Oven Curing

[james]

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

[dmun]

I'm not expert, but...

I think a lot of the discussion of curing the ovens overlooks an important point about the way concrete works. Concrete and ordinary mortars depend on their strength on creating a chemical-mechanical matrix between the aggregate (the stuff like gravel and sand) and the portland cement, which (to oversimplify) is a baked limestone powder. This process is called hydration, which to the casual reader might mean "get it wet", which is true enough, but what it really means is "keep it wet" for at least a week.

Concrete isn't like, say, drywall compound, which is a mud that dries out and it's ready. This curing process needs time to happen, and it needs to be in a fairly narrow room-temperature range, and it needs to have moisture to work. This isn't usually a problem with things like foundation slabs which are fairly massive, and in contact with ground moisture, but with mortar and bricks or blocks, drying out pre-maturely is a real danger, leaving you with a crumbly mess.

Getting your bricks wet isn't an optional step. Masons run a hose over the brick stack for an hour or so. They don't need to be sopping wet (that makes the mortar sloppy and weak) but they shouldn't sop up moisture.

So, for at least a week, don't worry about drying out your oven. Keep it wet. Keep it out of the direct sun, but don't wrap it with a tarp so it bakes. If it's dry weather (Not a problem we've been having on the east coast this summer) get out there with your hose on mist, and wet it down two, three times a day.

Once that first week has passed, you can worry about drying out the oven, with graduated fires and such.

[Fio]

A VERY timely post. . .

I just burned my second fire on the Fourth of July. It went well - the oven got up to 300 degrees briefly before dying down. A few small cracks have appeared near the vent, as expected, but no major cracking of the dome.

I have a question: How LONG should each fire burn? My first two fires lasted only a half hour. The dome got warm the first time, and hotter the second time. How long should I burn my third fire, and the fires thereafter?

Thanks,

- Fio

[james]

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

[TomB]

outside temperature

The top of my oven is getting pretty hot. I can not leave my hand but for a second. I am in the final stage of curing my oven and do not know how hot to expect. Do I need more insulation and what kind?

[james]

Tom,

Can you remind us what your insulating schedule is. It's possible, even probable that you are still driving out moisture, which is the source of the heat. But if your insulation is on the thin-side, you might want to bulk it up.
James

[TomB]

Hot top

I only ended up with 3 inches of vermiculite/concrete on the very top. There is much more down the sides. I am thinking of covering it with aluminum and pouring another layer of vermiculite/concrete.

[firepie]

I have 6 inches of vermiculite/cement mixture covering my dome. Last saturaday I had the interior of the dome at 750 degrees for over 2 hours, yet the exterior of the dome was cool to the touch.


Tom

[james]

I'm going to venture the guess that it's a little bit of both -- a little moisture left, and a little too thin on the insulation. After a lot of curing, 3" should be OK (but not great).

If you can add the extra vermiculite concrete on top, that should eliminate any worry.

I'm not sure you need the foil, but could just add the vermiculite concrete.

Anybody want to comment on that?
James

[arevalo53anos]

Tom, James:


My oven have only one inch of cladding over the bricks. Over it there is a layer of fiberglass and, then, nearly of three inches of vermiculite/cement (8:1).
In a normal baking day, the temperature on the hearth reachs 800/900 ?F with an internal ambient one of more than a thousand ?F.
The oven has 8 thermocouples and two of these are out of the isolation, in the upper and lower oven positions.
These thermocouples shows a little heat loss during the baking hours, going from ambient temperatures (around the 80's) to 120 ?F. When the oven was working during three days (last Christmas) the final temperature in the external surface of the isolation was 170 ?F (for reference, the hearth temperature, after 24 hs without fire was 150?F).
Very good, I think.
Of course, isolation will be never too much.
It is necessary to compare the expected lost temperature gradient with the cost and effort to instal the isolation, at some point (Cost/benefit relationship).
In reference to aluminium foil, it will be not a good ideia to put it between two layers of vermiculite.
Do not forget that the foil do not permit the humidity go up and out. And this is a bad thing.
If you could read a past thread (answering to Marcel) about this item, it will be possible to have more data to analize.
I hope this help.

Luis

[jayjay]

Fires (what tricks do the members use to get the oven hotter quicker)

My curing fires went well. But I do not think that I got my baking fire hot enough. How much wood does it take to heat a 3 x 4 foot oven with a wall thickness of about 8 inches. What size wood to most folks use? I am sure I am not using enough wood from what the post on this site are indicating.
I did not notice any cracks in the dome or in the oven walls. I did not see any cracks even near the chimney. I did notice that the smoke came out of the front of the oven, we have no door yet, and not up the chimney,
When the flames died down the smoke went mostly up the chimney, rather
than out the door.
I have been using light weigh dried mango wood. We did put a few good size
trunk peices in the fire, but most of the wood was on the smaller side, about an inch or more in diameter and about 12 to 18 inches long.
I have never seen a wood fired oven before. I never heard the term (wood fired oven) before and today I ''are one'', part of the old joke. You talk about being a dumb as a box of rocks, we I am that box of rocks when it comes to running this new oven. Help Please, the holiday of Thanksgiving is comming and I do not want to have an undercooked Turkey.
JJ
Philippines

[mgraban]

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

By "let it cool down", do you mean to knock the fire down so it goes out right away?

[james]

Re: Oven Curing

Hi Mark,
Happy Holidays. Just let the fire burn itself out and the oven will cool down naturally. You don't really have to "do" anything, just let nature take over. I always say this, but toward the end, your oven will be getting into the 400-500?'s, so you can start experimenting with roastings and grilling. Enjoy.
James

[Xabia Jim]

exothermic

Cement is exothermic, meaning it puts out heat to cure.

firing a cement product too soon will damage the cement so the longer you can "cure" it before firing the better.

Also, the wetter you can keep your cement initially, the better. I am constantly wetting new cement, concrete etc. so it cures, not dries. Drying cement will cause it to crack whereas keeping it wet will allow it to cure and it will become stronger.

Rember, concrete continues to set up for years! Old concrete is much tougher to drill or break up than younger products.

...in my experience!

Now, I was told by a Spanish builder to use the "yeso" which I think is plaster of paris or gypsum. He said this will strengthen with the heat of the fire. Time will tell.

Happy holidays.....the oven is warming for the weekend cooking!!!

[Hendo]

Moist Curing

All useful information here. From my experience, enough emphasis cannot be placed on the importance of keeping newly poured concrete moist. Two things affect concrete strength more than most people realise ? water in the mix, and water (or lack of it) during the curing process.

Additional water in the mix makes the concrete easier to pour but decreases its strength. Concrete delivery drivers seem to love adding water when they pull up at a site, sometimes without even asking. So the strength guaranteed by the concrete supplier goes out the window!

On curing, the Cement and Concrete Association of Australia, in its publication entitled Design, Control and Characteristics of Concrete states:

?The increase of concrete strength with age continues so long as drying of the concrete is prevented. When the concrete is permitted to dry, the chemical reactions cease. It is, therefore, desirable to keep concrete moist as long as possible. The best practice is to moist cure the concrete continuously from the time it is placed until it has attained the desired quality ? usually a minimum period of seven days is required.?

An interesting graph in this referenced publication shows that 28 days after pouring, concrete allowed to dry out will have a compressive strength of about 70% of concrete which has been continually moist cured.

After the concrete has firmed up after the pour, it should ideally be covered entirely with a plastic film to prevent it drying, and it will moist cure naturally. This is preferable to hosing with water periodically, as the concrete will cycle between wet and dry, rather than simply retaining the moisture present in the original mix when encased in plastic film.