will be sitting on.
Has anyone on this site used or have experience with these bricks to verify one layer will work or if I should turn them on the side for 4.5" or put a double layer for 5". I would hate get this thing built and find the slab getting too hot.

[UtahBeehiver]

For a perspective, CaSi board has a K value (thermal conductivity) 0.05 W(m/K) vs IFB of 0.15 so CaSi is three times more efficient, thermally wise, than IFBs. There have been several Aussie builders who have used IFBs for floor insulation.

Here is a link to Karangi Dude's big 48" build with a IFB floor insulation.
https://community.fornobravo.com/for...nch-oven/page2

[BenKeith]

Thanks for the link. Looking at the numbers he recorded and posted on the temps he was seeing with the IFB's under the floor, I might want to look at adding a second layer. After several hours, With less than 500F on top, he was reading 150 on the bottom of his fire bricks. At 800F, that might be a little more than I want on my slab. and a little more than I want to waste.
However, I also can't find what grade he used. There's a huge difference in the different grades. K-23's are up the grade scale pretty well and very pricey if one had to buy them from the average retailer. I've seen them going for almost $5 each plus shipping. That's probably more than most would want to spend to insulate under the floor.

[UtahBeehiver]

Based on the data of the following post from KD I do not see the high temps "under" the IFBs you are talking about. I see 66C at the concrete hearth when the top of the fire brick floor temp is 302C at equalised temps.
https://community.fornobravo.com/for...nch-oven/page8

[BenKeith]

Probe 6 shown between the IFB and the slab is the point I was referring to, and after a few hours, he's showing it to be 66C with the temp. With an air temp of 575 and a floor temp of approx. 560F, I would think by the time it was at approx. 800F for a while, that number at probe 6 would start getting a little on the hot side.
However, not knowing exactly which IFB he used, it's only a wag at what mine might do. Plus I'm using 2.5" of 2700 degree castable denscrete refractory on the floor and not sure which fire brick he used.on the floor will make a difference also.
Just to play CMA, I might go ahead and get enough brick to turn them on edge and give me 4.5" between the floor and the slab. I'll probably have to do some experimenting and see.
I will also be adding a couple of thermocouples to monitor the floor and air temp inside the oven, but not to the extent he went. I just want to know what the cooking temp of the floor and the air inside is.when using it. Plus I will have a manual thermometer sticking in it somewhere.

[UtahBeehiver]

If it is a concern, then on edge will work for the IFBs. I have TCs in my dome wall to check heat saturation but I have not used them for a long time, for government work, I just use my thermal gun to check temperatures and for all intents and purposes this is all I need for my type of cooking, basically, pizzas (day 1), then bread (day 2), then roasting of some sort (day 3). I did notice that there is not a huge change in IFBs K values between K23s and higher temperature rated IFBs, ie K25 or 26. There is a point of diminishing returns on over thinking material needs on these ovens.

[BenKeith]

It sounds like you use yours about the same as I plan to use mine. My daughter likes to cook on a grill so she's interesting in seeing how it does for that.
The thermal couples will just be for the initial learning about how it heats up and holds the heat. After the initial learning curve and get comfortable with how it cooks, I doubt I would be using them. I will use one good one in the dome, anything else will those cheap chines you can buy for little of nothing.
I know it's easy to over think this stuff, and I'm a good one for doing it. That's just like I started with plans to build a 42" dome, but then scaled it back to a 36" to reduce some of the cost. Now, half way through casting all my pieces, I keep getting the brain fart idea that I would like to go back to my original plans and do the 42". It took me a week to build the mold for the front 1/4 section with the door and vent section all molded into and have cast the piece. I was just looking at the mold today, seeing how much work it would be to change it for a 42". I'm casting a second one anyway so I figured why not, but it would be easier to build a new mold than try changing that one.
As for the fire brick. That's not something I can easily go back and change if I find it's getting hotter than I want and it doesn't cost that much to go ahead and put more in while I'm doing it the first time. Anything else like the blanket etc, I can play with before I close it up and I guess I could take the dome apart (I'm building it in modular sections), take the floor pieces back out (they are also in sections) and add another row of fire brick but that would a hellavalot of trouble just because I was trying to be a cheap scape to start with.

[BenKeith]

Well, I just had an oh, s**t! moment. Sitting here watching a little TV after casting another front section for my oven and realized, I forgot to rub the mold down with my release agent. That ought to make things a lot of fun when I'm trying to get it out tomorrow. I was actually thinking about casting one for my granddaughter but looks it probably won't be with that mold.

[BenKeith]

Well, as expected, the piece came out but the mold was trashed.
Here's where I'm at now.
The piece on the dolly was my first, it was going to sit on the oven floor that I was going to make 42" in diameter. Trying to figure out a good way to insulate the layout, I changed my design. I decided to put the floor inside the dome. To do that I have to make another piece that was 2.5" taller by adding onto the bottom, so I made the modifications to my mold and cast another piece. I made the floor 36" in diameter so they will fit inside. The layout I did for the floor was in a effort to keep it from cracking and all pieces have a 1" over lap on the joints. The Dome pieces will have a 3/4" over lap on all the joints.
Now I deciding if I want to do the rest of the dome in two or three pieces. I'm making them 3" thick and I have to handle this stuff by myself so doing it in two more is going to make them pretty darn heavy.

I guess I should mention. the floor pieces have a white looking coating on them because I'm using the paper used for beer and drink cartons to cast on and when I remove the pieces, some of the while outer paper layer, since its kaolin based, comes off on the pieces, and I haven't cleaned them up yet.

The joints also fit together very tight, They are just slid in position for the pic.

[BenKeith]

Finally, the last piece is cast. I cast this one where it goes on the oven with the floor and all other pieces in place. Since the floor is internal and using basic hand tools, band saw etc to make forms and cut molds\patterns and needing to maintain my 1/4 space between the pieces, I figured the best way to insure that was to cast it in place. You can't see them but there are 1/4" spacers between the joints of the pieces and the one I cast that will be removed tomorrow when cured. This way, there will be no doubt about the spacing and all pieces fit snugly against the floor. I did the dome in four sections, this was the last 1/4 section. With 10, 55 pound bags of refractory in the dome, each piece took 2 1/2 bags and weighs approx. 135 pounds.
You will notice I'm doing this on the back of my pickup. That way I can work under a carport for wife's VW convertible and then move it out and use front end loader to pick pieces up and move them around.

Also note as I mentioned once, when casting refractory dome with only an internal mold, moisture content is super critical. You might think it's dry just from looking at it, but when you start vibrating and packing it, it gets very wet, very quick, and if just a little too wet, it's not going to stay up. It's going to slide down faster than you can work with it. A little too dry and it's just not going to compact. Basically, if you see any sign of wetness after mixing, it's way too wet