HELP! Can I cure and test oven before enclosing?

[Nick]

My mason has a mind of his own. I gave him the Pompeii oven plans MONTHS ago an have talked to him about what I want (a neopolitan low-dome) but he pretty much works out the design in his own head and does what he wants. So far this has been okay. He really is a talented man. He started the dome this week. He's just about to the point where he'll fill the oven with sand to finish off the dome. (He decided he didn't want to do the extreme neopolitan, so we agreed on a hybrid.) Last night I went out and measured the cooking floor. It's barely 40" and it was supposed to be 42!

In and of itself, this is not an oven-killing thing. But since he has obviously not followed the instructions, I'm concerned he'll do something else that will result in a poorly-functioning oven. He's a very stubborn man.

My question is: Can I cure the oven once the dome is complete and then test it to see how it works? Should I install the insulfrax blanket first? Before curing or just before full firing?

There's always a chance the oven will work perfectly. But if it doesn't, I don't want to have to tear down the outer wall, roof, etc to rebuild it. I can live with just tearing down the oven and starting on the base again. If testing the oven at this stage (assuming a full cure as outlined in the plans) isn't possible, I will tear it apart and start over from the base.

Thanks for your help!

Dominick

[asudavew]

Re: HELP! Can I cure and test oven before enclosing?

I think most have concluded that it's best to insulate and do the curing fires.
(less thermal shock)

That can be accomplished with the fiber blanket.

There are other opinions though. Some have cured without insulation at all.

Good Luck!

[dmun]

Re: HELP! Can I cure and test oven before enclosing?

He's just about to the point where he'll fill the oven with sand to finish off the dome.

Just a point, I think this sentence means that he is using the sand as a form to build the last courses. There are traditions of INSULATING the oven with sand. Just make sure he's using real insulating materials, not cheap sand around your dome.

[Kemo]

Re: HELP! Can I cure and test oven before enclosing?

post some pics...

[DrakeRemoray]

Re: HELP! Can I cure and test oven before enclosing?

For the record, I cured mine before insulating, this helped me identify and fill cracks.

I have read the arguments about insulating before and how that may cause less thermal shock. I don't see how that could be true. If anything it would mean that the oven gets hotter when insulated, therefore more thermal shock...call me a doubter...
Anyway, I would cure it first and patch the inevitable cracks, then insulate and build the enclosure...

Drake

[asudavew]

Re: HELP! Can I cure and test oven before enclosing?

I have read the arguments about insulating before and how that may cause less thermal shock. I don't see how that could be true. If anything it would mean that the oven gets hotter when insulated, therefore more thermal shock...call me a doubter...
Drake

Good point.

I hadn't though about it that way.

If the blanket is on, I guess it would get hotter faster, but wouldn't it cool more slowly? Maybe heat it up with no blanket, then throw one on for cooling
down.

But apparently, either/or has proven to work.

[Unofornaio]

Re: HELP! Can I cure and test oven before enclosing?

Got to agree with Drake. Curing the chamber first exposes weaknesses that could potentially damage the insulation layer and provides the opportunity to fix them.
In addition when curing the oven it moves in all directions more radically than it does once cured.

Then again many have cured it after without ill effects.

[wlively]

Re: HELP! Can I cure and test oven before enclosing?

May or may not be a big difference with firebricks, but the thermal stress is greater without insulation, no doubt about that. The stress is greater because the temperature gradient across the brick or dome is greater and that gradient is what produces the stress.

Again, while it may or may not make a huge difference, I believe it is better to fire/cure after at least a blanket of insulation. That is what I did. If you have cracks or want to inspect it is no big deal to pull back the blanket. I am still 100% crack free.

To each his own.

[Nick]

Re: HELP! Can I cure and test oven before enclosing?

Thanks for all the great advice.

The sand was just for the form, not for insulation. We have several bags of vermiculite for that. Plus, I've ordered the insulfrax blanket from Forno Bravo. Not sure when that will arrive!

The mason finished the dome today. It looks great. I will post some photos once I figure out how.

I like the idea of curing the oven with the blanket, but then removing it to check for cracks and make repairs.

According to the plans, I need to wait a week before starting the cure anyway.

Thanks!

DB

[Unofornaio]

Re: HELP! Can I cure and test oven before enclosing?

[QUOTE=wlively;16429] The stress is greater because the temperature gradient across the brick or dome is greater and that gradient is what produces the stress.
QUOTE]


Not disputing this theory in any way I just want to understand more. Did you arrive at this with the fact that the insulation is holding the heat in mind? Meaning, since the insulation is, for lack of a better description, distributing..(actually encapsulating may be a better term) the captured heat its causing the dome to heat more evenly? Thus reducing stress.

[rlf5]

Re: HELP! Can I cure and test oven before enclosing?

I think that the rapid heat up and cool down which would occur w/o insulation has a bit more stress on the bricks than it would with the blanket. Its kind of like heating up a piece of glass and then sticking it in cool water...just not as severe. The blanket doesn't facilitate heat transfer well at all (which is why we use it as an insulator) so the brick has to transfer the heat to the air inside the oven, which is a much slower process and so less stressful. Granted the stress which would occur with no insulation may not pose any problems since many people have done it w/o any known ill effects, but its definitely a more aggressive route compared to having the insulation. Both ways have their advantages and shortcomings. I may keep the blankets off for the small curing fires to check for leaks, but put them on for the really hot ones.

[Archena]

Re: HELP! Can I cure and test oven before enclosing?

I'd think it would stress the thing less without the blanket. Brick heats (relatively) slowly with the heat transferring from the interior to the exterior. the thermal mass, not present in the glass analogy, should make that a slow and reasonably gentle process as opposed to the intensity of having most of the mass at full temp.


Or not...

~shrug~

[wlively]

Re: HELP! Can I cure and test oven before enclosing?

Not disputing this theory in any way I just want to understand more. Did you arrive at this with the fact that the insulation is holding the heat in mind? Meaning, since the insulation is, for lack of a better description, distributing..(actually encapsulating may be a better term) the captured heat its causing the dome to heat more evenly? Thus reducing stress.

Basically yes. The blanket keeps the temp of the dome more uniform by containing the heat inside the dome and forcing the temp to propagate thru the material instead of to the air and blocking the various conductive or wind currents which would carry heat away at different rates. In the same manner it also greatly slows the rate of cool down again reducing stress. I think an added benefit of keeping the dome warm longer helps continue to drive out moisture at a slow steady rate. I think we can all agree that moisture is the biggest danger for damage when curing.

No problem, don't claim to be a certified genius expert.

But I do work with high temp materials and need a decent understanding. Thermal stress is "Stress resulting from non-uniform temperature distribution or differential thermal expansion". So when a material is heated it expands with temperature. Depending on the size and thermal conductivity of the material different rates of expansion are present which generate both tensile and compressive forces. And even though we are talking a very small amount of expansion the forces generated can be quite large.
"If you put a solid piece of glass in boiling water, it will expand about
0.02%. Maybe thousandths of an inch. That is not a lot. But if you tried
to prevent it from expanding by putting a large clamp on it, you would need
to squeeze it with a force of 2000 pounds per square inch to completely
eliminate the expansion."

So we have stress building up inside the layers of brick and more importantly also the brick/mortar boundary. How fragile is this boundary or the brick itself? I don't know. Not being an expert on refractories I cannnot say how critical this all is or if we need to worry about it. My thinking is/was just why not avoid all the sources of stress that I could, within reason.

[ChristopherBlair]

Re: HELP! Can I cure and test oven before enclosing?

Hi, Guys,

I wanted to jump in here and ask a question. My dome is done and i have the insulation almost completed (vermiculite and portland, 6" thick) with no curing as of yet.

Couldnt you light small fires for a longer time to drive out most of the moisture? maybe do the newspaper fires for several days, then move on to the paper and kindling?
just curious if this would cut the stress down.

thanks,
Christopher

[maver]

Re: HELP! Can I cure and test oven before enclosing?

I'm not sure newspaper fires have enough time to really change the dome temperature much at all. Better off placing a space heater in the oven and partially occluding the door or just giving some time for the moisture to work it's way out. Remember you do want some moisture for mortar for a little time to ensure complete cure - don't dry it out too soon.