Oven base design and refractory cement questions

[clanny]

I've been on here for a good while now and have been collecting various bits and bobs that I can use in my build.

I've settled on a barrel/vault oven design for a couple of reasons:
1. I managed to blag a half pallet of fire bricks in 3 different sizes, the largest of which are 430 x 230 x 75. I think these will be prefect for the side walls of the oven.
2. I haven't enough small bricks for a circular oven and don't want to have to cut too many bricks so cutting them in half is undesirable. Using the bigger bricks seems a no-brainer.

I've laid a foundation, built the support walls, and cast a 6" base on top, mortared 8 thermalite blocks on top to insulate under the oven floor. My plan was then to use refractory cement that I got with the fire bricks (8 tubs) to cement 9 large firebricks, in a herringbone pattern, to the thermalite blocks. This would be the oven floor. My plan had then been to cement the oven walls on top of the oven floor (with the floor extending out past the bottom of the walls) and build the vault arches on top (3 arches of 10 bricks).

I've been reading people's opinions of whether or not to cement the oven floor down so I'm conflicted now. I could just place the firebricks on a thin layer of sand on top of the thermalite blocks (to help level them) but if I do that, should I reduce the size of the oven floor so the walls of the oven can then be cemented to the thermalite blocks, or will it be ok to build on the oven floor even if it's not cemented down? Or will I be ok cementing the floor down as planned?

Or could I just cement the outer edges of the oven floor slabs and leave some small exansion gaps between them?

I'm guessing firebricks will expand less than most materials because this is what they're designed for so will expansion be as much of an issue?

Secondly, the refractory cement I've got has been in my garage for well over a year now and when I opened the first tub up last nigt, there was a layer of about an inch of amber watery liquid sitting on top. I spend a sweaty 30 minutes mixing it all back together again and then tested it with a couple of broken bricks to see if it still went off. I'll check tonight and see if it's made a secure bond, or should I wait longer?

Obviously I'd prefer to use this stuff if it works ok but is there anyone out there with experience with this stuff? Does it have a sell by or use by date? Will it have died on the shelf?

[oblertone]

In no particular order: I'd lay out the first course of bricks on your thermolite base and mortar them end-on-end only, then lay a herringbone inside on a dry sand bed. Place a strip of cardboard between the herringbone and wall, this will burn out and refill with ash leaving you an expansion gap. Then I'd build the arches, remembering that the end walls should be inside the arch, not next to it. Your refractory cement is fine, just a bit OTT for a WFO as the maximum temp you'll get to is about 550c; do remix it to an even constituency before use.

The above is just my opinion, others will be along with theirs :-)

[clanny]

Thanks for the reply. I've probably not explained my design well enough. I'm thinking of just bringing the outer walls up on the outside of the concrete slab using concrete blocks and rendering them when I'm done. The Thermalite blocks don't reach out to where the outer walls will be. My plan was to build the oven, add concrete thermal mass which should also help support the side oven walls as the arches will try to push them out, then insulate with vermiculite just poured in the top with the outer walls keeping it in. Then add a roof to keep it all dry. Maybe this picture will help.


I was kinda hoping not to have to build the oven walls up from the thermalite as for some reason, in my head the oven wall heights would be right if I built on top of the oven floor. Although I can see how that design would be good for replacing blocks that get damaged, thermal expansion etc.

I've been doing some reading and I know there's an optimal height (63% of oven dome height) for the oven/flue transition but is there an optimal heigh/width ratio for barrel vault ovens? I've not managed to find any reference and can't remember any specifics.

Anyway, I've had a look at the refractory cement and it's cured but not completely. It's still a little soft inside when I broke off some from the edge but the bond is good and strong already. I'm happy that it should be good enough for jointing the firebricks.

[oblertone]

You're correct in that I've never seen a height / width ratio quoted for a barrel oven so I'd just go with the 63% ratio between dome and door. On reflection, most seem to a half circle, sometimes on a soldier course for added height.

I'm a little confused regarding what you call a wall ? Is this the containment or outer wall with the barrel oven constructed within it ?

[clanny]

Haha yeah, rereading I did say "wall" a lot! I've edited my post to differentiate between outer (concrete block) walls and oven (fire brick) walls.

I'll just go with the half circle or thereabouts which means the internal height should be about 60cms at the peak.

I'm interested to know how people make their arch formers. I tried to cut some from old decking boards using a jigsaw. I cut three and screwed them together but they were all over the place! How do people make them so accurately? Maybe a frame and bend some thin board round for a more uniform curve?

[oblertone]

Remember your formers don't have to be too robust, just enough to do the job. I used sheets of expanded polystyrene to do mine.