Casting a Transistion

[ftaylor74]

Hi All!
Thanks to the great information in this forum, and the exceptional advice that I've received from forum members, I've completed the first 5 courses of my 38 inch oven. I'm really stoked but I've reached sort of an impasse.

I picked up a very nice wood stove door last summer in Maine, and I had envisioned that it would make an excellent oven door for this oven. I did not envision the difficulties I would have transitioning from a rectangular opening into the dome.

After much gnashing of teeth, I have decided that the best approach is to cast a transition piece that will span the 17" opening, and connect and incorporate the the ends of the 6th brick course. Pictures are attached.

I've made a Styrofoam model of the transition piece. My plan is to make a mold of the shape in a box filled with plaster of paris. After the PoP hardens I will remove the styrofoam, and hopefully will be left with an intact mold. Once I've treated the PoP with a release agent, I will cast a refractory mixture into the mold and hopefully end up with a concrete replica of the foam piece.

Before I pour, I'd like to hear your thoughts on the feasibility of this approach. Also, I have a couple of questions... (as you have already guessed...)

1) Has anyone used AW Perkins Castable Firebrick in a similar application?

2) If I were to go with the Portland Cement/sand/fireclay/lime mix, is a pottery clay grog a suitable substitute for fireclay?

3) Can I substitute HeatStop50 for Portland cement in the above homebrew mixture?

4) Will two lengths of 1/2 " rebar be sufficient to insure the integrity of the casting, or should I use something like steel needles or other additives?

Any ideas will be greatly appreciated!

Regards

[Tonyp]

Re: Casting a Transistion

Looks like the oven is coming along very nicely! The brickwork looks very clean.

Very pretty door. How are you planning to build your vent area and outside arch with the door connected to the dome inside? Perhaps rethink the location? I would be more inclined to use it as a decorative door for the outside vent opening.

[ftaylor74]

Re: Casting a Transistion

Thx for the reply. tony

Its funny you should mention the location of the door and vent/chimney. I realize that the opening looks semi finished as it is, but my (current) plan is to extend the entry about 6 inches forward (wit the door remaining where it is, and incorporate a shallow arched chimney/vent per the traditional Pompei oven.

But the question has crossed my mind whether I really need a chimney. I understand that a chimney will vent the heat up above the pizzaoli's head, but if that's all it does I'm not sure its worth the effort. If I need the chimney to get a more efficient circulation of heat within the chamber, then by all means , I'll do it, but otherwise I'm not so sure.

I was going to post that very question soon, so I'm glad you brought it up.

I see you provided a link to your build. Thx, I'll check it out!
Regards

[cobblerdave]

Re: Casting a Transistion

G'day
The smoke coming out of a fully hot pizza oven is hot, 500C hot! And clear, you can't see it. Its a good way to loss to lose some fringe hair and eyebrows.
The secound thing to consider is the sides of the vent protect the oven from the wind. My friend has an internal chimney which to cook pizza is blocked off. All the heat comes straight at you, worst in any wind. I much prefer an oven with an entrance. Its worth the extra effort, but you can always add it later I suppose.
Regards dave

[ftaylor74]

Re: Casting a Transistion

I see your point, Dave. I've no doubt that the chimney would suck away some or most of the heat that would otherwise be smacking me in the face. But I hadn't considered the effect of crosswinds messing with the air/heat circulation within the oven. That seems like an important point.

Looking at the picture you posted of your friend's oven, I can see that a chimney would have prevented the blackening of the of the oven above the door. That would drive me crazy. So. A chimney it is!

By the way, I noticed that you have a continuous brick floor on your oven that extends past the opening and becomes the landing. Mine is also set up that way, but I am considering breaking that up. My concern is that it is a considerable additional thermal mass that will take longer to heat the oven. Even worse, the landing area will be cause the oven to loose heat even after buttoning up the oven with the with your very nice hebler backed insulated oven door.

How hot does your landing area get, and how quickly does the oven come to ambient temperature after you've stopped cooking and shut it down? i'd be really interested to know.

Thanks
Regards,
Frank

[ftaylor74]

Re: Casting a Transistion

Dave,
After viewing your entire build (the brick veneer is a thing of beauty), I see that the landing area is enclosed by your oven door. So your oven probably does not have the heat sink/heat loss problem I think that I will have.

Really nice work Dave. As I've been building my own, I've been telling myself that it is a shame that all of the brick work will be hidden by the insulation. The veneer really solves that problem. Thanks for the great idea!
Regards,
Frank

[david s]

Re: Casting a Transistion

Hi All!
Thanks to the great information in this forum, and the exceptional advice that I've received from forum members, I've completed the first 5 courses of my 38 inch oven. I'm really stoked but I've reached sort of an impasse.

I picked up a very nice wood stove door last summer in Maine, and I had envisioned that it would make an excellent oven door for this oven. I did not envision the difficulties I would have transitioning from a rectangular opening into the dome.

After much gnashing of teeth, I have decided that the best approach is to cast a transition piece that will span the 17" opening, and connect and incorporate the the ends of the 6th brick course. Pictures are attached.

I've made a Styrofoam model of the transition piece. My plan is to make a mold of the shape in a box filled with plaster of paris. After the PoP hardens I will remove the styrofoam, and hopefully will be left with an intact mold. Once I've treated the PoP with a release agent, I will cast a refractory mixture into the mold and hopefully end up with a concrete replica of the foam piece.

Before I pour, I'd like to hear your thoughts on the feasibility of this approach. Also, I have a couple of questions... (as you have already guessed...)

1) Has anyone used AW Perkins Castable Firebrick in a similar application?

2) If I were to go with the Portland Cement/sand/fireclay/lime mix, is a pottery clay grog a suitable substitute for fireclay?

3) Can I substitute HeatStop50 for Portland cement in the above homebrew mixture?

4) Will two lengths of 1/2 " rebar be sufficient to insure the integrity of the casting, or should I use something like steel needles or other additives?

Any ideas will be greatly appreciated!

Regards

1. I'm not familiar with that product, but there are many different castables available. What you need is a dense castable refractory not an insulating one (they lack strength)
2. No, grog has been fired and will act more like sand as an aggregate rather than giving the mix good workability.
3. No, Heatstop 50 is a premixed mortar not the cement component.
4. No, thick rebar is unsuitable. the heat will rush straight to it because it's more conductive, making it expand faster than its surrounding refractory resulting in potential cracking and casting failure. The reason ss needles are the preferred reinforcing for castable is that their greater surface area allow them to dissipate their heat to the refractory that surrounds them more easily.

[ftaylor74]

Re: Casting a Transistion

Thanks for the input, especially about the rebar. I think you have helped me avoid a disaster!.

A may have mispoken by calling the pottery clay "grog". I think the right word is "slip"... that is, the leftover wet clay that has been turned on a potters wheel, that did not incorporate into the vase or urn or whatever. I'm sure it has not been fired yet. Its very sticky and slick and smooth.

Anyway, now all I have to do is find a local source for either the needles or the Perkins Castable Firebrick!

Thanks again.
Regards

[cobblerdave]

Re: Casting a Transistion

G'day Frank
With any wood fired oven you only get back what you put into it in the way of heat. If my oven is fired to pizza temps 450to 500 C for say 2 hours then closed of on a fri evening with the insulated door next morning the temps would be typically in the 300s C. By the mid day it would have dropped to 250 to 280 C by the evening the lower 200s by next day 150 and finally sun night you could still expect to have still have 100C.
The temp drop is slower as it approaches the lower temps.
The heat stored deep in that brick is a funny thing too. You can take a 300 C oven take off the door and swab the floor and get that surface temp down . Put the door back on and within 1/2 an hour the temp jumps back up and you have actually only lost a few degrees.
Using it ,taking of the door, lots of bread does drop the temperature over time.
I don't always take the oven to pizza temps just to wait for it to cool . If you only want a bit out of her you only put a small amount in in the way of fire.
An hour burn gives me enough heat for an afternoon roast and bake.
My oven hearth and entrance is in one piece. The insulation continues right through . A bit old school now days as a lot of ovens include and insulated break at the hearth and also at the entrance way. What effect it actually has is hard to gauge accurately . I'm certain it would have some effect on heat loss but I'm certain it's not as much as say the complete lack of insulation.
The entrance /chimney is only connected to the oven by the brick of the oven mouth, a small amount of the total of the oven mass . The entrance/ hearth is preheated by the smoke and radiant heat from firing the oven as well. A Heat break would certainly help in heat retention but only in a limited way an insulated oven with an insulated door will work well enough
Watching you build with interest, that's a pretty impressive door.
Regards dave