Casting the flue gallery

[eigil]

Hi. My first post here, but I've been using the forum for a while.
I have built the stand and floor insulation (perlcrete 5:1) for my 40" brick oven.
I am planning to use homebrew mortar for the bricks, and I have been thinking alot on the transition from the vent landing over to the flue. I've tried finding a "square anchor plate" here in Norway, but it seems impossible to find.
Therefore I've been looking into casting the flue gallery using the same homebrew recipe, also adding some SS needles, and some PP-fiber. A positive side effect is also keeping the vent depth smaller.
My question is; if I build a sand mold, can I build the flue gallery by adding the cast bit by bit like vermcrete insulation on the dome, or should I make an outer form to keep the cast in place?

[david s]

My question is; if I build a sand mold, can I build the flue gallery by adding the cast bit by bit like vermcrete insulation on the dome, or should I make an outer form to keep the cast in place?

Yes you can, but if you've not had experience with working with a castable you may run into trouble. One big problem is getting the mix consistency right, this is particularly so if you don't have an outer mould. Too wet and the mix will slump, too dry and you'll end up with voids.The sides are the most difficult so you could create a hybrid version by doing these with brick and cast a top piece. The top piece should be formed like an inverted funnel so it acts to collect smoke and create a smooth flow. Also find attached my method for attaching the flue pipe which does not involve an anchor plate which are expensive and take up a lot of room as well as the problems created by drilling and fixing into brick or castable.

[eigil]

Yes you can, but if you've not had experience with working with a castable you may run into trouble. One big problem is getting the mix consistency right, this is particularly so if you don't have an outer mould. Too wet and the mix will slump, too dry and you'll end up with voids.The sides are the most difficult so you could create a hybrid version by doing these with brick and cast a top piece. The top piece should be formed like an inverted funnel so it acts to collect smoke and create a smooth flow.

Thanks David for the reply. I've read numerous post from you, and your solution was the reason I started thinking of casting the gallery.
For now my solution is this:
Use 3 full bricks as sides of the landing, use halv bricks for the outer arch, and when this is cured, I will make an elevated platform (3 bricks high), and build a sand mold between the dome and the outer arch that will form the inverted funnel.

I have seen some pictures from other builds that connect the outer arch with some wire in the joints that goes into the cast. As long as that works I'll do that.

Should I insulate the dome before making the flue gallery? (I have bought the ceramic blanket, and I'm going to use 2" of blanket and then perlite cement on top of that)

[david s]

Of course there are different ways of doing it, but I prefer to insulate after the gallery, leaving the outer decorative arch uninsulated. There are a number of advantages in casting a gallery vs brick construction. Namely, being able to keep it much shallower which makes the oven far easier to access. It can also be made lighter which reduces the heat sink effect. It is also far easier to construct and allows compound curves for better smoke flow.

[eigil]

I am now finished with the bricks and flue gallery cast. It took me 11 days, laying the floor and building the dome. I think i saved a lot of time not tapering my bricks. Of course the oven doesn't look like art work, but hopefully it will work the same. I used the arch form as a platform for my sand mold, and that worked very well. Also using a long clamp on the newly mortared arch, allowed me to remove the arch form soon after it was mortared, and made it easy to clean the joints on the underside.
I finished the casting this morning, and the question is how long should I wait before I remove the mold? I have covered it with plastic to let it cure. The temperature here is around 16C (60F)...
Thanks alot to all you guys contributing to this forum, and answering all the questions. I've learned alot from reading here!

[david s]

The curing of the casting depends on what materials you used in the mix. If you used castable refractory because it achieves its full strength quite early, remoulding can be done after 24 hrs, although I always give it 48. If you used homebrew as your castable, strength development is a lot slower, so allow 3 days before remoulding and then keep it moist for a further week for strength enhancement.

Not sure how you plan to attach the pipe to the gallery, but I don't use an anchor plate because a. they are ridiculously expensive. b. they take up too much space at the top of your gallery and c. they require fixing by drilling and fixings which create potential crack issues and corrosion problems. On the second two ovens I built I cracked the flue gallery castings because I cast right up to the pipe and the resulting expansion of the far more conductive ss pipe well before the refractory that surrounded it had time to also expand. I now ensure that the pipe is a slightly loose fit and pack around the joint with some6:1 vermicrete which has some elasticity to it. The attached pics may help you.
There is also a similar problem where the outer render meets the pipe. If you wrap some cardboard around the pipe before rendering up to it, when it has firmed up after 24 hrs it can be removed leaving a space that can be filled with high temp silicone. This will prevent cracking and a n entry point for water.

[eigil]

I used homebrew with SS needles and PP fiber so I'll wait 3 days.

[david s]

OK, then keep it moist by wrapping it up well for a further week.

[david s]

Just re-reading your thread and as you've built a 42" oven it really requires an 8" diam. flue pipe. By casting a gallery you should have provided a decent volume under the base of the pipe so that smoke will flow more easily to the pipe entry, so I think a 6" pipe will work ok for you. All new ovens are rather smoky when new and you are purging them of moisture so don't panic when you get excessive smoke in the early fires. Draw can also be increased by increasing the height of the flue pipe so another section added to it is another alternative.

[eigil]

Well, I planned for a 42" but ended up with a 1 m (39ish). The flue pipe I'm gonna use is 1 m high stainless steel. I worried the wind would catch it when doing the cast, so I used the top for making the "hole" to minimize the risk when the cast was curing.

[eigil]

OK, then keep it moist by wrapping it up well for a further week.

Maybe stupid question; but would the fiber blanket and vermicrete count as "keeping it moist", or should I wait?

[david s]

No, both the fibre blanket and vermicrete will want to draw moisture out of the casting. Wrap it in some wet burlap or towels and seal it up with plastic would be a better solution.