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Secrets for successful Clay Bread Oven

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davidjm's picture
davidjm

Secrets for successful Clay Bread Oven

 


Secrets to successful Clay Oven Usage


I'm still relatively new to this, but I haven't seen the information below in other places.  I welcome your comments and suggestions from your experiences as well.  They will benefit the whole community!


First, go ahead and buy the book by Kiko Denzer "Build your own earth ovens" (amazon.com $15)


Insulated Hearth Subfloor:


In Kiko's book, he recommends using plain sand as a subfloor for the hearth.  That is the cheapest way to do it, but for $50 more, you can have an insulated subfloor that will hold heat much better than sand.  Build a form the size of the top of you base at least 2" thick.  Buy a bag of Portland cement and 2 big bags of vermiculite from a plant nursery.  Mix the two at a 5:1 ratio (vermiculite:Portland) dry.  Then add water and mix until you get an oatmeal consistency.  Pour into the form.  Smooth out the top.  Make sure it's level!  Let dry for at least a week.  Then you will set your fire brick directly on top without mortar.  The clay walls will hold it in.  Ideally, you would have 4-5" thick subfloor.  I found that I loose heat out the floor faster than the walls with 2" thick subfloor.


Oven Dome:


Kiko, in one of his blogs, actually says the ideal height of the dome, no matter the size of the floor, is 16".  He plans to add it to the next edition of his book. In the present edition, he gives a percentage formula. 


Firing the oven:


After a couple miserable failures, and combing the web for advice, I finally figured out how to successfully fire a clay oven.  Here's what I learned.


You really need good seasoned oak to make it get hot enough. 


Buy an Infra-red thermometer (amazon.com $80).  It is worth it.  You'll need to chart out the heating behavior of your oven at least one time.  Then you can use it to give you a frame of reference during a heating. 


And, plan to spend at least 3 -5 hrs heating it up, depending on the size of your oven.  My oven floor is 28" wide by 31" deep, and 20" high ceiling inside.  It is a relatively large oven.  I found that I have to fire the oven for 4+ hrs to get the temp high enough. 


Think in terms of heat saturation of the clay walls and floor.  Noah Elbers at Orchard Hill Breadworks (orchardhillbreadworks.com) says he fired his clay oven 6 hrs before he attained proper heat saturation. 


The outside walls are a good guide as to heat saturation.  In my oven, I need the outside walls to gain 100 degrees in temp before I am near having proper saturation; even more if I want to bake a larger quantity.  (This is where an IR thermometer comes in handy!)


I think firing time depends on how much you are baking too.  If you are only doing a couple pizzas and no breads, then you don't need as much heating time.  But if you're going to maximize your baking potential, you'll want a long hot heating.


I took hundreds of data points of my oven during a firing, and I put my findings into a graph.



(The upper lines are inside temps.  The lower lines are outside temps.)


Couple observations from the graph:



  1. You see a big jump in internal temp at 75 minutes when I put in a few pieces of nice seasoned oak.  After which time, the internal temp continues to grow.

  2. Inside temp reached 1000+ degrees F at its peak.

  3. The rate of heating of the outside increased after the good oak was added and steadily gained in temp until the fire went down to coals.  (I rake the coals across the floor and let sit for 30 min to heat the floor uniformly.)

  4. After that time, the outside temp remained relatively constant.

  5. You can see clearly how after the fire is taken down to coal at 255 minutes (or 4:15 into firing), we immediately start losing inside temp at a steep rate.  Coals stayed down for 30 minutes and then raked out. 

  6. Once the oven inside temp reaches around 450, we see a leveling off of the rate of cooling.  I think that if I had fired the oven another hour, the inside temp would have leveled off at a higher temperature.  That would have given me addition time in the pizza and bread baking range.  As it was, I got about 90 minutes worth of baking time on that firing.  My max capacity in that firing was: 14 pizzas, seven 30" baguettes, and 6 whole grain loaves.


I hope this is helpful.


Let's hear some of your secrets!


David


 

Comments

davidjm's picture
davidjm

 I meant to add a link to a photo journal of building my oven for illustrative purposes, but forgot.  So here it is:http://www.facebook.com/album.php?aid=3471&id=100000101576256&l=009e1a6efc


If I had to do it over again, I'd make the subfloor thicker (4" instead of 2"), Make the opening out of brick instead of clay (see example: http://clayoven.wordpress.com/2008/09/08/5-the-oven-entrance-and-chimney/)


I'd also bring down the dome height from 20" to 16".  I think pizza would cook better.  Though I am torn.  When the fire is raging, the extra inches help contain it better.  Perhaps someone can offer more advice on dome height.


Lastly, I put my chimney at the top of the dome.  Mistake.  Should be on slope of incline downward.  What happens is the flames shoot out of chimney because of it's position.  If it were not at the top, but rather more on the slope down to the entrance, I think it would retain heat better, and contain the flames better.


So now you know everything I do about clay bread ovens!


-David

rcmullins's picture
rcmullins

So glad you posted something here.  I am looking at embarking on my first oven soon, and this is extremely helpful.


 


One thing I would like to know is what kind of wood did you use for your initial firing and in what quantities, before you added the Oak that is.  Which oak did you use, white oak or red?

davidjm's picture
davidjm

Glad this is proving helpful!  I use any sticks on the ground to get the fire going.  It takes a while to get it hot enough to actually start burning a real log (as you see from the graph above).  When you get to the point of burning logs, they should be short pieces (max 1 ft), and only about 2'' square at maximum.  Too big and it burns too slowly and takes too long to reduce to coal. It might take 8-10 2" square pieces to fire the oven, depending on interior size.  It use a bunch of sticks before it gets hot enough for a log.


I think I'm using red oak, but I don't think it matters, red or white.  Here is a helpful burn comparison chart I found online: http://www.chimneysweeponline.com/howood.htm


Let me know if you have any other questions.  You're probably not the only one asking it!


-David


 

mershadow1's picture
mershadow1

I'm new to the clay hearth oven also, in fact Still in the process of building mine.  Winter has set in so I'm at a stand still. 


But---the big thing I have learned about Kiko's design is that the hearth subfloor should NOT be sand. Maybe you have an older version of his book?   In the book I have he suggests a bottle/sawdust/clay mixture which is where I'm at now.  I've imbeded one layer of bottles and clay/sawdust, and next will be adding a solid block of clay before I lay my firebrick down.  This Hearth subfloor will be Hot!  I'm figuring.

davidjm's picture
davidjm

Thanks for your input!


I know what Kiko suggests.  But remember his premise "Build your oven for less than $100".  That is why he wants you to use bottles, sawdust, clay, or anything else you can find laying around.  It's not wrong, it's just not very heat efficient. 


What you need is thermal mass in the floor.  That is why I suggest the Portland cement:Vermiculite mix.  I took it from the brick oven sister of the K. Denzer clay oven.  The brick oven is much more heat efficient than the clay oven.  (Some report that the next morning after a fire, the oven is still around 400 degrees F!)


I'm no expert, but if you want to bake more than a few loaves of bread or pizza, then I'd recommend doing a real insulated sub-floor.  Download the instructions for a Pompeii brick oven from the forno-bravo website.  (FREE)  Read about how they insulate the floor.  I think in the end, you'll be much happier with the results - provided you plan to do more than a few loads per firing.


 

DownStateBaker's picture
DownStateBaker

I've been meaning to make graphs like that at work to help newbies see how the oven heats up and cools.