Oven Curing

[tulowulo]

I have been curing my oven for the last week and I'm using the low, long and slow method. The first 4 fires which I increased by 100 degrees each day I used lump charcoal. I was taking temperature readings with an inferred thermometer on the brick at the top of the dome and then on the sides as well. On day 5 I started using small pieces of wood.
My question is do you take a temperature reading when there is a flame or wait until after the fire has died down a little. In other words, is it the temperature of the brick that needs to reach the desired temperature or is it the peak temperature of the fire.


Any information you can provide me with would be greatly appreciated. Thanks!

[SableSprings]

You are looking to keep the firebrick temps slowly increasing. Within each brick the temp is going to vary as the side towards the fire will slowly transfer heat to the deeper parts of the brick. Driving moisture from the entire brick and supporting/covering insulation is just a slow, even process. You will notice the apex of the dome will heat relatively faster than the sides with the cooking floor being the coolest until the last of the moisture in the base insulation is driven out. Taking brick temps only at the peak is going to give you the highest readings, most likely far hotter than the sides & base of the dome while the fire is active. I like to see an evenly maintained fire (for several hours), keeping the approximate average dome temps consistent. Once that fire dies out, take readings at top, sides, and over the cooking floor. As you approach final curing, those temps should be pretty similar. Also as the peak of the dome turns white later in the curing process, you'll notice it moving down further with each fire. Dark areas indicate that moisture is still present in that area. The outer, lower dome perimeter and the cooking floor are normally the last areas to "cure".

That's the long winded answer to your question, I hope it helps. Also if you have an oven door, let your day's curing fire burn out, remove the ash, & close the door for an hour or two. Doing a temp scan of your oven after this period of temp equalization will give the best idea of where moisture remains. Stay patient & you'll be enjoying pizza soon.

P.S. Once your oven is holding temps of +300F you can be using that residual heat to bake...

[tulowulo]

Thanks for the response.
I've been covering my oven with a tarp on the nights that there was a forecast for rain. The fire and coals were out for several hours before I covered the oven. In the morning I would pull the tarp off and start my next curing fire. The curing fires were going well and I didn't notice any problems with the tarp covering the oven overnight. There didn't seem to be much, if any moisture that collected between the tarp and the outside of the dome.
Yesterday I did my sixth curing fire aiming to reach 600 degrees and I decided to leave the tarp over the dome of the oven. I'm not sure if that was a mistake or not. I know that some people suggest doing that to get a gauge on how much moisture there is remaining in the materials. After several hours I checked the tarp and found that it was noticeably wet and when I pulled the tarp off the dome there was steam rising from the pcrete. The inside of the dome is now noticeably blacker. The soldier course is still clear but from about midway up the first course of bricks it becomes a dark black. I had to cover the oven again last night because of a forecast of rain but I think the oven was sufficiently cooled down.
Today I decided to return to using lump charcoal and a lower temperature of 500 degrees since I was unsure if I caused any problems by leaving the tarp over the dome. I'm getting very consistent temperatures throughout the oven. The dome and walls are all pretty close to 500 degrees. I'll try your suggestion of removing the ash and coals and taking a temperature after a couple hours.
I also thought it might be wise to do several of these 500 degree cures with lump charcoal before attempting to increase the temperature with smaller pieces of wood.
Any input is welcome. Here is a picture of the blackened dome that I was talking about.

[UtahBeehiver]

The carbon does not burn off until the 700 plus range. However, if you are physically seeing steam off the pcrete you are heating too hot and too fast, Liquid water when it sublimates to vapor increases by 1500 plus times the volume which can cause pressure build-up and possibly crack the pcrete. Do you have the recommended vent in the pcrete?

[tulowulo]

I do not have the recommended vent in the pcrete. I must have overlooked that in the Forno Bravo Pompeii Oven Plans. Is it too late for me to install one? If not how do I do it?

Also, I am not seeing steam off the pcrete when there is no tarp over the oven. I only saw steam rising after the tarp had been on the oven while curing and then I removed the tarp.

[UtahBeehiver]

The FB plans are a good baseline but dated, the use of the vent has evolved since the plans were published. You can do a search for vents and since you are at the pcrete stage and not stucco you can fairly easily install one. Here is a picture of the one Gulf and I used, available at any car part store for under 10 bucks, 1/2" mpt end that you can thread into a PVC bushing. But there are many other types

[UtahBeehiver]

[tulowulo]

I found a zinc plated iron breather vent from McMaster Carr and installed it in the top of my dome. I returned to the process of curing my oven but I scaled down my curing fire to 400 degrees with lump charcoal. All seemed to be going fine. I waited three days after the 400 degree fire to do the next fire at 500 degrees and it too went just fine. Today I fired it to 600 degrees and I noticed a crack along the base of the pcrete and the cement slab. I'm concerned about this because I'm not intending to build a roof over my oven to keep it dry. My plan was to put an acrylic stucco over the pcrete to waterproof it. Any recommendations as to how to address this cracking so I can keep rainwater from going underneath the pcrete and possibly saturating the ceramic FB that is insulating the hearth?? The crack is only on one side of the oven so far.

[david s]

As the perlcrete is an insulating layer it really doesn’t matter if it cracks a bit. The acrylic stucco over the top will do the weather proofing. Clean up the base of the perlcrete and supporting slab so you get a good bond there with your acrylic stucco. If you haven’t already, make some holes through the supporting slab so the under floor moisture, which is the hardest to get rid of, can find its way out.

[tulowulo]

I was unaware of the idea of holes in the supporting slab to allow under floor moisture to escape. Makes sense though. How many holes would you recommend on a 34" oven and what diameter holes should I drill? Also, approximately where should the holes be drilled.

[david s]

In my build I only use one, about 20mm in diameter covered with insect screen to keep crawlies out, but for your maybe 3 x 15mm near the centre of the floor.

[francisb]

I've been following the curing process as detailed in my Casa 80 installation manual. I've done the 500F fire now and the inside of the dome is still jet black; I assume this is correct as it shouldn't have hit 700 to burn off yet. I'm thinking it would be prudent to do a 600 as per this thread before getting it up to 700. Once I go for the big 700+ fire how quickly does the carbon/soot burn off? Many thanks. (I'm assuming here the temperature is that measured on the dome above the fire once the flames have died down which seems to be about 1-200 higher than the floor?)

[david s]

Yes, when you are pushing the moisture out there will be a huge temperature difference between the top and bottom of the dome. This means that there’s also a big difference in the thermal expansion of the two areas. The moisture holds the temperature back so it is vital, to avoid damage, to take it slow. As the chamber dries out it will fire way more efficiently and the temperature rise is likely to go too fast. You may notice a persistent ring of black around the base of the dome. This is an indication that you still have some moisture there. Try to avoid big flames impinging on the dome.

[david s]

Forno Bravo recommend not to use charcoal. I don't know why and they have not provided an explanation. When wood burns you are left with charcoal and this of course continues to burn so their recommendation is strange. To reduce the flame impingement on the dome we recommend what are known as "Heat Beads" here, which are small briquettes designed for barbecues. They burn much slower than wood and provide a less intense and more prolonged heat,, but do require some wood to get them burning when the oven is moist.
Attached is the firing schedule I offer for my 21" ovens, a larger oven will probably require a larger volume of fuel. If you have a Forno Bravo oven you should follow the instructions they provide.

firing schedule before render.docx

[Gulf]

Forno Bravo recommend not to use charcoal. I don't know why and they have not provided an explanation.....

Actually, they have. We called their hand on that back in 2015. Here is the full discussion in the thread: Has anyone used charcoal in their WFO?

It all boiled down to the misuse of terms. "Charcoal" for "Coal". Two very different materials by definition. However, "Lump" Charcoal, Would have always fit the "purist's" intentions of the forum.

A full read of the linked discussion will explain.