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phippsj 11-18-2010 10:35 AM

Thoughts on curing a brick oven from a pretentious newbie :)
 
So, after getting through the 'crack panick' that I guess all of us newbies go on, I had to sit down and try to figure some things out (I really tried hard not to, but I am one of those engineering types, so....)

1) I think the idea of actually putting fire within the oven for the first three curing events is probably not the way to go. I see a lot of of folks had talked about charcoal brickets, but I was afraid of using those because I just don't want accelerants baked into my brick. Finally, I realized, that what I should have done was built a fire outside of my dome and places some embers into the dome with shovel. Temps would have been easier to control, and it could have been distributed evenly.

Taking that thought further, the heat differentials felt at the top of the oven are greater than those at the bottom of the oven. Spreading embers our evenly, with no open flame, should minimize that impact. I also think that oven temp measurements for that part of the curing process should be taken at the cieling. The increasing fires may indeed push water from oven core to oven crust (via brick and mortar), but the overall process is curing the mortar and brick from the top down (at least it did in my case).

2) I think it is theoretically impossible to not have cracking, the question is how big they will be. If the 'nice' ovens from FB, I could see why curing is so sensitive (need even expansion for a dome that has all the same material, as since heat goes up that expansion will be uneven so it needs to be as controlled as possible from top down). With our 'bulky' firebrick ovens, I think it would be pretty hard to break a brick. In that case, it comes down to mortar and bricks, which are bound to expand at different rates. Personally I believe the brick expands at a more rapid rate than the mortar. If the bricks are wedge cut (which it seems like everyone does), then that expansion pressure goes out, and hence cracks. Curing AFTER insulation would be beneficial according to that line of thought as well, because bricks and mortar would heat more evenly and thus expansion would be more even.

All of that makes me think that cracks are not the issue as much as crack management is. In that case, I think adding more and more embers from external fires for the first three days is good, then start fires on the next three days. There will still be cracks, but they should be as minimal as it is possible for the oven. I will only repair a crack if I can see it all the way through the oven, and so far I have not found one.

So, words of non-wisdome from a completely inexperienced newbie who is just finishing a first oven cure! A whole bunch of ways to say "I don't care about cracks, bring the fire on and give me some pizza!"

Tscarborough 11-18-2010 10:41 AM

Re: Thoughts on curing a brick oven from a pretentious newbie :)
 
This is all you need to know about concrete, masonry (units held apart by mortar), and stucco:

It IS going to crack, proper design tells it exactly WHERE to crack.

GianniFocaccia 11-18-2010 11:15 AM

Re: Thoughts on curing a brick oven from a pretentious newbie :)
 
Quote:

It IS going to crack, proper design tells it exactly WHERE to crack.
Knowing this, wouldn't the proper curing process include a short curing session before insulating the dome (to identify repairable cracks) and a more comprehensive curing session after insulating the dome?

md.guthrie 11-18-2010 11:24 AM

Re: Thoughts on curing a brick oven from a pretentious newbie :)
 
I used the rotary kiln and incinerator curing schedule which is 50 degrees per hour increase. I used propane as the heat source because it's easy to control the temp. Gas works just as good as wood...... all your doing is driving off the moisture in the structure. It cost $18.00 for the complete cure process and I have not a crack after 2 months of normal high temp operation.

phippsj 11-18-2010 11:42 AM

Re: Thoughts on curing a brick oven from a pretentious newbie :)
 
Quote:

Originally Posted by md.guthrie (Post 102410)
I used the rotary kiln and incinerator curing schedule which is 50 degrees per hour increase. I used propane as the heat source because it's easy to control the temp. Gas works just as good as wood...... all your doing is driving off the moisture in the structure. It cost $18.00 for the complete cure process and I have not a crack after 2 months of normal high temp operation.

On the inside or outside? I think that you probably have very small external cracks, maybe barely visible... unless your mortar is so strong it is preventing the brick from expanding faster than the mortar itself is. Either way, it's not an open wood flame, which makes a lot of sense. I wish I had done that method!

Tscarborough 11-18-2010 11:43 AM

Re: Thoughts on curing a brick oven from a pretentious newbie :)
 
I fired mine before insulating it, the day after I built it in fact, and I didn't pussy foot around with little fires either, it was cold when I was building it so I built normal fires. Note that for the oven itself I used a fire-set wet refractory cement, not a dry or home brew. I have zero cracks in the arch*, but the homebrew stucco cladding cracked in several places as expected.



*Mainly because of the particular and peculiar method of construction (timbrel construction).

GianniFocaccia 11-18-2010 12:38 PM

Re: Thoughts on curing a brick oven from a pretentious newbie :)
 
Quote:

*Mainly because of the particular and peculiar method of construction (timbrel construction).
Tom,

I've been curious how your arch was handling the oven heat/expansion cycles over time. After seeing your arch, I researched and fell in love with Guastavino vaults and arches. These were beautiful, architectural marvels well ahead of their time and far ahead of today's modern reinforced concrete projects.

Given the lateral loads a classic low-dome oven places its side walls, wouldn't it be feasible to construct a circular Guastavino vault that could be placed directly on top of a course of soldier bricks, thus directing the thrust straight down instead of sideways? I've been imagining constructing a 3.5" thick dome of this type using brick splits approximately 2.5"x5"x.75". I'm convinced it would be strong enough. The only unknown is how well the mortar would hold up to thermal cycling.

John

Tscarborough 11-18-2010 12:46 PM

Re: Thoughts on curing a brick oven from a pretentious newbie :)
 
That is why I used the wet-mix. It has finer aggregate and on the inside my joints are almost zero.I filled all gaps between the layers with the wetset, but used home brew as cladding because it has better flexural strength. As it stands, all of the mortar could fall out of my arch and it would not fail.

For an elliptical dome built with timbrel construction, you would either have to use very thin tiles cut very small, or have formed tiles, and then I think the cycling would become an issue because of the amount of joints.

edit-I still don't have any interior cracks. Also note that for the ends I notched them and didn't fill the inside gap with mortar to allow them to expand differentially, as it would certainly have cracked there had I not.


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