Steam and Cracks!!!

[kypreos]

Hi guys, hope you can help with some questions about steam and cracks after a first good firing...

My pompeii is by no means finished - still needs render, but I thought I'd give it a proper firing and cook last weekend... I did a lamb and potato roast - a variation of Greek Kleftico !

The vermicrete insulation has been set and drying for about a month and I have lit the 7 curing fires exactly as per the pompeii guide...

After a good two hour firing, I put the food in. Whilst the food was cooking, I noticed steam coming from one particular area of the oven in particular - it was from a 10" roundish patch just behind and to the right of the dome's peak... Temperature there was 60 degrees C rather than the 25 - 30 deg C average elsewhere... The steam poured for several hours... I can't think why that area would be getting steamy.... Is this normal??? and is that steam from the structure or the food?

Also, I have some rather nasty cracks on the arch - to the point where the keystones have loosened and three of which have fallen down by a cm or so... I think I need to retro-fit a lintel of some sort - any experience of this that anyone can share, pls?

Any help much appreciated as always!

Thanks,
Kyp.

[cannyfradock]

Hello Kypreos

Sorry to hear about the cracks. Which Pompeii guide are you using?...perhaps some of our members have used the same guide and can relate to it.

It sounds like there was still a lot of moisture in your oven when you fully fired it. The patch with the problem may have been an area where some water may have held. There is no exact amount of time for curing the oven. Some builders soak and cut their bricks....others don't. If the curing fires were started slow and frequent ....as soon as the dome was completed, this would help release the moisture from the bricks and mortar before the insulation layers went on. If curing starts after the dome is complete with insulation layers, then a longer slower approach to curing your oven is needed. I always recommend 2 weeks of slow curing fires...a couple of times per day.

Cracks very often appear as people don't realise how much moisture must be driven out before fully firing the dome. Cracks however can be repaired and wont affect the performance of the oven.

The cracked and dropping bricks of the arch can also be repaired....it's difficult to give advice without seeing a photo of the arch. Without seeing the arch I would (if it were mine) use the same arch former used to build the arch (or build another one exactly the same) and place under the arch. Using large folding wedges force the former up so as the bricks go back into place. Then use a 4" angle grinder to chase out the cracks and re-fill with refractory mortar (use a thin jointer/trowel to force the mortar well into the crack or use a tube of fire-cement in a skeleton gun...again to squeeze the cement deep into the crack.

The pure nature of a dome or arch structure means that as long as it was built correctly, then it can't go anywhere. However shallow the arch, as long as it forms part of a perfect circle, then it shouldn't move. If your arch is very shallow or closer to horizontal , then after repairing you may need to lay some ex-met over the arch and bed it in refractory mortar.

Not sure on the lintel approach as you would probably have to disturb some of the dome structure?

Hope that's some help and hope you get a bit more feedback from the guys on board.

Terry

[bookemdanno]

Hi Kypreos, Terrys comments are spot on, and most things can be repaired, it's just the method you use to go about it.

If your camera can handle the stress, and you also in sending some pictures of the "damage" then we can all help, i'm sure.

I suffered a similar fault in my oven. I was far too enthusiastic and let my early fires get too big, etc, etc. Then fired up early on for the Vermicrete, with little plumes of steam erupting from it hear and there, but all stayed in place! To top it all off i then fell foul of some early frosts whilst i was away which damaged by lovely Lime Render finish i was so proud of!
My oven is fairly unique for here, in its Old Red and hydraulic lime construction but still uses the same principles.

I think the Oven arches of these lovely ovens really get some stick with the heat passing out of them and up the chimney! Mine has "character cracking" to the main arch in to the oven, with the arch now in four sections. No brick slips as they're all cut as voissoirs (angled), so will always hold in place due to being bigger at the top than at the bottom but loose none the less. Then cracks radiating out from the chimney in the outer render, which is again heat related through expansion of the metal chimney.

These will all get raked out in the Spring, filled and the whole rendered dome coated in some Limewash to get her back to her pre-used glory once more! I fully expect to have to do it again sometime but i feel it'll all be less, as the oven has had its settling in period.

Its all under cover for the winter now, as i don't want the winter weather making things worse through freeze thaw actions.
I found that a Garden Table cover worked superbly for a cover, perfect size!

[kypreos]

Thanks for the replies Terry, Danno - much appreciated.

I've attached a picture showing the arch damage:

As you can see, there's no chimney, so all the heat comes straight out of the door. It's also a very shallow arch and the keystone angles are not great...

The lintel option I was considering was a 2-odd inch strap of steel bent to the shape of the arch and attached within... the first keystones on both sides are tied in using steel ties, so they should always hold. the rest of the theory was pretty much as you described, Terry - i.e. grind out the mortar and replace with KOS fire cement from the tube...

Attachments:

[tonyb]

Kypreos, although you can repair it as per the guys instructions, my own view would be to leave it as it is and see if it settles down. Doesn't look too bad to me.

[kypreos]

So, I've now got myself a steel support for the arch - a 2" strap made into an upside-down 'U' shape - and I'm going to have a go at the arch repair tomorrow, if the weather's ok... the damage is actually quite bad - four bricks in the middle of the arch are looking ready to drop out if it gets any worse...

I'm planning on using the ready-made KOS fire cement that comes in a tube for use with a mastic gun. The instructions say that this needs to be cured with fire or heat...

Does anybody have any experience of using this stuff?
And is there a limit to how thick you can lay it on?

Thanks!
Kyp.

[encasta]

Hi KYP - your right the KOS tube stuff is a bit tricky to apply as it is like a cookie consistency mix - warm water with a small amount of fairy liquid dipping your finger in will help make it easier to tidy it up once you have put the metal band into place. dont be shy when putting it on - it will react with the heat thats what sets it and with the air - youll notice it going off a bit the longer it is out the tube - youll also get a fair bit of steam coming off the insulation blanket when you apply it - dont worry though that is normal too.

Danny.

[kypreos]

Thanks Danny - I will apply liberally!

Is this ready-made fire cement any good then? Anyone else used it?

[cannyfradock]

Kypreos

I haven't used the KOS tube fire-cement. I have 2 concerns....read the tube and see how thick you can apply it. Once you have the arch back in place, remember the principle of arch construction. As encasta has said, the stuff will react with the heat.....but you will also get normal movement in the arch once the heat gets to it. ...An arch supports itself when ALL bricks are wedge shaped in uniformity...or straight bricks are used with wedge shaped mortar.

I would still try to rake out the joint either side of your keystone....or 4th brick from left....or right.....and the perp either side of the 2nd brick from the right (even if you have to use a 4" angle grinder) and tightly pack with the fire cement.

My comments/suggestions may be a bit OTT as the 2" strap should do the trick regardless. I just think it's just a shame a small setback is stopping you from firing that beautiful beast up.

terry

[kypreos]

Dec 30, 2012 13:52:44 GMT cannyfradock said:

I would still try to rake out the joint either side of your keystone....or 4th brick from left....or right.....and the perp either side of the 2nd brick from the right (even if you have to use a 4" angle grinder) and tightly pack with the fire cement.

Cheers Terry! We did exactly that today - i.e. ground out the 'old' mortar on the keystones with a 4"grinder, secured the strap and squeezed in plenty of the ready-made cement in every cavity. Then fired it up to a stable temp of about 600F. It will get a full firing on New Year's day too to complete the job...

So far so VERY good! The KOS cement is an absolute pleasure to work with... If it holds, then I can't recommend it enough.

Will report back with a picture and an update... Many, many thanks to all for your help!

Kyp.