Fio's Oven Curing Confessional

[Fio]

Chronologically, this is a continuation of my oven building thread, but I decided to continue here briefly to stay topical.

After the dome construction, I ripped out the form and dried the inside for three days using a ceramic space heater. I put a thermometer and hygrometer in there to monitor the temp and humidity. It got fairly warm, but at the end of three days, the floor bricks in the doorway were still damp. Turns out, they stayed damp through three firings.

[Fio]

FirstFire!

Having completed the dome, and dried it out for three days, I could now commence with the time-honored rite of passage, indite in the lore of ovenbuilders over the centuries: The Holy Sacrament of FirstFire! (Perhaps I have embellished the sanctity of this ritual in my own mind, but I was quite excited).

Here's where I wonder if I started screwing up. I am told to build a "small" fire. How small is small? I used newspaper and a few hunks of wood. The inside temperature near the floor reached about 200 degrees.

Subsequent fires grew larger - larger than I wanted, in some cases, and we'll see the effects.

[james]

You'll be fine!

If you go slow, don't get the fire too hot, or let it run to long, you'll be fine. Gently up and down, up and down. You're seeing how much moisture you have in the oven. Shoot for adding 100F each day, just letting the oven reach the temp you want, then letting it burn out.

Let us know how hot you think your oven is when your metal thermometer explodes.

Are you thinking of getting an infrared?

James

[Fio]

Subsequent fires got hotter - too hot?

I didn't take notes, but here's what I remember of the max temps recorded by the analog cheapo Taylor oven thermometer on the floor of the oven. It's an easy consistent way to measure a given, fixed temperature data point (infrared thermometers vary by a hundred degrees when you move it a half inch):

FirstFire: Max temp 200 degrees.
Second: 300 degrees
Third: 300 degrees (Wood was damp and/or I lost patience).
Fourth: 500 degrees (Yikes! It got super hot really quickly!)
Fifth: 400 degrees
Sixth: 600+ degrees (Oops! It was at 500, I turned my back then it hit 6 Benjamins!)
Seventh: 500 degrees, then I knocked it down and cooked a chicken.

The hotter fires burned portions of the apex of the dome clean white. I know this is what you shoot for when cooking pizza, but I'm wondering if it's too early to hit those temps.

While my measuring standard was the floor-mounted Taylor thermometer, I was poking around with the infrared thermometer. I LOVE that thing. It gave me a good idea what was going on in the chamber. There was some seriously obscene heat going on.

As expected, cracks started appearing. Small at first, then a bit larger, then more of them. Each new fire brought more cracks. More on this later.

James asked if I have an infrared thermometer - I do. I got it at Home Depot or Costco. Max temp is 1K which is fine for me.

[james]

An all-white dome in less than an hour

Fio,

This all sounds right to me. Over the next series of firings, you are going to see the oven heat faster, and more fully. You'll know the oven is cured when you get the ENTIRE dome clear/white in about an hour. I always think that's the best part of firing your oven. You see a small white (clear) spot in the top of the dome, right where the fire hits, then is grows down the sides. You want to see everything clear before you start some serious cooking.

Is the outer mortar layer of the dome smooth and without cracks? Or are you getting some cracks big enough to let out small bits of hot air and smoke?

James

[CanuckJim]

Fire in the Hole

Gentlemen,

I know, I know, it's very tempting to start building those fires. However, three days drying time is not enough. A week or two is more like it before any flame appears.

Patience is difficult, but it is a virtue.

Jim

[Fio]

Fio,


Is the outer mortar layer of the dome smooth and without cracks? Or are you getting some cracks big enough to let out small bits of hot air and smoke?

James

Most of the cracks are hairline cracks. I notice they grow when the oven is hot; shrink when it's cool. Thus, when I patch the cracks, I'll force in furnace cement when the dome is warm, to get a better seal.

None of the cracks in the back or top of the dome leak smoke, but one of the larger cracks near the vent manifold does leak smoke.

[james]

Hi Jim,

Three fires definitely doesn't do it -- but happily Fio's done seven. It does seem like the vent is the spot where the largest cracks happen. It sounds like all is well, with a little repair and a little more curing.
James

[Fio]

Addicted to Crack[s]

Given my arguably aggressive (perhaps too aggressive, in the eyes of some) curing schedule, my firebrick construction (as opposed to a cast FB component oven), and the fact that I chose a sub-optimal cement-sand-fireclay mortar mix (instead of HeatStop or Refrax), I knew cracks would be inevitable. It appears we all hate them, yet we are all resigned to accept them as a natural part of the curing process.

I expected cracks, and even wanted them. In my view, cracks are natural fault lines, giving the oven a place to heave and expand which, if established slowly, will provide a fixed place for the oven to swell and shrink with changes in temp. It's similar to having airplane wings that bend. Have you ever looked out the window and seen an airplane wing bend with the wind? They're supposed to do that; otherwise, they'd snap off.

Of course, I am probably rationalizing. If I had managed to avoid cracks altogether, I'd probly be singing a different tune.

The bottom line is, I am optimistically confident that once my oven hits a state of stable expansion equilibrium, it will still be strong.

So far, I have had seven firings. Small cracks appeared after the first firing, and more appeared in subsequent firings. I have marked them with chalk to monitor their progress. Some existing cracks have grown, but I am expecting that in future firings, no new cracks will appear. When this occurs, then I'll seal them up with furnace cement. I'll do this while the dome is still warm and "swollen."

As James observed, the most active cracking occurs near the vent manifold. Most cracks are "hairline," with the widest being 1mm in width.

[DrakeRemoray]

Where do you get furnace cement? I have not seen it at HD...

[Fio]

You need to find a "REAL" Hardware Store

Where do you get furnace cement? I have not seen it at HD...

Furnace Cement is one of many things that is not stocked by Home Depot. I went to a real-live Ace Hardware store to get it. They sold me my ceramic space heater, furnace cement, and oven floor thermometer. All on a blistering hot day in July. HD had none of these items.

I am tempted to vent my spleen about how Big Boxes like Home Depot and Wal Mart are ruining America, but for now I will zip it.

Nothing like the pleasure of going to a REAL hardware store.

[FlGator]

If one use only furnace cement for all the dome, do you still need to build up to heat? Besides price, why not only use furnace cement for the dome?

[Marcel]

100 pints of furnace cement

If one use only furnace cement for all the dome, do you still need to build up to heat?


(M) Yes! Furnace cement gains strength from a "heat cure"


Besides price, why not only use furnace cement for the dome?

(M) You'd be buying so many small containers of furnace cement that you'd go crazy disposing of all the empties. I have not seen furnace cement available in bulk.

(M) Also, regular portland based refractory mortar will gain some strength even after a few hours; the furnace cement may not be able to support your bricks before it is fired and you would start to losen your bricks as you built up your courses.

(M) If it were a viable alternative you'd see chimneys that used furnace cement between the bricks rather than mortar.

Ciao,

Marcel

[TomB]

My 7day old oven

Here are pictures of my new oven. I cant wait to build a fire in it. oops How do you reduce the size of a jpeg for posting?

[dmun]

edit pictures

Most picture editing programs have an "edit image" pull down menu, and one of the choices will be edit image size. If you make your picture 500 pixels wide, you should be good to go.

I use and recommend "IrfanView" to edit pictures. It's easy and free. Any picture editing utility, including the one that came with you camera, should do the same thing.