Longmont, CO new WFO build - casted over sand

[david s]

The thicker it is the slower you need to go. Avoid direct flame impingement on the dome in early fires by using a combo of small sticks and BBQ fuel. If you see visible steam, back off you are going too hard.

[cnegrelli]

small fire late yesterday - but didn't have much success in getting the charcoal going.
This morning, I rigged it up to get it off the floor and get some oxygen and I've got a nice charcoal fire going.
Large differentials between dome peak and lower walls, which I've heard David state many times means there's moisture down below. No doubt.
So I'll be going at this slow.

Current Temp readings:
Floor: 100 F
6" up Wall: 140 F
Top: 220F

[fox]

I managed to do two small fires a day for the first few days but then the oven would not cool down enough so I went to one fire every 24 hours.
I used kindling for the first few then added a few blocks of wood but kept the fire going for an hour or so after around 8 fires I was getting 250c on the floor then just feeding it for around 1/2 an hour.
I made the first cooking fire after 7 days & got the floor to 400c.
The dome went white alter my second cooking fire, I held around 400c for two hours.

[cnegrelli]

I'm seeing floor staying around 100 degrees yet dome top up at 250. Not seeing steam. When these things are full go (later) what sort of differential is to be expected? Heat rises, no?

[david s]

I'm seeing floor staying around 100 degrees yet dome top up at 250. Not seeing steam. When these things are full go (later) what sort of differential is to be expected? Heat rises, no?

Yes, that's correct. When the top of the dome reaches around 300 C it burns off the soot and will become dry. This means that it will heat even faster and the temperature difference will be even greater. You can spread the coals out to the sides in an effort to heat the bottom rather than the top. It is always difficult to maintain a fire in a new, wet oven and it's common for the fire to go out or get the charcoal or brickettes to light. You will probably also find it rather smoky. As more of the black burns off you will have a persistent ring of black at the base of the oven which indicates that this part is still not dry, so don't get too eager at that point. Because most of the oven will be dry the fire will gallop away easily.

[cnegrelli]

Yes, that's correct. When the top of the dome reaches around 300 C it burns off the soot and will become dry. This means that it will heat even faster and the temperature difference will be even greater. You can spread the coals out to the sides in an effort to heat the bottom rather than the top. It is always difficult to maintain a fire in a new, wet oven and it's common for the fire to go out or get the charcoal or brickettes to light. You will probably also find it rather smoky. As more of the black burns off you will have a persistent ring of black at the base of the oven which indicates that this part is still not dry, so don't get too eager at that point. Because most of the oven will be dry the fire will gallop away easily.

David - thanks again for all the advice.
My dome is very dark, almost black in color before the fires (that's the color of the Castmax 28 refractory) so I'm going to have a lot of trouble knowing when I've cleared the dome!

It started to sprinkle a few times today for minutes at a time, and I'd cover it with a tarp. When I took it off, there was quite a bit of moisture collecting in just a few minutes, so it's plenty wet in there. It's been 17 days since I cast the dome, 10 days since I cast the gallery/flue, and 5 days since I wet it up to patch. I honestly thought it would be pretty dry given our humidity here has been very low. I was warned.....

Had a charcoal fire going today for about 10 hours at 250F at the top of the dome. 140F about 8" up on sides. Floor stayed about 100.

Working on the enclosure as much as I can. Damn Octogon is driving me crazy!

[cnegrelli]

Continuing the curing fires with charcoal and seeing some progress in terms of lower temps closer to dome top temps as I move the fire around the periphery. Keeping the dome around 300-350 and just going at it for hours at a time and being patient right now.

Meanwhile, finally have the front of the enclosure the way I want it (the engineer in me knows I should have taken the time over the winter to learn a CAD program, but alas, I was lazy). Plus I'm and Electrical, not a Civil or Mechanical, so there is that. Here's the latest pic.

[cnegrelli]

So I continue with curing fires, reaching 450 F for long periods, keeping warm overnight. Dome has a distinct ring around bottom about 4" up. I've been using fires contained in a basket that is an inch or so off the floor to boost oxygen. What is this ring line and what does it mean to me? Zero visible cracks from inside so far. Any help appreciated!

[jonv]

That ring is below the level of the smoke - it'll always be there and shows the layer of clean air drawn in to the oven whilst the smokey hot air exhausts above. Nothing to worry about

[david s]

That black ring is what I posted about before in my previous post https://community.fornobravo.com/for...204#post399204

The soot burns off at around 300 C, so that black ring indicates that the refractory there is below that temperature. This is quite common on new ovens. If you have an IR thermometer it should confirm this. Don't push the fire too fast until it has all burnt off.

[cnegrelli]

I'm seeing lower temps below that ring... With IR thermometer. I'm only at 300F. Am I confused, perhaps I will have to recheck.

[cnegrelli]

That black ring is what I posted about before in my previous post https://community.fornobravo.com/for...204#post399204

The soot burns off at around 300 C, so that black ring indicates that the refractory there is below that temperature. This is quite common on new ovens. If you have an IR thermometer it should confirm this. Don't push the fire too fast until it has all burnt off.

Question, the black ring that starts 4" up the wall indicates the temperature is below the 300C below that mark, correct? It's all black above.

So I'm no where near 300C, I'm more like 300F with the top of the dome perhaps around 400-450F and the bottom much cooler. Several days where I had long fires going where the floor is in the low 200s, the walls about 12" up around 300 and the dome 400. I've had spikes above that, but only for short periods.

I believe I picked up moisture in the CaSi from keeping the dome wet for that first week. I put a lot of water on it. There's been no rain and 3 weeks of drying out. 5 days of fires, 8-10 hours at a time (today only 4-5). Should I be proceeding towards 300C or slow and steady as is?

[david s]

From what you describe it sounds like you can build the fires bigger and try pushing coals towards the outside to get those areas hotter.

[Gulf]

Also, pull some of the coals toward the door and build a berm or levie across the door opening during the later stages of the firing. That will raise the drawing air up a little above the floor level (which is where the dampness is coming from, helping to dry the floor) If the oven is dry, and it reaches a saturation temp, all of the black will clear.

[cnegrelli]

Thanks gents!