Oven Entrance-Firebrick or regular?

[DavidK]

OK, I'm going close in my dome and work on my entrance this weekend. I plan on using clay brick for the entrance arch and chimney (I think). So, should I use firebrick to make the vent transition or are regular bricks OK to handle the heat? I've seen it done both ways here on the forum and would guess regular brick is fine, but it never hurts to get opinions.

I had also planed on using a clay liner with a brick facing for the chimney, but I wonder if the liner is really necessary given the use (outside, away from combustible materials, etc). Thoughts?

Last question, what about mortar for the entrance and arch bricks? Refractory mortar, right?

Thanks for any guidance...

[Marcel]

Omitting the liner will allow you a larger flue.

(M) David, if you omit the liner then you will have the option of making your flue that much larger. I'm not sure if that is an advantage as after a point I think the flue could allow too much heat to escape. But that is an "equation" with many variables such as the volume ratio of oven to flue height of chimney, distance of opening from fire, etc.

(M) I'm constricting the final exit of smoke somewhat with a hand-made chimney pot. I read that decreasing that exit acts as a venturi and accelerates the smoke's exit and therefor increases the draw for incoming air to feed your fire. How much to decrease is a question for a Lab. mathematician. Quien sabe?

(M) Although I will be using the same firebrick for my arch as I used on the hearth floor, I'm sure that the only real consideration is personal aesthetic. So many have used red brick without reporting a problem that it sounds like that choice is entirely open.

(M) I would use refractory mortar if for no other reason than the fact that it will allow a consistent coefficient of thermal expansion for all joints. It seems to also be very strong. If I remember correctly, the ratio is 8 parts sand, 3 parts cement and 2 parts fireclay.

Ciao,

Marcel

[dmun]

C/e

Speaking of coefficient of expansion, I seem to recall that standard brick has a much greater C/E than fire brick or refractory cement. I know that some cracking of masonry is expected, but it might be a greater problem with mixed brick.

The reason that common brick isn't used in fireplaces, and so forth, is a problem called spalting, where a part of the surface of the brick chips off in the heat.

For me, I plan to use refractory materials where ever there will be contact with fire or smoke, but that may be overkill. In my case, the oven will be part of a building and the building inspector will be involved, so I have to be a bit more careful.

David

[DavidK]

Marcel & Dave, thanks for the quick response.

I'll probably use the regular brick and go straight up to avoid any disimilarities in expansion. I don't know how much larger the flue will turn out but at the least, it will gain the thickness of the liner. Plus, I won't have to engineer its support. As for spalling, if and when it happens, at least it will be in the throat and shouldn't get into the food.

M, I really like the idea of a chimney pot, but dang, they're expensive! And I don't know anyone with a kiln and pottery skills... I bet I could cast one and pigment the concrete to look like clay...

David, it's interesting. Miami-Dade County's codes are some of the most stringent nationally, but because it isn't attached or adjacent to any living structure, they were disinterested in the chimney's construction. Clearly, around here the concern is a hurricane blowing it away but given the oven's inherant weight, that's pretty unlikely. Of course if my brick work stinks, the chimney could collapse...

Thanks again!

[Marcel]

I agree that you could cast a mortar chimney pot

[QUOTE=DavidK]Marcel & Dave, thanks for the quick response.

<snip>

(D) "M, I really like the idea of a chimney pot, but dang, they're expensive! And I don't know anyone with a kiln and pottery skills... I bet I could cast one and pigment the concrete to look like clay... "

(M) I agree on both points, David. I think that by the time your smoke gets that high you won't have to worry about a cracked chimney pot, especially if you put a layer of refractory mortar on your (plywood?) form, followed by wire mesh and then more refractory mortar. And yes, you could pigment it too. Consider getting a white cement so you can get the color you want with your pigment.

(M) The only really difficult part is getting the form to release after the mortar dries. But, if you shape it as a pyramid with the top cut off (Trapezoid in 2D) and grease the plywood it should slide out after the cement sets up for a few days. The serendipitous advantage is that by sloping the sides a bit you will also get the Venturi effect.

Ciao,

Marcel