Author Topic: curing without fire  (Read 833 times)

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Offline scarbo

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curing without fire
« on: July 31, 2014, 07:40:02 PM »
I'm assembling an FGM 950B and about ready to start the curing process. I've seen a few build threads with pictures of electric work lights placed in an oven and aimed up at the dome.  Was this to imitate a curing fire, and if so, how well does it work? Obviously, no light can duplicate the work of flames rolling across the dome, so the last two fires will need to be authentic. But the first two are low-heat (150 and 160 C), and the goal is to keep the flames low enough to avoid touching the dome. So for these fires, it would seem that any heat source that produced the target dome temperature would be as good as any other.  I welcome anyone's input.


Offline Tscarborough

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Re: curing without fire
« Reply #1 on: July 31, 2014, 11:02:58 PM »
The goal is to raise the temperature of the oven gently enough to eliminate thermal shock.  Done slowly enough you can fire the oven to 1000 degrees the first time you put a fire in it.  Refractory cement and castable refractory both retain residual moisture that is not utilized by the chemical reactions that "set" them.

The residual moisture is free moisture and will leave the material, well, freely.  The instructions are stated as they are to idiot proof the process as much as possible.  Indirect heating (meaning actual flame not touching the masonry) is preferred to remove as much of the free moisture as possible, but the main point is to gently bring the mass up in temperature so as to decrease temperature differentials between the fire-face and the outside of the mass.

Offline stonecutter

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Re: curing without fire
« Reply #2 on: July 31, 2014, 11:20:53 PM »
And, the mortar and units are cured...you're drying the oven, not curing it.  A low fire maintained for as long as possible will drive out more water than hitting a high number and then letting the oven cool down.
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scott123

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Re: curing without fire
« Reply #3 on: August 01, 2014, 12:55:41 AM »
I use a completely overkill approach to curing, and, so far, with the people that I've assisted, I'm two for two without cracks, so until one of my assists produces a crack, I'm sticking with my pain in the butt approach.

When water boils and converts into steam, it expands about 700 times.  To me, this is the biggest threat to oven integrity, so my process stresses drying (sub 212F temps) and avoids boiling (above 212).  I've also found that most approaches, even electrical ones, tend to produce very localized heat. If, say, you're able to keep the top of the dome at 200 for a few days, but, during that time, the edge of the dome isn't exceeding 100, you may still have water where the oven is cooler.

My answer to this? I combine strong electric (a double hot plate) with a computer fan- 24/7 for a week. I find that the hot plate can typically be cranked to full and the moisture that's already in the dome/floor will naturally keep the temp low a few days, but I still monitor it closely.  After a week, then I move on to curing fires, basically a grid of single twigs spaced every foot or so, then, once the oven is starting to cool a bit, doubling twigs and decreasing the spacing, tripling the twigs, etc., until the floor is covered- and then moving on to actual logs. Ideally, you want it to take about a week to hit 300 using electric and then about a 100 deg. increase each day. Give or take :)

Once (if ;) ) I see a crack, then I'll subscribe to the 'cracks are inevitable' theory and tow the company line.  Until then, though, if someone asks me how to cure, this is how I'm instructing them.
« Last Edit: August 01, 2014, 07:35:41 AM by scott123 »

Offline TXCraig1

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Re: curing without fire
« Reply #4 on: August 01, 2014, 02:08:57 AM »
I used to think there is no “overkill.” There is only “open fire” and “I need to reload.”

I was wrong.
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Offline stonecutter

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Re: curing without fire
« Reply #5 on: August 01, 2014, 07:31:30 AM »
I use a completely overkill approach to curing, and, so far, with the people that I've assisted, I'm two for two without cracks, so until one of my assists produces a crack, I'm sticking with my pain in the butt approach.

When water boils and converts into steam, it expands about 700 times.  To me, this is the biggest threat to oven integrity, so my process stresses drying (sub 212F temps) and avoids boiling (above 212).  I've also found that most approaches, even electrical ones, tend to produce very localized heat. If, say, you're able to keep the top of the dome at 200 for a few days, but, during the time, the edge of the dome isn't exceeding 100, you may still have water where the oven is cooler.

My answer to this? I combine strong electric (a double hot plate) with a computer fan- 24/7 for a week. I find that the hot plate can typically be cranked to full and the moisture that's already in the dome/floor will naturally keep the temp low a few days, but I still monitor it closely.  After a week, then I move on to curing fires, basically a grid of single twigs spaced every foot or so, then, once the oven is starting to cool a bit, doubling twigs and decreasing the spacing, tripling the twigs, etc., until the floor is covered- and then moving on to actual logs. Ideally, you want it to take about a week to hit 300 using electric and then about a 100 deg. increase each day. Give or take :)

Once (if ;) ) I see a crack, then I'll subscribe to the 'cracks are inevitable' theory and tow the company line.  Until then, though, if someone asks me how to cure, this is how I'm instructing them.

Impatience is the biggest reason for premature cracking. 

You are doing extended heat up times to slowly dry out the oven, that is the point.   Your target tempurature range is going vary depending on the climate, how much water was used in the mortar, if the bricks were soaked..etc,etc.   I like your method because it takes time, and taking your time is going to help produce quality masonry work.
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When we build, let us think that we build for ever.
John Ruskin

scott123

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Re: curing without fire
« Reply #6 on: August 01, 2014, 07:45:39 AM »
I like your method because it takes time, and taking your time is going to help produce quality masonry work.

Thanks, although you will soon see that there are those who feel differently :)

Offline stonecutter

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Re: curing without fire
« Reply #7 on: August 01, 2014, 08:24:34 AM »
Thanks, although you will soon see that there are those who feel differently :)

It's not so much the method but the final result that matters most.
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When we build, let us think that we build for ever.
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Offline TXCraig1

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Re: curing without fire
« Reply #8 on: August 01, 2014, 10:29:04 AM »
Thanks, although you will soon see that there are those who feel differently :)

I was with you up the part about the twigs. Maybe twig means something different to me.
"We make great pizza, with sourdough when we can, commercial yeast when we must, but always great pizza."
Craig's Neapolitan Garage


scott123

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Re: curing without fire
« Reply #9 on: August 01, 2014, 10:43:42 AM »
twig = small branch

One twig is basically a very small fire.  As long as they're dry, their bark helps them burn well.  Where's there's trees, there's usually twigs, although store bought kindling will work just as well.

Offline TXCraig1

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Re: curing without fire
« Reply #10 on: August 01, 2014, 10:46:27 AM »
That's pretty much what I envisioned. How do you light all those small fires? Are you just feeding the oven endlessly for hours on end?
"We make great pizza, with sourdough when we can, commercial yeast when we must, but always great pizza."
Craig's Neapolitan Garage

scott123

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Re: curing without fire
« Reply #11 on: August 01, 2014, 11:22:37 AM »
Not endlessly, no.  For the first fire, I put a twig at the edge of the perimeter, every two 'hours'- 12 o'clock, 2 o'clock, 4 o'clock, etc. etc.  then three more twigs towards the center of the floor. That's 9 very small fires. You can light them with a long twig, or a stick- all at once.  If the oven has been drying electrically/convectionally for a week, these 9 very spread out fires should produce a reasonable bump in dome temp- ideally around a 100 degree bump (to around 350-400).  If you can hit 350, then you close the door to just a crack and let the oven cool for a while, such as overnight. The next day, double the twigs.   You don't want to stack them, but they can touch, so you're not lighting twice the fires- imagine a grid of twigs.  The day after, then a greater number of twigs (or kindling) to create a denser grid work.

It's not an exact science. Depending on the oven, in order to hit 100 degree increments, you might need thicker twigs  or more of them.  But spreading them out spreads out the heat- both to the dome and the floor. Whatever coals you end up with, if you're motivated, you can move each back an hour to an open spot of floor.

To be honest, I think, after a week building up to 300 with a fan and a double hot plate, you're going to have some leeway after that.  The electric phase of the process if far more critical, imo.

Edit: put another way, my fire phase is basically the same thing as the curing fires you see in most oven instructions- just spread out rather than localized.
« Last Edit: August 01, 2014, 11:26:56 AM by scott123 »

Offline TXCraig1

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Re: curing without fire
« Reply #12 on: August 01, 2014, 12:49:51 PM »
Why not burn some wood in in a grill, barrel, hole in the gorund, etc. and place coals in the oven rather than mess with a bunch of tiny fires that have to be a royal PITA to keep lit.
"We make great pizza, with sourdough when we can, commercial yeast when we must, but always great pizza."
Craig's Neapolitan Garage

Offline TXCraig1

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Re: curing without fire
« Reply #13 on: August 01, 2014, 12:52:32 PM »
Better yet, why not use a large propane burner like I use to light my oven - starting very low and scaling up? You could easily control the temp with good precision from barely warm to 800F+.
"We make great pizza, with sourdough when we can, commercial yeast when we must, but always great pizza."
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Offline stonecutter

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Re: curing without fire
« Reply #14 on: August 01, 2014, 01:20:27 PM »
Why not burn some wood in in a grill, barrel, hole in the gorund, etc. and place coals in the oven rather than mess with a bunch of tiny fires that have to be a royal PITA to keep lit.

Some guys do that with briquettes.
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scott123

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Re: curing without fire
« Reply #15 on: August 01, 2014, 03:26:36 PM »
Why not burn some wood in in a grill, barrel, hole in the gorund, etc. and place coals in the oven rather than mess with a bunch of tiny fires that have to be a royal PITA to keep lit.

Coals tend to be more about conduction and radiation than flame which I believe gives you greater convection. Heating the floor is good, but I think it helps to have heat flowing faster to the dome, as you find with a flame.

I use twigs all the time for kindling.  Perhaps it depends on the species of wood I'm using, but I've never had a twig go out.  Bark loves flame.

scott123

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Re: curing without fire
« Reply #16 on: August 01, 2014, 04:06:23 PM »
Better yet, why not use a large propane burner like I use to light my oven - starting very low and scaling up? You could easily control the temp with good precision from barely warm to 800F+.

Between the hot plate and computer fan and the twig fires, my entire focus is spreading the heat out as much as possible to every part of the oven.  For instance, with the hot plate and fan, when the dome is 225, the coldest part of the oven, the remotest crevice, is 200- and that's a stable 225 dome for 24 or even 48 hours.  Your propane burner is way too unidirectional, imo.  You can wave it around a bit, but you're not going to see the kind of low even temps I'm achieving- either with electric or twigs.

Your burner + a fan might provide even heat, but you can only use that fan up to a certain temp. I've taken one up to 225, and, honestly, I didn't think it was ever going to survive.  If it can't go much north of that, it's not going to work with your weed burner.

Last year, I asked about an affordable high temp fan that could be place inside a wood fired oven, and nothing panned out from that conversation.  If you have an idea for a fan, or have another way to gently and evenly heat an oven, I'm open.  The hot plate and computer fan took a great deal of trial and error, so, for that phase, I'm pretty settled in.  But for the twigs, it was just the first thing that came to mind. I'm certain it can be improved upon.
« Last Edit: August 01, 2014, 04:10:25 PM by scott123 »


Offline TXCraig1

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Re: curing without fire
« Reply #17 on: August 01, 2014, 04:19:42 PM »
Perhaps it depends on the species of wood I'm using, but I've never had a twig go out.  Bark loves flame.

Birch bark or cedar bark. Most hardwood barks don't burn all that well.
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scott123

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Re: curing without fire
« Reply #18 on: August 01, 2014, 04:30:04 PM »
Birch bark or cedar bark. Most hardwood barks don't burn all that well.

I took a tree identification class at summer camp as a youngster, and, while I forgot most of what I learned, birch and cedar could be one of the two most easily recognizable barks.  I've never burned birch or cedar twigs.  If I had to take a guess, based upon the predominant trees in my area, the twigs that I've burned were either oak and/or maple- and they burned like there was no tomorrow.

And, just to be clear, when I talk about twigs, I'm not talking about cutting one off a tree. I'm talking about walking around the property and grabbing twigs off the ground- twigs that have been on the ground for a while. A green twig, bark or no bark, will burn just as poorly as a green log.
« Last Edit: August 01, 2014, 04:33:11 PM by scott123 »

Offline TXCraig1

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Re: curing without fire
« Reply #19 on: August 01, 2014, 04:54:50 PM »
I took a tree identification class at summer camp as a youngster, and, while I forgot most of what I learned, birch and cedar could be one of the two most easily recognizable barks.  I've never burned birch or cedar twigs.  If I had to take a guess, based upon the predominant trees in my area, the twigs that I've burned were either oak and/or maple- and they burned like there was no tomorrow.

And, just to be clear, when I talk about twigs, I'm not talking about cutting one off a tree. I'm talking about walking around the property and grabbing twigs off the ground- twigs that have been on the ground for a while. A green twig, bark or no bark, will burn just as poorly as a green log.

Remember you are talking to a guy who has started about as many camp fires as you have baked NY pies.
"We make great pizza, with sourdough when we can, commercial yeast when we must, but always great pizza."
Craig's Neapolitan Garage

Offline Tscarborough

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Re: curing without fire
« Reply #20 on: August 01, 2014, 11:12:38 PM »
I have never seen steam pops on firebrick, or any brick or mortar in my career.  I HAVE seen ice pops though, and aggregate pops from heat in concrete.

Some people are going to cure the oven with a "Now that's a FIRE, Gus" fire, and others are going to lay out origami twig diagrams, cooled by computer fans.  Either end of the spectrum will work, but riding just to the left of origami on the bell curve will probably be more efficient than either.
« Last Edit: August 01, 2014, 11:16:01 PM by Tscarborough »