#11  
Old 10-03-2007, 04:53 PM
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Default 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.
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  #12  
Old 10-03-2007, 06:56 PM
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Default 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~
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  #13  
Old 10-03-2007, 08:49 PM
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Default Re: HELP! Can I cure and test oven before enclosing?

Quote:
Originally Posted by Unofornaio View Post

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.
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  #14  
Old 10-04-2007, 11:03 AM
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Default 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
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Old 10-04-2007, 09:48 PM
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Default 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.
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  #16  
Old 10-04-2007, 10:23 PM
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Default Re: HELP! Can I cure and test oven before enclosing?

Define 'newspaper'. Seriously, newspaper 'logs' can burn reasonably long and hot. A grocery bag worth makes a nice campfire - never mind how I know this.
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  #17  
Old 10-05-2007, 06:37 AM
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Default Re: HELP! Can I cure and test oven before enclosing?

Ever leave a sheet of plywood out and it got rained on? It bows up on the wet side because it expands due to soaking up water.

If one face of most materials are exposed to heat (like the wet side of the plywood) and the other is at outside temperature (like the dry side) the material tries to flex due to the difference in stress on the hot face and the cooler face. The amount of stress relates to the difference in temperature between the hot side and the cooler side. With materials like brick that do not flex - this stress can create cracks in bricks or mortar joints.

Insulation helps to keep the temperature difference between the hot side and cold side smaller - which results in less stress due to thermal expansion.

That being said, there are ovens centuries old that had no insulation or used poor insulation and they are still standing. I can't say that insulation pre or post cure is a requirement.

One school of thought is to fire without insulation - give the oven a higher stress than it will ever see while in use. Use that stress to find cracks that may occur in the weakest parts of the oven and patch them.

Another school is to fire with insulation and expose the materials only to the normal stresses that will be found during future use. Cracks may still occur.

Christo
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Old 10-05-2007, 08:48 AM
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Default Re: HELP! Can I cure and test oven before enclosing?

It would seem, that with all the reasonable opinions regarding the different approaches, that the only way to be sure would be to build several identical ovens side by side and cure them with each of the various methods. Then, take them appart and do a forensic examination to see which held up the best after years of use.

This is really going to delay my oven.

Sheesh.
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Old 10-05-2007, 09:28 AM
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Default Re: HELP! Can I cure and test oven before enclosing?

Either/or should work just fine.

Don't sweat it!
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  #20  
Old 10-05-2007, 04:50 PM
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Default Re: HELP! Can I cure and test oven before enclosing?

Yea, with all do respect this subject has gotten W-A-Y to technical.

The fact of the matter is that either way is not going to destroy or seriously damage the overall structure. Keeping the curing fires low and slow as they should be is the most important factor in either scenario insulated or not.

When I built my test bread oven years ago (3X5) I cured it with the concrete and vermiculite layers on. Several years later when I built the bakery oven (8X10) I let the vault, hearth slab and insulation under the hearth cure for 30 days prior to moving forward. I fired the oven to cure the fire-clay then added the concrete cladding and insulation. Both never had issues.
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