Curing methods?

[Rodneyf]

I had a long talk to the guys at Field Furnace in Sydney and they build ovens, kilns and furnaces. They said they always use heat beads ie; charcoal because they can take 8hrs to get the oven to 200C and hold it there for 1hr per 50mm of lining. That means that if we have a 100mm thick oven we should be looking at 8hrs to get to 200C and hold that for 2hrs. They say that you can then increase the temp by no more than 200C per/hr till you have reached the desired temp. The theory is that a fire no matter how small the fire, it will produce a flame that will lick the dome with 750C (the flame temp) and this heat will produce hairline cracks. I think that by doing it their way I should be able to keep more control over the temps and not get the heat up cool down cycle of several curing fires but instead get a slow, controlled release of moisture from the dome. They also said that this must be done after the insulation and any render has been finished so as to keep the oven heating evenly. I have included the heat up sheet they gave me and would love to know what the forums experts think of the idea.

[david s]

Re: Curing methods?

I think they are spot on. I use a gas burner to heat the oven slowly (24 hrs) to about 250C then go to wood fires and heat beads. You need a fair amount of energy, expended slowly to convert a lot of water to steam. eg How long would you have to boil a 5 gallon tin of water to boil it dry ?

[MK1]

Re: Curing methods?

I'll be using this schedule and technique tomorrow, if it doesn't rain! I'll let you know how it works out. I fairly dread cracks. Don't be careful.

Mark

[Rodneyf]

Re: Curing methods?

Mark, that sounds like you are about three weeks in front of me so I am really interested in hearing about your results, keep us posted.

[MK1]

Re: Curing methods?

Well, it rained on and off all day but Saturday looks possible. I have included pictures of my "heat stick". I used a few flare fittings and a 3' black iron pipe. I also have a heat shield for above the burner to disperse the flames.

Mark

[Rodneyf]

Re: Curing methods?

Looks good and I hope we are on the right track but it all makes sense to me. The guy at Field furnace said that the render on the outside will get very hot during curing but will actually cool as the oven dries out during the process, he also said if you see steam you are going too fast and to just knock down the heat a little. Good luck.

A couple of pics of where I'm at right now.

[MK1]

Re: Curing methods?

Rodney,
That's an important tip, thanks. I assume the outer render gets hot because the vapor is going into the insulation as well as up the chimney. I incorporated a stainless vapor barrier so I don't think my insulation will heat up much.
Your workmanship looks excellent. That top plug makes me think you might have a large core bit or you're damn patient with the grinder.

Mark

[Rodneyf]

Re: Curing methods?

Grinder old son,grinder! It didn't take long actualy I just did a rough cut with the saw then made the plug by grinding it down to size. When is the curing happening?

[MK1]

Re: Curing methods?

Today. I'll let you know how it turns out.

Mark

[MK1]

Re: Curing methods?

Rodney, it was a longer process than anticipated, but worked like a charm. Driving out the moisture at the 200c stage took a full 24 hours. I have 1.5" of refractory mortar on the outside of my bricks and I believe that was the never ending source. I was up every hour or so over night, but I could have just let the propane hold it at 200c and slept a full 6.
After the 200c stage I started shoveling in the heat beads for the 50c/hr ramp to a bit above normal operating temp. I used about 30-35lbs of propane for the first 30 hours and then 120lbs charcoal briquettes (heat beads) for the final ramp and cure at 1100 f. I haven't found any cracks so far. I don't have my metal dome covering the kaowool yet, and that was a good thing. I would advise you let an opening for vapor to escape because the moisture goes both ways.
My son helped the whole time and made a batch of bread for on the way down after we raked the ash.
We put the bread in at 420f hearth temp, at that was just a little low, (playing it safe) it turned out fantastic. I've never had good spring in a regular oven, but these loaves almost doubled.
I'm still a little dotty from the last 2 days so I might be forgetting a few minor details.
We had some rain, thus the comical rig over the oven.
This is a fantastic method of curing. Thanks.

Mark

[Gromit]

Re: Curing methods?

Mark,

What was the indication that you had driven off the moisture after 30 hours? Was it steaming the entire while?

[MK1]

Re: Curing methods?

David,
I never had any steam from the inside of the oven. I have a stainless vapor barrier on the outside of the masonry, under the insulation. The vapor, (I hesitate to call it steam because it wasn't hot) came from the joint I have between the stainless and the chimney,(not finished). The vapor started after about 3-4 hours of temps at 400F. It steamed steadily for 26-27 hours and then just stopped. That's when I started with the charcoal.

Mark



PS I just looked at your build again, (haven't for a little while) and I think it's fantastic, particularly the vent to pipe transition. Shame to cover it! I do think the oven should be cured like a giant piece of pottery.
After seeing my sons bread evenly done on all sides, realizing how well balanced the dome charge is to the thickness of the floor I have to suppose "The James Gang" has been through all this before and have perhaps found it's a waste of time.
I just wonder if I'll get some cracking when I fire with wood.

[Rodneyf]

Re: Curing methods?

I am so glad you have no cracking and can't see why it would crack now. The way you went about it looks good and I don't have a burner so I think I will stick to the beads. How long did your dome take to build, how long did you leave it before you cured it and what temperature was it during the time between the brick work completion and the curing fire. I am trying to compare your drying times with mine, we are going through a dry period right now and the daytime temps are around 19C but we are down to around 4C overnight. Lets see some pics of the bread next and thanks for for the update.

[MK1]

Re: Curing methods?

I started almost 3 months ago, was about half complete right away, got busy with work and then finished about 2 weeks before the curing fire. It's been in the 80s F and humid most of the time, 60s at night.
Have your source of beads at hand. Without a burner you might need 200lbs. Pics on the way.

Mark

[Rodneyf]

Re: Curing methods?

That is a lot of beads! I don't know if I want to spend the $130 on beads yet so I might see if I can get a burner. My dome will have been drying for 4 weeks by the time I start my fires so it may have dried quite a bit in that time. The temps here have been pretty good this week and the WFO has been uncovered all day every day in full sun with a bit of wind around so it must be drying well. I have done a dry fit on the chimney and will finish all the brick work tomorrow then fit the insulation and render the week after.