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-   -   My take on curing (http://www.fornobravo.com/forum/f16/my-take-curing-17586.html)

SCChris 04-12-2012 01:43 PM

My take on curing
 
It's just my opinion, but I think it’s easy to break oven curing into 2 major phases, drying the oven and breaking in the oven structure.

Drying of the oven.
For simplicity, oven drying relates to available, not chemically bonded, water. Unbonded water will be converted to steam over its boiling point, 212F or 100C at sea level, at altitude the temp change of course.

It’s easy to forget that even if the interior of the oven is above 212F / 100C the exterior will be something less, especially so when you’re salivating for your first pizza. The top of the dome will be hotter than the bottom of the dome structure. These differences in temperatures in the beginning are differences in wetness, and this creates a big opportunity to stress the oven unevenly and crack the oven. A wet brick will absorb heat to a greater degree than a dry brick and therefore hold the temperature down. This differential allows the dry brick to expand faster relative to the wet brick and I feel that it’s this difference that creates the majority of dome cracking seen during curing.

To avoid these cracks, dry the oven using low, slow, even heating. Ideally, though not particularly practical, the oven would be heated from all sides to just under 212F/100C and maintained at this temperature while dry air is circulated around the whole oven until the water is baked out. As I said this isn’t really practical, but my imagination goes to using some sort of oversized Christmas fan inflated snow globe with heat. The more practical answer is to warm the oven and keep it warm and dry while circulating air around the structure, passively or actively, until most of the available water has been removed from the brick.

Simple methods of gentile warming are placing light bulbs and space heaters in the oven while keeping a make shift door ajar. There are other methods of heating that can be found by digging around in the forums. Fires by their nature are more dynamic and so somewhat less controllable, but using fire was the only way that curing was done for thousands of years and done successfully. And, just because most of the water is gone a light bulb isn't likley to get your oven above that magic 212F/100C point. At the point where your temps are steady with a bulb, your going to have to graduate to some controllable fire. Be very conservative with these fires, go low and long if you can.

Breaking in the oven structure
In the second phase, the major problem is stress related cracking from thermal shock. The oven swells and contracts during curing cycles and the bricks, like shoes need a bit of time to fit just right. Heating the oven with a big fast fire, don’t allow the bricks of the oven structure to comfortably find their respective “happy places” within the oven structure. A large fast fire will heat the interior to high temps before any real change in temp occurs in the exterior of the oven. I envision that when the oven is young it’s like a great pair of new boots that hasn’t had time to become flexible. Again, Be very conservative with these fires, go low and long if you can.

You will be rewarded!



Chris

PS. Regarding insulation and curing;
It’s my opinion that early in the curing up to say 250F, insulating the dome has some downside relating to air movement and drying, I could be wrong here. After 250F or so, I think there are advantages to loosely insulating. The insulation should maintain a more even dome temp and therefore should keep thermal stresses to a minimum. Another advantage is that the oven will stay hot longer and this should help to dry the structure in a more homogeneous way. I could be wrong.

SCChris 04-12-2012 02:23 PM

Re: My take on curing
 
I look forward to your info. I remember when I was a youngster and was active in ceramics that 2 points were critical during the heating of the kiln. 212/100c and around 1000F. It's been a very long time, but the high temperature point was where chemically bonded water was finally freed. At both of these places, if you pushed too fast, some pots would break due to steam pressure. Feldspar, a mineral in Grainite, changes at this point and breaks down, cheap sand too.

Chris

david s 04-13-2012 05:09 AM

Re: My take on curing
 
Here is a good explanation of chemical water.
About Wood Firing :: Gary Hootman - Clay, It's In The Hands Studio

When using wood flame to drive off the water it heats the top of the dome up far too quickly leaving the bottom of the dome and the floor still pretty wet and cold. If you have the time allowing the oven to rest for a week will allow some of the moisture to migrate more evenly into the whole structure once more. Then you can do another fire. This is not so practical because most of us are so excited we want to get to the cooking stage as fast as we can.
From my experience clay pots tend to "blow" at around 250 C or so if there
is excess moisture in them, not at just over 100 C as you would expect. I think this is due to the fact that the moisture in the clay keeps the temperature down so the outside of the pot wall is hotter. Also there is considerably more expansion of the steam at 250 C than there would be at 100 C.
My point here is that you shouldn't think you are safe just because the oven is over 100C (212F)

SCChris 04-13-2012 07:47 AM

Re: My take on curing
 
"When using wood flame to drive off the water it heats the top of the dome up far too quickly leaving the bottom of the dome and the floor still pretty wet and cold."

Yep! It's so easy to have a fire go too far too fast at this stage, and as the curing goes along the same small fire that hardly did anything early on, now pushing the intended temps way beyond the target before you're aware of it. It's hard to relate to the situation until you've been there, and slow seems sooo slow.

As a friend use to tell me "make haste, slowly!"

Chris

C5dad 04-13-2012 07:58 PM

Re: My take on curing
 
looking back at my cure, I would side with Chris!

Curing low and slow is the way to go (think smoking ribs!) Having a small fire over long periods to dehydrate the water in the mortar without rapid expansion is critical to avoiding cracks... I know that the crack in my dome was caused by direct flame impingement on the dome from the weed burner at too high a temp.

Drying for a few days at ambient, followed by low temperature heating that is sustained will drive the moisture off without pushing the water too fast. Ideally, allow the mortar to set for 28 days and then heat would work (TS - comments?) I know in the molten metals industry where I work, I hammer the slow heat up principle over the Dump and Run philosophy!

SCChris 04-14-2012 08:18 AM

Re: My take on curing
 
I googled up this info at "http://pottery.about.com/od/temperatureandmaturation/tp/tempclay.htm". Here is a portion of the info that seem to pertain to our curing temps.

"Chemically Combined Water Driven Off:
Clay can be characterized as being a molecule of alumina and two molecules of silica bonded with two molecules of water. Even after the atmospheric water is gone, the clay still contains some 14% of chemically bonded water by weight. The pot will be substantially lighter, but with no physical shrinkage.
This chemically combined water's bond loosens when heated. Overlapping the carbon and sulfur burn off, the chemically bonded water escapes from the clay body between 660⁰ and 1470⁰F (350⁰ and 800⁰C). If the water heats too quickly, it again can cause the explosive production of steam inside the clay body. It is for all these changes and more that the firing schedule must allow for a slow build up of heat."

Although this info is targeted at ceramics, the 660F to 1470F (350⁰ and 800⁰C) temp range can also cause dome damage if curing is pushed too fast. So much for the "shoe" analogy..



Chris

ronh 04-14-2012 11:02 AM

Re: My take on curing
 
I find the LP burner method to be working beautifully as I enter day two of my cure. I can't imagine being able to control a low/slow fire as accurately as this is working. Used a part of my old turkey fryer, with an old saw blade on the top to defuse the flame and provide a more even heat up inside (read that on here from Chris, wasn't sure that's what was meant, but it worked!). Would highly recommend this method to anyone, sure its a little propane, but I feel like it is soo much more manageable for most rookies like me!

Aussie Pete 04-15-2012 01:46 AM

Re: My take on curing
 
sounds like a good curing plan ronh, any pics to share of how it all looks.

ronh 04-16-2012 02:30 PM

Re: My take on curing
 
4 Attachment(s)
Here's some pictures of my makeshift curing process. Working like a charm, you can really control temps consistently for long periods of time.....and no problems with the hose heating up for those thinking that, I've checked it constantly.

Pompeii Nate 05-01-2012 05:48 AM

Re: My take on curing
 
Chris,

Great idea!

Ron, thanks for the pictures.

Now I just need to find an old turkey fryer.

Nate


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