First Fire in my Oven

[Leao]

Hi everyone at Forno Bravo,

I have almost finished my dome. It looks brilliant.

I'm aware I should do a fire to let the bricks and mortar expand before adding the insulation and final layers. What quantity of wood should I use to fire it? It's 70 cm in diameter (35 cm high). I don't want to overheat it but I haven't installed any thermocouple sensors.

Thanks again for all your invaluable help!

Leo

[Lburou]

The subject of curing your oven is a frequent subject here. The recommended solution is here:

Originally Posted by James, of Fornobravo.com

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

While there are 'other' curing strategies, this one is the most conservative, and probably most often used.

[Leao]

Are these instructions about curing the oven at the very end of the job or also just after you have finished the dome (but haven't yet put any insulation)? I understand one must cure it just after the dome has been done. Should I wait a week until doing this or is the recommended week long wait just referring to when the whole oven is done (insulation, outer coat etc).?

Thanks Lee!

[dmun]

Are these instructions about curing the oven at the very end of the job or also just after you have finished the dome (but haven't yet put any insulation)?

The point of curing is to drive off water. If you are using vermiculite/perlite concrete, which contains a LOT of water, you should cure after insulation. If you are using the blanket insulation, it's your choice. I think it's better to heat it with the insulation on it: Less thermal differential between the inside and outside of the dome means less potential for cracking.

All this takes place a week after you finish the dome: the initial week it should be kept damp to cure your mortar.

[azpizzanut]

Hi All,

I'm asking this question for the benefit of new builders. (My oven is already cured and I've been using it).

Where is the best place in the oven to take the temperature readings when curing? Could a builder average readings from several locations or simply point the IR at the back wall? What is your advice?

Cheers,