Oven Curing

[winnsb]

Re: Oven Curing

James we have contacted tech support and have sent pictures. The story, I took a week off of work and sat by my oven 12 hours a day during the curing cycles. Everything went exactly as you described. I did not start curing my oven Casa 2g90 for 6 weeks after the build. It is enclosed in a water proof brick sturcture.

We have had our first pizza night and are very pleased with the results, until clean up and discoverd a 1 inch by 8 inch piece of the left rear interior lip laying in the ash. What now, you cah see the out side half of the joint and the seam?

[winnsb]

Re: Oven Curing

Curing your oven is an important step in the installation of any brick oven -- whether it is a Forno Bravo precast oven, a Forno Bravo Artigiano brick oven, or a Pompeii brick oven. Heating up your oven too fast can lead to cracks. You have invested a great deal of time, money and energy in your oven, so go slow, and cure your oven properly. If at all possible, don't schedule a pizza party the weekend your dome is finished.

After you have installed your oven, there is still a great deal of moisture in the mortars, hearth concrete, vermiculite, and the oven chamber and vent themselves. Each of these oven components was recently produced using an air-drying, water-based process. Simply letting the oven stand for a week does very little to "cure" the moisture out of them oven. In fact, the Forno Bravo precast oven producer recommends letting the oven stand for a week after it has been assmebled before "starting" the curing process. Thicker sections of concrete can take many weeks to cure.

You are trying to avoid two problems. First, any mortar or concrete that dries too fast shrinks and cracks. These cracks can let hot air and/or smoke escape from the oven chamber. Second, if you bring your oven up to heat while there is still sufficient moisture in the oven dome or mortars, you will actually create steam, which can produce hairline fractures, or even cracks in your oven. I heard a story (possibly an urban legend) from one of our installers who used to work with one of our competitor's ovens, where the home owner lit such a large fire in a non-cured oven that a chunck of the oven dome actually blew out the front door. Hmmm. Maybe.

Also, using a space heater can help, but only so far. It is not an alternative to fire curing. We ran a space heater in an assembled Forno Bravo precast oven for two days, then quickly heated the oven up, (don't do this at home -- it was an experiement to see what would happen to an oven that we have here) and we found that we created a very large amount of steam from the oven, mortars and vermiculite, which went on for hours and hours.

To be safe, here is a good curing schedule.

1. Let the oven sit for a week or so after you have finished the dome.
2. Run a series of seven fires, starting with a small, newspaper-only fire.
3. Increase the size of the fire each day by about 100F
200F
300F
400F
500F
600F
700F
800F
4. Let the oven fall back to cool as soon as you reach the temperature you want. It is important to bring the oven up to heat gently, then back down to cold, each time.
5. If you don't have an infrared thermometer, try this schedule:
Newspaper only
Newspaper and a little kindling
1 stick of 2"x3"X16" wood
2 sticks of wood
3 sticks of wood
4 sticks of wood
5 sticks of wood

James

james your tech spec only suggests a 5 day cure and your people suggest maintaing heat no mention of a cool down

[indiebands]

Re: Oven Curing

Hi All,

I don't have FB oven, but a 4 piece modular Vesuvio oven from Zio-Ciro in Italy. The builders used a perlite-cement mixture instead of the loose vermiculite (perlite) on top of the insulating blanket, which was placed on top of a 10cm perlite-cement mixture above the dome. I am wondering what impact this will have on my curing process and curing time. I have a 60watt incandescent bulb in there at the moment with the door mostly closed and am eager to start the curing fires, but want to be conservative given the amount of water that is in the whole structure. Any suggestions or recommendations? I realize my oven is pretty far from the specs recommended by oven manufacturers ;-(

Thanks in advance.

[SCChris]

Re: Oven Curing

indiebands,

Curing is about time at temp. Whatever the material, it must be raised to temperature long enough to dry. The 100C/212F point is critical, not only because when water flashes to steam, it can damage the oven, but also because of the temperature differences that can occur between the wet areas and dry areas and the damaging stresses that these temperature differences create.

There is another temperature point where the oven materials finally release more tightly bound water. I don't remember exactly, but this is somewhere between 260C/500F and 426C/800F. By moving the oven temperatures up slowly and allowing the oven to saturate, the water will have time to migrate out of the structure in a nondestructive way and the damaging stresses will be avoided.

Slow and steady are the guide words for curing.

Chris

[indiebands]

Re: Oven Curing

indiebands,

Curing is about time at temp. Whatever the material, it must be raised to temperature long enough to dry. The 100C/212F point is critical, not only because when water flashes to steam, it can damage the oven, but also because of the temperature differences that can occur between the wet areas and dry areas and the damaging stresses that these temperature differences create.

There is another temperature point where the oven materials finally release more tightly bound water. I don't remember exactly, but this is somewhere between 260C/500F and 426C/800F. By moving the oven temperatures up slowly and allowing the oven to saturate, the water will have time to migrate out of the structure in a nondestructive way and the damaging stresses will be avoided.

Slow and steady are the guide words for curing.

Chris

Thanks a lot, Chris. How long should I leave the 100C temperature (assuming I can hit it with a Quartz Halogen lamp 500-600W)? A few days, a week? Thanks. I know there is a lot of moisture in the structure above and below the oven.

[SCChris]

Re: Oven Curing

indiebands, you want to hold the temp long enough to dry. I think you should be good after a couple of days at 70C. The time will differ depending on your weather. If you live in a humid environment the time will be longer. When I cured, the oven had been finished for a few weeks and we had a week of very dry weather.

In the early part of curing you want air movement over the exterior of the dome, as you progress past 100C, I think you want to insulate. This insulation will make it easier to hold even temperatures with less fuel and minimize the uneven stresses that can crack the dome. As your oven dries the amount of fuel to get to temp and hold these temps, will drop. This can catch you by surprise, be warned, you need to add fuel in small amounts.

If you choose to use wood, try to use the same type of wood and roughly the same sizes. This will help you to anticipate what the next bit of wood will do. Also I recommend investing in a infrared thermometer and take the temperatures of the dome at the base of the interior wall, mid way up and at the top of the dome. All of these measurements are building your knowledge about the oven..

Slow Slow Slow..

Chris

[Lrby]

Re: Oven Curing

I am on the second day of curing. My oven floor separated from one of the side walls. Is that normal?

[SCChris]

Re: Oven Curing

Is you oven Modular or Brick?
The modular ovens that I have seen have a separate floor and many times when a brick oven is built the floor is independent of the dome structure. Can you post a picture?

Chris

[Lrby]

Re: Oven Curing

Chris,

Thanks for replying. Your previous posts have been very informative. It is the strada60. I think it s just the expansion from the heat. I have less space as the oven has cooled down.

Jamie

[thomasshore]

Re: Just reach the temperature

Hey Fio,

Just reach the temperature, then let it cool down. You fires will get a little longer, but the idea is to go up and down without stressing anything.
James

James,
I just placed my Primavera 70 on my home built stand. For curing I should start with newspaper (how much, loose or rolled) and just get to the 300 mark and then close the door? Day 2 repeat to 350, Day 3 a little wood and paper once again to temp and then close?
Thanks,
Tom

[Mooveebuff]

Re: Oven Curing

First off, thanks to all of you that post on here....it is extremely helpful to those of us just starting out.

I won't lie to you and tell you that I've read this entire thread, but I've read large portions of it. I have finished my dome as of Thursday, 3 days ago. My oven is not a brick oven, but rather a clay oven. My dome is made of 1 part fire clay mixed with 3 parts sand.

I understand the process of curing, but I was wondering, do any of you know how much of this is applicable to a clay oven? I imagine the process is similar, but I'm assuming clay will be more delicate than concrete/mortars. I actually have a break from the rain and the sun is out today, so I'm hoping to pull the sand form out from inside of the dome tomorrow.

Is a week long enough to let the oven sit before building fires? I've made it to this point without any disasters...I'd like to keep that streak going! Thanks in advance for any help/advice.

[SCChris]

Re: Oven Curing

Mooveebuff, Curing a clay oven is going to be a bit different than brick.. Although firebrick starts as clay and sand that won't break down in heat. Firebrick is quite porous and so water can move out of the brick and mortar relatively fast compared to an all clay structure. The variables that you're going to be dealing with are how thick the walls are and how fine the clay is. Clay is microscopic planes of material and when water is added the water occupies the space between these planes and this gives the material plasticity and this water also forces the material to expand. This is why building on clay, expansive soil, is not a good idea.

Some ceramics, Raku comes to mind, can be wet and fired in this condition and because of the large proportion of heat stable aggregate in the body, the steam can vent, and with fire, magically you have a cup or bowl. My point is that if the clay you used has a significant portion of aggregate, then steam and vapor will move and the structure will dry relatively quicker than otherwise.

Ok, because you are using clay, if it were me, I'd be going very slow and carefully. The center of the clay will not dry easily. Start with a quartz work light and move air through the oven and over the exterior of the oven. The light will warm the interior and help to move air. As the interior dries you can start to close the entry and this will help to contain the heat.

You spent time with the build, take more time than you'd like with the curing and you'll have long use of your oven.

Chris

[david s]

Re: Oven Curing

It is too late now, but one trick is to add fibres to your mix that are designed to burn away at low temp leaving a network of mini pipes that water vapor can escape through. This is what proprietry refractory castables contain.Polypropylene fibres are usually used and the fibres have a melting point of 160 C. I think adding horse manure to the mix for a cob oven has a similar effect. Chris, while I agree with almost all you have said, it has been my experience that clays with a high proportion of grog are actually slower to dry than finer bodies. One trick to use is to throw some plastic over the dome to test whether there is any condensation under it. This will tell you if water is still being expelled.

[GreenBldr]

Re: Oven Curing

I put a heat lamp in my FB oven for days, but at some point you will have to have fire. Keep the fire small and resist the impulse to make it larger even if you have kept it small for a long time. What will happen is that a small fire will continuously drive off water, tending to hold the mass at a certain temperature. If you are monitoring temperature, it will seem like you aren't getting anywhere. Once the water is driven off, the temperature of the clay will start to rise again, without having to increase the fire size. Keep it small and steady

[C5dad]

Re: Oven Curing

I used a propane weed burner and adjusted the flame as needed. Also: Make sure that you have the flame hit some bricks placed in front of your bricks as a flame barrier. You do not want to have the flame hitting your brickwork and make a nasty old crack (like what happened to me on day 3 or 4 and I determined the solution!)

As a side note: I use my weed burner to start the fire as well - works like a charm.