curing the oven

[ocafra]

Okay I looked through numerous threads on this and keep bouncing back and forth on what is meant.

I let the oven dry for probably two weeks then started the curing process. What I am not sure about is the definition of "temperature"

It this the temperature of the core of the fire, the top, the inside bricks, the outside of the bricks. With a core temperature of 600 degree, the top of the fire could be 300, inside walls 100, and outside 50. So which one are we talking about?

I have not insulated yet so I can repair anything that might not be correct. Chimney is not capturing all the smoke (70/30), so may need to do some grinding there.

BUT, in heating the oven up each cycle, what temperature are you measuring. I have come to the conclusion it is the inside of brick temperature, as I can not possibly keep a fire below 300 degrees for more than 15 minutes let along for four hours.

Given this, I have warmed the internal bricks to 300, 350, and 400 now (obviously the fire is hotter) for 1-2 hours a cycle. Anyway, today a fine (1/32") crack appeared vertically along a grout path, but again I have read this is normal, yes?

But before i continue I wanted to make sure what I am doing is good for the oven and not killing it.

I can only heat it for 2-3 hours, not the four needed. Tomorrow I was going to heat the oven to 450, but again wanted to get some feedback as that would require some hot fire temperatures.

Also do people really get their oven up to 800+ before insulating it. Seem like you would need a LOT of wood and time to accomplish this.

[jeeppiper]

Re: curing the oven

I wouldn't get too hung up on following the exact temperatures and times. The curing schedule is simply a way describing the process by which you want to slowly and gradually cure the oven. ITs always a good idea to be as gradual and gentle when firing your oven to allow any trapped moisture to escape the brick and mortar.

[ocafra]

Re: curing the oven

Completely agree, trying trying to figure out a good baseline to start from....hence my question....300 fire, inside bricks, or outside. Also interested as to when most people apply the insulation.....

[Tonyp]

Re: curing the oven

I am at the point of starting to cure the oven. I had a 1500 watt electric heater blowing into the oven for about 4 days. The outside of the brick was warm to the touch and no moisture was condensing on a little plastic wrap I put over the outside of the oven. I then had some charcoal burning for about 5 hours and it was warmer but didn't have an IR thermometer yet so not sure how warm.. I could touch the single wall vent pipe though. Picked up an IR thermometer and more charcoal and put one layer of the ceramic blanket on the dome. Will see how things go.

[ocafra]

Re: curing the oven

You will definitely let me know how it goes. I have currently let the inside bricks get to 450 (with a hotter fire) and the outside to reach 150 before putting the fire out, closing the door and letting the oven bake. I don't plan to put the blankets on until I reach 550-600 in brick temperature in order to allow as much moister as possible to escape easily without the risk of reabsorption. Also want the ability to repair any cracks that may developed, but so far so good other than a few spider cracks, that have not allow any smoke to pass.. The fire found two tiny holes on my first fire that were acting like little geysers(areas that where never filled with motor). I was thinking I could make a face around them on the roof and make the smoke come out the mouth of the face. Took 3 seconds to think about the work involved and filled them shortly thereafter ;-)

[Tonyp]

Re: curing the oven

Hi Ocafra,

Did you happen to continue monitoring the outside temperature after putting the fire out? I would be curious to see the maximum temperature outside reached. 450 seems pretty warm, I would venture a guess and say there isn't much if any moisture remaining in the bricks.

[cobblerdave]

Re: curing the oven

G'day
The object is to dry the oven.
Use the same size fire and same time "burn". At first the temps it shows no difference as the heat is used just to get ride of the water, the heat will rise without a "bigger fire" as the moisture drops.
When the temps stop rising its time for a bigger fire and burn duration.
Every ovens different, so there is no hard and fast rules.

The temperature will rise as the oven drys. Use that to gauge the next size fire.
Don't just try to increase the size of fire to suit your temp gauge.
Regards dave

[ocafra]

Re: curing the oven

Ahhh very interesting.....makes complete sense. Appreciate that, as I was wondering how big the fire would have to get. Currently I leave small logs (2-3" in there and just let them simmer). Thank you.......I did notice it is smoking a lot less, so looking forward to higher temps. Currently I keep the first small as I don't want to risk things.

[ocafra]

Re: curing the oven

Tony P......

I monitor it almost every 15minutes for the two hours just to make sure the bricks inside on the dome do not get to hot (not above 300,350, 400 with 50 degree intervals each day). I did wait two weeks before starting, and at this time all that has appeared is a couple spider cracks. When you say 450 is warm...in what reference, based on what I have read 450 is spot on for the third firing let alone the fourth. The outside bricks have not reached 200 yet., as the inside seems to cool down faster than the outside is warming up, but then again the temperature outside in the morning is in the 50's right now warming to 75 in the day on average with a slight breeze.

Based on what cobblerdave noted, I am just going to keep running with my same small fire that makes is 450 inside for at least two week (probably overkill, but in no rush). Then I will insulate and let it start warming up even more.

[Greenman]

Re: curing the oven

What Dave says is right. As the oven dries it will get hotter with the same size fire and at some particular point it will tend to spike quite quickly.

There are two processes involved. One is shedding the moisture in the bricks and mortar and the second is curing the oven. Constant lower temperatures will eventually dry the oven but it is the cycling through the regime of increasing temperatures that cures it.

Two weeks of smaller fires probably is overkill and given the ambient temperatures you are dealing with I would be inclined to insulate the dome and then carry on with the firing. You will still be able to drive out moisture easily enough and you will find it easier to achieve the temperatures you will need to fully cure your oven.

The use of a plastic sheet from time to time will tell you when you have driven out all of the moisture.

Just my .02.

[Laurentius]

Re: curing the oven

What Dave says is right. As the oven dries it will get hotter with the same size fire and at some particular point it will tend to spike quite quickly.

There are two processes involved. One is shedding the moisture in the bricks and mortar and the second is curing the oven. Constant lower temperatures will eventually dry the oven but it is the cycling through the regime of increasing temperatures that cures it.

Two weeks of smaller fires probably is overkill and given the ambient temperatures you are dealing with I would be inclined to insulate the dome and then carry on with the firing. You will still be able to drive out moisture easily enough and you will find it easier to achieve the temperatures you will need to fully cure your oven.

The use of a plastic sheet from time to time will tell you when you have driven out all of the moisture.

Just my .02.

The elimination of the moisture in the bricks and mortar is in essence curing the oven! Unlike the curing of meat where it a two fold process of driving off the water and penetrating the meat with salt or smoke to kill pathogens.

[ocafra]

Re: curing the oven

Thank you Greenman\Laurentius...tied the pieces together. Based on that I will add the insulation tomorrow and heat to a lower level to insure the outside bricks do not heat up to quickly. I have five cycles in. So starting tomorrow I will run 300 internal temperature for six hours (like I did today), and 400 Monday, 500 Tuesday. Then will come the real test, bringing it over 500 for the first time. I have had a hot fire just not hot bricks.

I like your metal door. Did you weld that? How did you cut the metal, with a grinder maybe? Neat idea to protect the wood door!!!

[Laurentius]

Re: curing the oven

Hi Ocafra,

Don`t add insulation yet, drive off as much of the water you can before hand. The heat on the outside of the brick has no bearing on anything, the heat penetrates from the inside to the outside, that its nature. Remember the meaning of insulation, if its not there, then more moisture is released into the atmosphere. The insulation is a barrier and slows down the evaporation process, just wait until water get under your hearth. It takes forever for it to dry.

[Greenman]

Re: curing the oven

I like your metal door. Did you weld that? How did you cut the metal, with a grinder maybe? Neat idea to protect the wood door!!!

The metal part of the door is aluminium and is riveted. I cut it with an angle grinder and cutting disc. It is easy to work and I found new offcuts of 1.6mm at the scrap metal dealer priced very reasonably (by weight).

[david s]

Re: curing the oven

U

What Dave says is right. As the oven dries it will get hotter with the same size fire and at some particular point it will tend to spike quite quickly.

There are two processes involved. One is shedding the moisture in the bricks and mortar and the second is curing the oven.

Your first statement is correct. As the water is driven off in the upper part of the dome the oven quickly heats up. Unfortunately the base of the dome and underneath of the floor are still moist and therefore their temperatures are held back by this moisture. This results in a massive difference of temperature in the top and the bottom of the oven which of course results in uneven expansion. This is the very vulnerable time to create damage. You think it must be all done because the temperature is at last climbing quickly, however it is also the time when you may get some cracking. Allowing the oven to return to ambient allows the temperatures to even out so you can start the process safely again. It probably wouldn't hurt to leave the oven for several days here to allow moisture lower down to migrate up to the top of the oven somewhat, but nobody has the patience at this point to wait any longer, it's too exciting because you're nearly there.

I disagree with your second statement though.

I have always disliked the term "curing" as applied to the first firings. It is confusing as we use the term to describe the holding in of moisture to "cure" concrete to enhance its strength. Driving out the water from the refractory is actually the opposite of this process. Water, both mechanical and chemical is driven off slowly to avoid steam spalling. For the temperatures we fire to there is no sintering of the material. You would have to get the whole thing over 600C to even begin the sintering process. You might get some of the surface at the crown of the dome up to this temperature but certainly not the whole oven. Don't try to get it this high by the way because once you hit the 500C mark unfired refractory must be increased extremely slowly to allow the sintering process (and especially the quartz inversion at 573 C) to go through the required changes safely. You can't control the temp increase properly with a wood firing and the results are likely to be more cracking.
All we are doing is "driving off the water". This would be a far better term to use for this process rather than curing.