#41  
Old 03-20-2008, 06:35 AM
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Default Re: Curing strategy

David, where I'm coming from is thinking about the full range of the heating and cooling cycle as it relates to the thermal expansion and contraction. By cooling to ambient, you get a broader range of expansion during the cure. Since you will do this many times over the life of the oven, I'd think that would be good....but I could be bass ackwards, maybe it's better to do that after you've driven out the moisture.

From my days making ceramics and pottery I remember that bisqueware is usually dry. As I recall wet bisquit will surely crack. Neat analogy to WFO's
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  #42  
Old 03-20-2008, 08:55 AM
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Default Re: Curing strategy

Well, I have lots of cracks along outside perimeter, marathon types. Long distance, perimeter distances, and all shooting to the top. 4-5 of them. I have a 1/4 to 1/2 inch mortar as shell, not helping, but the cracks don't run to the inside, just to the mortar within bricks on outside of dome. I cannot see any cracks on the inside though. I cannot verify this part because the inside is as black as a coal miners face. I am proceeding with the curing strategy, and I will use furnace cement with some grinding once the curing process is complete.
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  #43  
Old 03-20-2008, 10:14 AM
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Default Re: Curing strategy

Quote:
Originally Posted by Xabia Jim View Post
But it's the Italians that started this new cure recommencation 3 months ago!
Precisely.............I read James first post on this thread as the Italians putting FB users right on curing ovens...........something that I was never informed about when we built our oven. Thank goodness I found FB forum.

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  #44  
Old 03-21-2008, 07:46 PM
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Default Re: Curing strategy

My point about the biscuitware analogy was that although it may appear dry there is still water in it and that takes energy to remove it, which is still being eliminated well over waters boiling point of 100 C . Why would you want to deliberately make the material expand and contract at this delicate stage? It doesn't make sense. Just a gradual expansion is a better way. Castable refractory firing schedules suggest a very slow and steady increase. Theyre the manufacturers so they should know their product.
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  #45  
Old 03-21-2008, 08:28 PM
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Default Re: Curing strategy

David, I don't believe you are being questioned. I believe we are evolving with techniques best fit to drive out the moisture, eliminate or minimize cracks, and strenghten the oven with best practice. The old philosophy for ovens was updated recently upon world practices. Should this method you speak of be a new and evolving technique to revolutionize the curing process, then that is AWESOME! My idea is for you to apply this practice you speak of to an oven you plan to build, even a guiny pig mini oven, does not matter. If it works with no cracks, then wala...
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Old 03-21-2008, 09:17 PM
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Now, to the new technique recently updated, and practiced. I believe that after the 4th cure, at 600 temp day, for 10 hour sessions, totaling 40 hours into the curing process, that I may (or may not) have answers to help others for crack prevention. This also applies to the building process, which leads to the curing cracks that I believe are impacting me.

I thought I had a disaster on my hands when I saw the spider web of cracks on the outside dome, and the 3 cracks along my arch, one on each side, and one up top. I believe my approach to building the oven was careful, with consistant mortaring of bricks, using patience and practice. I thought that by just applying a thin amount of mortar to the outside dome (1/4 to 1/2 inch) it might help the bricks to limit movement. I did not mortar or add support to the outside of the arch sides. For the dome, the bricks all crossed bricks below with middle point positioning. I believe that the Refmix mortar I used was well worth the cost, it did not break up or crumble meaning that heat in the area was right for the environment during the COLD and snowy winter, and no bricks are cracking. What I did by packing each bricks sides, keeping the bricks moist, then moving to the next brick allowed strong bonding between brick and mortar. The weight of the dome is also great and limits movement due to pattern of bricks laying upon each other with great balance. I built my arch with the idea of having an angle for looks, but I also attempted the best way to keep strength as well.

Now for the cracks, I look at where the cracks are and believe I may know the answer.

A: Dome cracks are only showing on the outside, not on the inside. I believe that due to the shell of mortar and expansion, that it was a waiste at this time, but after the curing process would be possible. The bricks were well mortared together and needed to bond with moisture being driven out. No cracks are going into the dome, none are going through bricks. I believe I would have none, or fewer crack concerns upon exterior of dome if I would have waited with this detail.

B: The arch has cracks along the wall which I believe is because of limited weight with no true mass of supporting bricks above. Looking at the dome, lots of weight is balanced by brick upon brick, altered upon each other. My cracks on the sides occur after the 3rd brick up which prepare for the arch to meet the wall. There is no real weight at the front face area either, just one brick above.

C: The arch at the top center is also full of exposure with no support above or on the sides.

D: For the arch, so much is expose to the cool outside compared to the dome, and the heat measurement at the arch is approximately 30 percent of dome temp. To me, this is where the root issue is to be answered.

When dealing with heat expansion from within the oven for 10 hours plus at 300, 350, no issues occured to the dome or arch. Upon entering the 500 temp, the heat finally reached the outside dome and arch enough to allow varing heat to create the varying expansion of bricks while the mortar cured. Being that they have different heat affecting the cure, I believe that seperation of bricks and mortar resulted. I also believe that if I did a rectangular arch, My issue may have been prevented as well. The curves of the arch seem to expose curing differences and weight balances during the cure, that I believe were the cause of the cracks. Now, this is a new practice, and I am merely documenting the details. Am I wrong, who knows, I am not a mason, I have only built one oven.

Tomorrow is day 5 with 10 hours of 700 temp. I may or may not have further issue with cracks. I may or may not have cracks on the inside of the dome.

Once the curing process is done, I will then go into the plasma state where my oven will once again be tested with a new dimension of heat. I may go up to 900 for a couple hours for a few sessions, and do some foods. This will not be long hour curing temps, just heat temps. After a few sessions, I will then go to Plasma state and saturate the bricks to allow for pizza. I will likely do my first pizzas on Sunday, April 13th. Saturday is wood day.

I can only hope this detail helps someone?
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  #47  
Old 03-21-2008, 10:35 PM
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Default Re: Curing strategy

David..........I can see where you are coming from with this. It would make sense to initially go up to cooking temps more slowly and therefore limit rapid expansion. Unfortunately most are not going to be maintaining fire in the oven so the heat/cool cycle must continue. Members have reported improved performance many firings after the curing period. Clearly driving out moisture continues quite some time. I don't feel that a lot of the cracks we are seeing are avoidable given expansion coefficients but neither are they detrimental to performance of the oven nor in most cases appearance, IMHO.

Someone once said ........"perfection is a journey, not a destination". Long may we all share it.

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  #48  
Old 03-21-2008, 11:57 PM
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Default Re: Curing strategy

Inishta,
Heartily agree. I don't think it is possible to eliminate fine cracks entirely. Theyre there and the open abit when firing but they disappear when the oven cools and they dont seem to get any worse. So what ? What would you expect when the whole structure expands so much. It doesn't affect the performance. All ovens and kilns for that matter have cracks. In fact that is precisely why the ovens are made of bricks so the cracks can occur at the joins where you want them to.
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  #49  
Old 03-22-2008, 12:21 AM
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Default Crack science

Quote:
Originally Posted by Xabia Jim View Post
David, but I could be bass ackwards,

From my days making ceramics and pottery I remember that bisqueware is usually dry. As I recall wet bisquit will surely crack. Neat analogy to WFO's
Thinking about this further, bisque firing was done at lower temperatures, then the clay was glazed and fired at higher temperatures. Bigger items have more of a tendency to crack than smaller ones.

From my days as a Geologist I learned that clays were 40% air space (sand is more like 25%) so in drying a clay, it takes a long time because it's comprised of tiny microscopic plates all jumbled up.....drying clay shrinks and cracks but potters are able to make crack free clay vessels.

We cycle our ovens during curing, but could a steady increase in heat be even better?.......searching for the VMO
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  #50  
Old 03-22-2008, 02:27 AM
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Default Re: Curing strategy

The recommended cure rate for castable refractory (which is the same material essentially, as fire brick or refractory mortar),is (for our WFO wall thickness) 50 C/hr I .believe that this is a good guide for a curing stategy. Simple enough
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