Acoma's cure

[Acoma]

Re: Acoma's cure

I have been applying furnace cement to the outside of my dome yesterday and today, and firing temps to upper 800's with outside reaching 120's. This of course is without the blanket. I believe now that the cause of the cracks is the varying methods applied to the dome, not the arch.

For the dome, I believe every course from solder to top should be mortard. I did not do the solder to 2nd because I was going straight up, that was not smart.

I also had that period where courses 4-6 had huge mortared backsides 1 1/8, not smart.

I then had courses 6-8 with shaved inner faces to get my keystone to its 21 inch.

Looking back, with these applications, I believe the oven had irregular weight distributions and expansion rates. I screwed up on these areas because I was not thinking. My mind was set on how damn cold it was out there, the limited space, and the area to work with. I wanted it done, even though I was attempting patience.

Suggestion, plan smart, and stick to the design. Changing a design while in build will not create any benefit.

I still love my oven, and now, with my medical (oven) degree, I can help heal patients (ovens) with more care

[david s]

Re: Acoma's cure

Acoma,
Many kilns are made where the fire bricks are loose stacked ie they have no mortar, so the structure is free to move under expansion and contraction. Don't worry too much about fine cracks. It is not a good idea to start firing just after you've added mortar. This is asking for trouble. I believe that a lot of the problems occur at relatively low temps (ambient to 250 C) where there is considerable expansion and its hard to keep your fire very small at this early stage.

[Acoma]

Re: Acoma's cure

Thanks David. Actually, the furnace cement is fine after a little while. It's been tested yesterday and today, and is holding great. Agree about not firing right after adding mortar. Les is doing his oven on the mortarless principle, so this will be a fun oven to see with completion and burns.

[gjbingham]

Re: Acoma's cure

I dunno Robert. You can second guess your work until the end of time. You did a really nice job and went far and above what many others have done! How many ovens have you seen recently that don't have cracks? I think builders now are just more willing to point them out when the build is done. Kind of like exposing yourself in public, and not living up to the measurement.

Look at you temperature differential from inside the dome to outiside. If the dome heated evenly, it would be much less prone to cracking. You'd need to apply heat to the outside too to make it cure and expand evenly, which is not how these ovens operate. You can't pretend that the physical properties involved in the materials you used don't have an effect. The oven gets heated from the inside, and will warm outward. Every brick in the dome will expand greater towards the interior of the dome faster than it does on the exterior of the dome. There's incredible stresses on placed on the bricks and mortar by heating on just one side.

Perhaps dmun's thinner dome walls are really a good idea - cutting down the thickness of the bricks in the dome. Faster transmission of the heat from interior to exterior equals more even heating and less stress on the brick and mortar. (Silly really - it makes totaly sense now and I'm kicking myself in the butt)!

Just a WAG (wild ass guess). Embrase your cracks!

[Les]

Re: Acoma's cure

Thinner does make sense. Hind sight is incredibly clear... I wished I went with 1/3 brick cuts. The benefits are obvious, less weight, less material, and faster heat up times. And as you point out, may reduce cracking.

Les...

[Acoma]

Re: Acoma's cure

DMUN, sounds like your logic to making thinner bricks is making more sense. We now have more debates for future builders

[mfiore]

Re: Acoma's cure

That's what I was thinking. For those of us that haven't built yet, just one more variable to reconsider

[david s]

Re: Acoma's cure

I recall a wood fired kiln I made about 30 years ago, that was made using loose stacked bricks. I had a thermocouple (or pyrometer) rigged up to it and had it full of kids pottery (for a biscuit firing). I had them go out of class and fossick for wood for a greater understanding of the whole process. Because I was teaching class I was forevever running up and down the back stairs of the art room to throw on wood. The temp was going up and down like crazy. The result was a lot of fine cracks in the pots. This tells me that refractory materials don't like temp fluctuations (especially at low temps) We like to get our ovens up to temp as quickly as possible when we should probably always do it slowly (ie do not exceed 100 C/Hr) But who has time for that. Live with small cracks. Sorry about the long story, but it is a good illustration of what happens.

[Acoma]

Re: Acoma's cure

David, thank you for sharing your history on the subject. Definately gets us to know more of each others past history on the subject. I bet the class was grateful for the experience.
Robert

[david s]

Re: Acoma's cure

Another observation I've made regarding WFO temp is that generally flame will increase your temp while a good bed of coals will simply maintain the temp. and the removal or not, of coals, once your oven is up to temp, makes little difference to temp drop.

[Acoma]

Re: Acoma's cure

David, I was thinking about that logic while doing a pizza party last night. I would take out 80% of the coals and replace with fresh log or two to maintain nice flame and heat balance throughout. I used the coals to sread over the front area inbetween pizzas, and shifting coals, logs to opposite side every 3rd pizza. Worked great to keep the heat perfectly high on the floor and dome.

[david s]

Re: Acoma's cure

Method perfection. Sounds like you've got it nailed.