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Πέμπτη 21 Νοεμβρίου 2013
πως να χτισετε εναν φουρνο με πηλο-Build a Clay (Cob) Oven

Build a Clay (Cob) Oven in Your Yard!

How to build a wood-fired pizza/bread oven using local natural materials
If you are looking for a small project to get your hands (and feet!) dirty testing out some natural building skills, then building a wood-fired oven is a great place to start.  If you have a little help, it takes just a couple days to build, then a few weeks to let it dry out (during which time, you can sculpt your oven to any shape), and then you're ready for a pizza party!!
What is a cob wood-fired pizza oven??  Well...it's a baking oven that is heated by lighting a fire inside, the fire warms up a thick clay oven wall, and the clay wall remains warm for hours after the fire is pulled out.  So you build the fire in the same oven area that becomes your baking space.  The beauty of this type of oven is a) the oven is simple to build using local, natural materials and b) the oven temperature remains very even throughout, with no hot or cold spots.  Plus, it's a fun project to do with a bunch of people and you can celebrate your accomplishment with a pizza party!
So, what do you need to know to ensure that your oven project is successful?
decide what size oven you want to build.The appropriate size for you will depend on how you intend to use your oven.  Here are the variables effected by size:
  1. The larger the oven, the more materials you need to build it.  For example, an oven that is 36" wide inside takes about twice as much clay, sand, & straw as an oven that is 24" wide inside.  And more materials translates to more building time as well.
  1. The larger the oven, the longer it takes to heat up.  For example, a 24" wide oven takes about 2 hours of fire to heat up, whereas a 36" oven takes about 3 hours.
  1. The larger the oven, the more mass, soooooo, the longer the oven stays warm.  This means you can cook in it longer each time you fire it up.  Especially if you a good insulation layer on your oven.
  1. And obviously, the larger your oven, the more pizzas you can bake at once!  (Or whatever you are cooking...)
So, think about how long you want to wait for your oven to heat up, how long you want the oven to stay hot (larger oven for pizza party use, smaller oven for personal use), and think about how big of a project you want to take on (do you want to build a small oven over 2 days or go for a larger oven and spend longer to build)??
Typical sizes are 22-1/2", 27", or 36" (these sizes work out well with the size of standard fire brick).  Of course, there are mini ovens as well as massive ovens, but those are mostly for special use applications.
THIRD, decide if you will build a roof over your oven.A roof will help protect your oven from the elements, and allows you to bake even when the weather is sucky.  Rain, especially, will erode a clay oven over time.  You can either allow that, replaster your oven every year, put a tarp over your oven when it's not in use, or....build a roof to protect it.  If you decide to build a roof, those materials will be in addition to those listed below.  Build your roof so you have plenty of room to stand underneath, and to clear any smoke out.  I recommend at least 7 feet of clearance under the roof.FOURTH, gather your materials.You will need the following materials to build your oven:
  1. Clay:  Clay is your essential ingredient, because it is the binder that holds all the materials together.  When wet, clay is sticky.  When it dries, it is strong & hard.  You can use clay-soil OR you can purchase dry, bagged, pottery clay.  (see video below on how to test your soil for clay content.)  If you are using clay soil, you will need to determine the proportion of clay in your soil (it may feel like it's 100% clay, but it rarely actually is...usually there is sand in there as well).
  1. HOW MUCH?  The amount of clay needed depends on the size oven you are building.  Here I am talking about total clay, so if you are using soil with clay in it, you will calculate the amount of clay based on the percentage of clay in the soil  (So, if your soil is 50% clay & 50% sand, then every bucket of soil = 1/2 bucket of clay & 1/2 bucket of sand.)  So total clay needed is about 25 gallons for a 22-1/2" oven, about 35 gallons for a 27" oven, and about 50 gallons for a 36" oven.
  1. Sand:  Sand is your aggregate.  It reduces shrinkage of the clay as it dries and it adds total strength to your oven walls.  You need to use angular sand, not smooth sand or silt.  Concrete sand is pretty cheap & works great.  I also use sand to build the form for the oven (this sand is taken out at the end and can be used to make plaster if you finish your oven that way).
  1. HOW MUCH?  Plan on about 300 to 500 lbs of sand if you are using clay soil with at least 50% sand content; if you are using bagged pottery clay, double the sand.
  1. Straw:  Straw is used to create an insulating layer for your oven.  It is also helpful to stand on the bales as your oven gets tall.  Make sure your straw is clean, dry, and mold-free.
  1. HOW MUCH?  You need about 2 to 3 strawbales for a small oven and 3 to 4 strawbales for a larger oven.  If you plan to sculpt your oven into a fun shape, make sure you have ample straw.
  1. Firebrick:  This is what I like to use for the floor of the oven, because they don't split in the heat of the fire and they have extremely squared edges, so they make a really smooth floor.  Typical firebrick are 4-1/2" x 9" x 1-1/2".  You can lay out the bricks for your desired oven size to see exactly how many you need, but below is what I use as a reference.
  1. HOW MUCH?  I use 15 firebrick for a 22-1/2" oven (12 for the floor + 3 for the door opening), 22 firebrick for a 27" oven (18 for the floor + 4 for the door opening), and 37 firebrick for a 36" oven (32 for the floor + 5 for the door opening).
  1. Water:  you will need a running water source to wet the clay binder.  (and is helpful for clean-up)
  1. Newspaper (optional):  I use this as a layer between my sand form & the first layer of clay...it lets you know when to stop digging out your sand so you don't accidentally gouge your oven wall.
  1. Stones or brick or urbanite (optional):  I recommend building your oven up on a base so you don't have to kneel on the ground to tend your fire & bake.  A comfortable height is typically 24" to 36" off the ground, but choose whatever height is comfortable to you.  You can use any kind of masonry material that is available to you, and you can make cob (clay, sand, and straw) to make a strong mortar.  Just make sure that your oven base is very stable.  Once the base is built & dry, don't forget to fill in the center (with something sturdy & non-compressible), so you have something solid to build your oven floor on.
Note: I highly recommend collecting extra material than you think you need so you don't run out of anything mid-stream.How to test your soil to see if it has clay in it:

And this is my tools list when doing an oven workshop:
  1. buckets - I like to have ample 5-gallon buckets; you these to transport and measure your materials; to me, 5 buckets is a minimum, but if you are working alone, one bucket will work
  1. tarps - I like to have 2 tarps, but one works; 10' x 10' seems to be a manageable size
  1. shovels - if you are working alone, one shovel is fine; if you are going to have a bunch of people, have at least one shovel for sand & one for clay (more if you will have lots of helpers)
  1. sifter - if you are using clay soil dug out of the ground, I find it easiest to sift it roughly through a 1/2" screen to remove any rocks & to break up the clay and make it easier to mix; I like a table screen that fits over a wheelbarrow
  1. a wheelbarrow is useful to transport material, but is not essential


FINALLY, build your oven!STEP ONE: Build your base
I like to make a shallow foundation filled with gravel to help keep the oven from moving with freeze-thaw cycles in the ground.

You can build your base with any masonry material - this example uses old cobblestones from the streets of Philadelphia, built with a cob (clay, sand & straw) mortar mixture.
STEP TWO: Fill in your base & build the oven floor
Fill in the base of the oven with a non-compressible material, like tamped gravel.  Then add a 4" or so layer of sand that extend just above the sides of your base.  Tamp and level that sand.

Then build your oven floor with the firebricks, making sure they are nice & tight to each other.  Tamp your bricks and make sure they are nice & level.  Spend some extra time here...the nicer your oven floor, the fewer nooks & crannies that will cause you potential headaches when you are baking.
STEP THREE: Build the form for your oven cavity
Basically, you are building the most boring sand castle ever: a nice dome.  The dome width is the diameter of baking space you want.  The height of the dome will be 75% of whatever your width is.
Add a layer of newspaper over your sand so that when you dig out the sand at the end, you know when to stop digging (before gauging your clay layer).
STEP FOUR: Build the clay mass layer of your oven
 
The first layer of your oven is cob without the straw, so just sand & clay.  You want enough clay so the mixture is sticky, but enough sand so that this layer doesn't shrink (and then crack) a lot.  For most types of clay, that means between 20% and 25% total clay content.  I like to make this layer 4" thick, all the way around your sand dome.  I do not worry about the door at this point...I carve that out later.
STEP FIVE: Add the insulation layer
Your insulation layer is mostly straw, with just enough clay to act like hairspray to hold the straw together.   The idea is to keep the heat inside the oven for as long as possible, increasing the efficiency.   What to do: use a clay mixture that is 50% clay & 50% sand (if that is the proportion in your soil, then you can just use your soil), wet the clay/sand mixture so it is quite soggy...like a chocolate milk shake, then add lots of loose straw, until all of the straw is coated with clay.  Then smoosh it in place.  I use about 6" thick of insulation.
STEP SIX: Cut the door
I like to sculpt the door in the insulating layer.  The straw has really good binding properties, which make it easy to sculpt a nice arch for your door.  If you are not doing a chimney, then the height of the door needs to be 2/3 as high as the inside dome.  This allows fresh oxygen-rich air to come in the bottom of your door opening to feed your fire and the exhaust exits the top of the door area without smothering the fire.  If you make a door ahead of time, you can use that as your template for sculpting & cutting your opening.
STEP SEVEN: Let the oven dry for a few days, then pull out the sand
There are a few strategies for taking out the sand.  I like to carve out the door the day the oven is finished, then pull out about 1/3 of the sand, and then let the oven sit for several days to let the clay mass layer dry out a bit.  Then you can pull out the remainder of the sand without risking any collapse of the dome.  But don't worry, if you pull out the sand too soon and part of it collapses, you can just patch it with the same materials.
STEP EIGHT: Sculpt your oven as desired
Use clay, sand, and lots of straw to make a firm, cob mixture that you can use to sculpt your oven into any shape that makes you happy.  If your oven has begun to dry out when you start sculpting, be sure to wet down the surface before you add your sculpted pieces, otherwise they will not bond to what is existing.  You can also put mosaic tile on your oven, or plaster it with clay or lime plaster.  (This photo shows a lime plaster that is fairly weather-resistant.)
STEP NINE: Let your oven dry out completely & then have a pizza party!
Fire the oven for 2 to 3 hours, with a nice hot fire.  Then let the fire go to embers.  If you made a fairly large oven, you can distribute the coals around the back perimeter of the oven to keep it hotter longer.  Otherwise, pull all of the fire out (into something non-flammable, non-meltable)  and clean the oven floor with a damp cotton mop or cotton T-shirt.
Put the door on for about 20 minutes to let the temperatures come to equilibrium, and then bake pizza, bread, casseroles, cookies, pies, and anything else your heart desires.  As the oven gets to around 100 degrees, you can even use it to culture yogurt.  And keep a well-fitting door on the oven to keep the heat in.



Still not clear?  Watch these videos that show you the step-by-step process for building a cob oven.Building the floor of the oven & the sand mold for the oven cavity

Building the thermal mass layer (that will heat up when you build your fire) & the insulating layer that keeps the oven hot longer.

Digging out the sand form & baking your first pizza.

Happy baking!! ΠΗΓΗ-http://buildnaturally.blogspot.gr/2013/06/build-clay-cob-oven-in-your-yard.html

Mud oven Project - technical notes.

  • Basic oven is a simple, dome shaped shell of mud and sand
  • we built a simple one layer oven - however you can build up to 3 layers
  • dense thermal layer
  • less dense layer with straw
  • finish layer
  • to figure how big to make the oven - start off with deciding how often and for what you will be using it. lay out loaf/pizza pans to see how big a circle you need to contain it - thats how big the oven needs to be.
  • location of site - you need to think about:
  • wind - direction
  • wood - storage
  • water - proximity
  • roof - protection
  • materials list:
  • water
  • dirt - subsoil - amount will depend on clay content (don't use top soil)
  • sand to mix with mud - sharp sand not round
  • sand for oven form - needs to be able to hold its form
  • straw if doing an insulating layer
  • foundation - use bricks, wood, concrete, urbanite (old broken up concrete) etc - height determined by how low you want the door
  • floor of oven - firebricks (expensive), standard red bricks (approx 28) new or used but must be smooth and flat
  • wood for door
  • newspaper
  • note on waterproofing oven: Denzer says an earthen oven is like a living thing and must be able to breathe. When baking it exhales steam, by putting a non breathable finish on like paint or concrete on top of the oven it is like putting on a rubber suit - it traps moisture. and the steam will condense and soak back into the oven - possibly cause collapse. the best sort of water proofing is a roof - leave at least 3inch ventilation gap.
  • Mud: dig down to where the subsoil layer is - hard solid mass of earth. recognising subsoil - its hard, it doesn't crumble. add water and you can mould it, when dry it is hard and not crumbly, it feels sticky and greasy when wet.
  • final cob mix needs a mix of 75% sand and 25% clay
  • give mud/cob mix test to see if it is right consistency - dampen mix lightly - not paste, listen to it crunch - squeeze and hear grinding. add more sand if it doesn't.
  • then the snow ball test - make it dry not too wet, pack it into a firm ball - this might take a few minutes. drop it onto the ground and it should hold its shape if it crumbles add more clay, if it goes flat add more sand, if it holds its shape - perfect.
  • more technical details in the book about shrinkage and strength, which we didn't really bother with.
  • foundation: must support the weight, be at the correct height, protect against the elements.
  • see our post on building the foundation out of wood and using prefab concrete at a base.
  • read the book for urbanite solution
  • about the sand form for the oven void: draw the circle around the brick floor - and fill with sand - it should be moist enough to form into clumps and hold its shape, shape it by walking a wooden board around and moisten with a light sprinkle of water. till it is smooth and firm.
  • measure height of oven, the door must be 63% of height. - why 63% research of traditional clay oven found that was the optimum ratio for perfect baking.
  • dome height is 60-75% of dome diameter
  • making mud: mix ratio of sand and clay to form fine granules - get all the clumps of clay out. put dry mix on a tarp and stamp and twist on it, working the clay and sand together. roll the tarp over the mix back and forth until it looks evenly mixed. add water slowly adn mix with tarp again, and then jump in and twist about breaking up the clay.
  • apply mud in layers around sand form. press handfulls around the base and press down with thumb and fingers - being careful not to damage the sand form - at least 3 inches thick - use your hand a guide - thicker oven requires more fuel but will stay hot longer.
  • when its all covered - take your flat board and whack or tamp it until smooth and solid. to add another layer rough this one up first. then add straw mud layer and then sculpt.
  • cut door and remove sand - remember your dimensions.



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