Oven Curing

[That's Amore]

Re: Oven Curing

I think you are making this a little more complicated than it needs to be. The top of the oven always will be hotter that the sides so if you can control the heat at that point then spread the coals around to try your best to keeo an even heat. The point of having a continuous burn for as long as you can is that the oven will eventually get to temp and with that control and time laps the top of the oven will surely have disposed of most of the water. I know when you built the oven it's your first time but with the low and slow and take your time method you will have no problems. Cheers,

[GreenBldr]

Re: Oven Curing

Maybe you have never used a "pointing" thermometer on your oven. The temperatures are radically different in different areas, particularly before the oven is cured.

No amount of coal spreading can change that. You can't even get coals without a fire big enough to make the ceiling hotter than minimum temp in the first place, and the floor might only be 120F.

I'd just like to know where/how the temperatures are taken for the curing schedule so I can do it the same way.

[That's Amore]

Re: Oven Curing

I was just trying to let you know that when the top gets hot that is where you would regulate the temps and if you keep the burn going for long piriods then the heat would make its way down the sides the top will have evaaporated the water and so you may then increase the heat and when you near the end of the cycle you will have evaporated all of the water. So as the fires get to be a little bigger then spread he coals around just to help the process. I know you worry about cracks but I did mine over 2 week just because I was also worried. Just trying to help ypu out don't be nervous it will work out in the end and so you know the top of the oven will always be hotter than the sides and floor but that is normal. I did use a infrared for oven as well and it worked great. Good luck

[david s]

Re: Oven Curing

Try for a compromise and take the reading about 1/3 way down the dome. If you bought a kit from someone, ask them. The manufacturer surely provides this advice.

[SCChris]

Re: Oven Curing

Greebldr, Amore has it right. Interior top third of the dome is where I chose to measure temps. Remember you’re going to have hot spots and cold areas and as the burn goes along the heat will increase in the cold areas if you don’t build the fire too big. The curing is a bit like bringing a pot roast to a simmer. The fire can start a big or small but when you approach your target temp in the top third of the dome back the fire down to slow the rate of increase.

I took the temps at about 4 places at the top of the dome; the top interior, the back and both sides a few courses down. What I was looking for was how even or not the heat was. At the top of the soldier course 6 to 8 inches up from the floor, I watched the temps to give me an idea of how the whole oven was curing. Even in a fully cured oven used for many years this lower area is going to be cooler from the upper third of the dome, but the difference won’t be nearly as great as in a new "wet" oven.

As I’ve said before how much fuel you use in the curing relative to getting your target temps will change. As the oven dries the amount of fuel needed to get to your previous targets will decrease. This is likely to surprise you and it’s likely you’ll exceed your target. This is not usually a problem as long as you don’t allow the temps to really run away.. Watch the temps often until you have some feel for the process.

Chris

PS consistency is what you're trying to achieve. Watch the same areas relative to the fire and use these to do your curing. As long as you're consistent and conservative and adhear to the curing protocol you'll do fine. Remember too that cracks are pretty normal and not often a real problem.

[GreenBldr]

Re: Oven Curing

Thank you, guys for the good advice.

I think the strategy of averaging temps over the top third of the dome might help newbies who are wondering the same thing that was bothering me. It's an FB oven. I really think FB should amend the curing instructions with this information.

I am applying an additional 50 lbs of FB mortar which was just received yesterday. I am hoping it will achieve the following;

1. Plug what appeared to be a pinhole in the dome casting. Pretty sure it was smoke coming out of that one and not steam when I started the first curing run.
2. Better seal the edges of the mortar bands that "glued" the dome sections when I assembled it. The wisps coming out the edges were most likely steam, but just want to make sure.
3. More thermal mass toward front 1/3 of oven dome.

The plan is to drape the patch areas with wet cloth for a few hours prior to beginning. The "partially cured" dome seems to be ever so slightly water resistant and want to make sure the new mortar has the best possible chance to adhere.

Is it advisable to mix the FB mortar as per instructions and then take a small amount and add just a bit more water to use as a "scratch coat" or transition between the old mortar and the new?

[fornoa]

Re: Crack repairs - is it necessary if no smoke is escaping?

I waited a month before starting to cure my oven yesterday. The oven has been uncovered during dry days and covered each night.

The oven is built of firebrick resting on a layer of insulating firebricks. It is a vault design with a square floor area of approx 1.3 sq meters. Also, I bagged the external part of the vault and wall with approx 1 to 5mm of render after finishing laying bricks. I have not yet insulated the oven, preferring to cure before doing so. My idea being that any residual moisture would not be trapped under the insulation and render layers.

I started the curing fire in the middle of the hearth with just a few sticks at first, keeping the fire small. Using my infrared thermometer gun, I found that the fire was barely heating the dome after 1 hour with the temperature just reaching 100C at the top of the dome and only 70-80C at eith end of the vault. The walls were even cooler at around 40C.

I decided to ramp up the fire a little and soon after I notice thickish (0.5mm) hairline cracking along the spine of the vault and further down either side of the vault, with one crack near the doorway being more severe than the others at approc 1mm. The temperature at the top of the vault reached 170C.

I kept checking for any escaping smoke or steam, but there was no evidence of either.

Considering that no smoke was escaping from the cracks created, I assume that I would not need to make any repairs. Does anyone have any advice regarding this?

[Wesleckner]

Re: Oven Curing

Fornoa,

I would leave the cracks that small alone. Just keep an eye on them and if the grow larger you can fill them in.

[david s]

Re: Crack repairs - is it necessary if no smoke is escaping?

I waited a month before starting to cure my oven yesterday. The oven has been uncovered during dry days and covered each night.

The oven is built of firebrick resting on a layer of insulating firebricks. It is a vault design with a square floor area of approx 1.3 sq meters. Also, I bagged the external part of the vault and wall with approx 1 to 5mm of render after finishing laying bricks. I have not yet insulated the oven, preferring to cure before doing so. My idea being that any residual moisture would not be trapped under the insulation and render layers.

I started the curing fire in the middle of the hearth with just a few sticks at first, keeping the fire small. Using my infrared thermometer gun, I found that the fire was barely heating the dome after 1 hour with the temperature just reaching 100C at the top of the dome and only 70-80C at eith end of the vault. The walls were even cooler at around 40C.

I decided to ramp up the fire a little and soon after I notice thickish (0.5mm) hairline cracking along the spine of the vault and further down either side of the vault, with one crack near the doorway being more severe than the others at approc 1mm. The temperature at the top of the vault reached 170C.

I kept checking for any escaping smoke or steam, but there was no evidence of either.

Considering that no smoke was escaping from the cracks created, I assume that I would not need to make any repairs. Does anyone have any advice regarding this?

From what you have described you have gone too hot too quickly.Never mind, the damage has now been done and your oven will not fall apart. Finish the water elimination slowly according to the schedule advised to prevent more damage. Trying to fill the cracks is usually unsuccessful but you can try. Your oven will still work and now has somewhere to move when expanding and contracting.

[Erdbeereis]

Re: Oven Curing

I was thinking about getting a pre-assembled Primavera 60, I still have to cure this correct? Where do you measure the temperature? The floor, roof? How long should I maintain the fire, just let it go out or keep it burning all day?

Erd

[Wesleckner]

Re: Oven Curing

Yes, you need to cure it. Take you time in the curing process, and I like to measure the temp about 2/3 of the way up the dome, which gives an average temp. Enjoy the fun!

Wes

[SCChris]

Re: Oven Curing

If you have a self made oven, in the early stages of curing, to about 180F or 80C or so, you can give yourself a leg up on the process by using a shop light or even a naked light bulb in the oven for a few days. Using the bulb to heat the oven, you have the advantages of moving dryer warmer air around the inside of the oven with little concern of cracking the oven. Use a few spare firebricks or plywood to block the doorway leaving room for fresh cool air to enter and warm humid air to exit. By using this method you should be able to raise the internal oven temps to close to the 100C and 212F water to steam barrier and by using the makeshift door, the oven will be warmed in a very evenly. The oven exterior will also dry from the interior heating. All of this gives you an advantage over using a fire early on.

I need to be clear that I can't speak for manufacturers of precast ovens, they will have a specific curing schedule that may or may not bless using a light in this way. If you have a precast, ask.

Chris

[Erdbeereis]

Re: Oven Curing

Yes, you need to cure it. Take you time in the curing process, and I like to measure the temp about 2/3 of the way up the dome, which gives an average temp. Enjoy the fun!

Wes

Thanks Wes!

Would I try to maintain the temperature all day, or just get to the temperature and let the fire burn itself out?

Erd

[SCChris]

Re: Oven Curing

Curing is about both time and temperature. Your oven needs the time, with heat, to move the water out of the brick or casting. So long low burns that allow the oven to be heated gently and evenly are better than a quick superficial burn. The oven needs to be heated gentily so water migrates out of the interior of the bricks or casting and into the surrounding air. What you don't want is water flashing into steam, this flashing will damage bricks and castings.

Another problem with quick heating are cracks from uneven heating and / or thermal shock. If the oven is heated to quickly the heating will be uneven and this will stress the structure, and if the heat is uneven enough it will crack the structure. Some cracking during curing is likely if not inevitable but much can be avoided. When the oven is new, the water will retard the heating of wet areas as the dry areas heat unchecked and the oven ends up stressed due to this uneven heat. By curing with small burns, you give the oven a chance to even the heat out before the next burn and the next burn and so on.. By burning long and low the very slow heating allows the oven structure to warm evenly and once heated the water starts moving out and taken away by the convection of air moving around the oven.

My opinion is to get the water moving and keep it moving. The trick here is in not letting the fire run away and heating the oven more than what is outside of an acceptable curing routine. Some people use briquettes, also known as ?heat beads? by our down under friends to control the heat. Early in the curing I like the idea of using a high intensity light, a quartz shop light and even an incandescent bulb generates a lot of heat and is safer to walk away from than a fire. It?s also dependable with regard to the heating it provides.

Chris

[GreenBldr]

Re: Oven Curing

I've now added an additional 50 lbs of FB mortar to the top and seams of my Casa 2G90 oven and have started the second curing run....

Just a note about adding the mortar; it appeared that there was a substantial amount of mold release agent on the oven shell that was preventing mortar from sticking well. On the initial assembly I scrubbed the shell with a wet rag in order to get it moist for the mortar. This time, I tried a wet hard bristle scrub brush, but the surface was still not wetting well, so I mixed up a very runny cup of mortar and used the abrasive mixture to scrub the surface and that was finally successful.

Now, I've used a shop light, a 1500W electric heater, a weed burning torch and finally, a real wood fire to get the upper dome temps up to about 500F over about 5 days.