28" homebrew cast oven in walled enclosure Belgium

[Kris S]

I'm not sure I completely understand what you mean, english is not my mother tongue...

I found this pic of one of your shallow flue galleries, I'd like to do something similar.
(here's the thread: https://community.fornobravo.com/for...lue-size/page2)




Trying to figure this out, is this diagram somewhat correct?
So you put the mould right up to the oven mouth, and the back edge of the chimney is also in line with the oven mouth to save space.
So the surface onto which the back cut out tab of the chimney rests is cast a couple of centimeters over (on top) of the oven mouth edge...?

If this is correct, I could shorten the flue gallery from 23.7cm to 15 + 3.5 = 18.5cm...

[david s]

There is a stack of room at the top of the flue gallery to set the pipe further back. If you make it start even higher by funnelling the gallery towards the base of the pipe you will have even more room to set it back further, making the gallery entry even shallower. The funnelled gallery will draw better, producing a smoother smoke flow and the shallow gallery makes the oven much easier to work, moving things in and out of the oven chamber. Because you don’t need the thermal mass in the gallery it can be made thinner. Around 35mm should be plenty of strength.

[Kris S]

Thanks for reassuring me David

Next question : I've been tinkering with the flue gallery mould, it's 23.7cm deep (9.3") for a 15cm (6") chimney, so I have 4.4cm (1.7") on either side.
It looks kinda deep, is it possible to reduce the depth by either:

* going even narrower than 4.4cm on either side,
* or by casting a bit more on top of the current oven mouth? is that strong enough, more likely to crack when heated? I could reduce the depth with 4.4cm that way (see second picture pls)

or just leave it as it is, because It won't be worth the trouble to gain 4.4cm?

note: chimney pipe is not the actual one I will use, and the mould isn't finished yet either.

[david s]

No, don't hose it, you don't want to get the underfloor insulation wet. Some of the water in the mix gets used up chemically in the hydration process, and for this to continue it needs a certain amount of free water to continue the reaction. It looks pretty good, just remove the sand tomorrow and cover it if it looks like raining. Once you've filled the voids keep it well covered for a week.

When you remove the oven mouth mould plate, use it to make the flue gallery mould plate, an inch larger all the way around. This ensures that there'll be room to fit your door in place.

You've just built your dome that would take the brick builders months to get to the same point, and it costs peanuts.

[Kris S]

I removed the plastic under which the oven was covered last night and today.
I expected to see the underside of the plastic covered in condensation, but this is not the case...

Should I be worried about that?

Should I give it a good hose?
I didn't do that yesterday, should I have done so?

here's a close up of the surface. I don't see any cracks or crumbly bits...

[david s]

You will find out when you remove the sand. It looks ok from the pics though. Too dry and you'll get more voids, too wet and the castable will slump and perhaps get some shrinkage cracks.

[Kris S]

Looking at some pictures of the casting stage from other forum members I get the feeling my homebrew looks a lot dryer when I put mine on...
I was cautious not to wet it too much, fearing it could sag under it's own weight.

[Kris S]

Next day was casting, I used the 3:1:1:1 homebrew mix with PP fibers, but no SS needles.
I first mixed all the dry components before adding water. I worked with batches of ~ 10kg dry and added about 2,5 L of water.
Starting at the base and working up to the top, went quite easy. I tried to push/press onto the newspaper in the hope to get a as clean as possible inside without too much wrinkes and voids...
There was quite a strong wind throughout, so fearing the newspaper would dry out and blow away I always kept the moist blanket on top as I worked my way upwards.

[Kris S]

Next step was super frustrating: getting the strips of wet newspaper to stick on the dome was nearly impossible with winds up to 60 km/h.
Had to call the missus to assist and after a stressy 20 mins we managed to get the dome covered.
Used more layers of paper than I had planned (a little extra weight so it wouldn't blow away immediately) I also ended up with quite a lot of wrinkles... curious to see what the inside of the dome will look like on tuesday...
Then I immediately covered the dome with a wet blanket and a plastic sheet for the night.

[Kris S]

Building the sand castle was fun, much easier than I anticipated.
I first shortened the central stick as suggested by David to assist it's removal when I will empty the sand.
Put some bricks and styrofoam blocks to save on sand.
Still ended up using about three 25kg bags of ~ 1 - 2 mm fine white sand.
Using the quarter arch pivotting on the screw on top of the stick made it easypeasy to shape the dome.

[Kris S]

I presume you mean the voids on the inside of the dome, after removing the sand 48hrs after casting.

I'm worried about protecting the outside of the dome from the rain...

Anyway it's raining now and I've covered the dome with a plastic sheet...

[david s]

"The voids need to be filled when the cast is somewhere between wet and dry to get a good bond" I'll let you be the judge as I I don't know your weather conditions.

[Kris S]

Allright thanks David!

Just finished casting the dome, all went well I believe. Pictures will follow.

just to be sure: we're having typical varying spring weather right now and next week: 15-25°C winds up to 60km/h and sunny and rainy spells...

It's very likely to rain in the next 48hrs, so should I still leave the cast uncovered for 2 days??

[david s]

I'm a little confused now, so is this procedure correct please:

after I cast tomorrow I cover the the cast directly with cling wrap or plastic sheeting
--> or do I need to wait 48 hrs before wrapping it in cling wrap?
--> do I need to first cover with wet blanket and then cling wrap or plastic sheeting?

48 hrs later I remove the sand and fill any voids as suggested by david
--> what if the paper is stuck to the cast and I can't remove it to inspect for voids?

5 days later (so one week after casting I remove the cling wrap (and / or plastic sheeting)
(I presume this won't be stuck to the dome by then?)

Then I wait for at least 2 weeks before starting the slow 5 day curing fires.

In the meantime I can cast the flue gallery


Thanks so far for all the help guys!

Directly after casting it will be quite wet. Wait for the casting to set before covering. After 48 hrs the casting will still be quite moist. The voids need to be filled when the cast is somewhere between wet and dry to get a good bond. If the newspaper is hard to remove attack it with a scrubbing brush. Once you've filled the voids you can then start the gallery. Build the sand form in front of the oven mouth and cast the gallery. Then keep both dome and gallery covered for at least a week to damp cure. Don't start any fires while the dome and gallery are damp. Wait until both are dry, then insulate and after the insulation has dried you can start the drying fires. Fires in an uninsulated cast is inviting cracking.

[Kris S]

I'm a little confused now, so is this procedure correct please:

after I cast tomorrow I cover the the cast directly with cling wrap or plastic sheeting
--> or do I need to wait 48 hrs before wrapping it in cling wrap?
--> do I need to first cover with wet blanket and then cling wrap or plastic sheeting?

48 hrs later I remove the sand and fill any voids as suggested by david
--> what if the paper is stuck to the cast and I can't remove it to inspect for voids?

5 days later (so one week after casting I remove the cling wrap (and / or plastic sheeting)
(I presume this won't be stuck to the dome by then?)

Then I wait for at least 2 weeks before starting the slow 5 day curing fires.

In the meantime I can cast the flue gallery


Thanks so far for all the help guys!