As we arrived in Clos du Doubs, we met Krishan, an old and wise clay-oven-builder.
He was there visiting Niccolas to build with him the new oven for the upstairs-rooms.
The oven would be used to heat the upper rooms and for baking food and drying fruits.
We came totally at the right time,
they had already built up the fundament for the oven and made some skizzen,
but the were only at the beginning.
Some of us were very interested in learning how to build an oven made of pure nature clay and sand,
so we started to help them build the oven up, brick by brick.
Whilst building Krishan told us about ovens and we learned in practical way.
In the evenings he gave us lessons in oven-building-theory and how he started to build up clay-ovens.
Here you can find an instruction for building the oven that we built:
Chapter one: Chimney¶
A chimney is a rope or a duct which normally goes through the house vertically up to the roof or sometimes just straight out of the oven itself. The chimney has the function of sucking out the air and the smoke of the oven and sucking new air into the fireplace to heat up the fire and keep it alive.
so the higher a chimney is, the more sucking-power it has.
But why is that so??
The nature tries to balance everything. When you put hot water in cold water it mixes it up and balances the temperature.
It is the same with the air pressure.
In higher areas we have lower air pressure than in lower areas.
At the fireplace we have more air pressure than at the top of the chimney, so the air pressure automatically presses the steam and smoke of the fire up through the chimney to balance the air pressure-differences.
At the same time:
The air gets heated by the fire which is more hot than the outside air.
Hot air is more light than cold air and rises straight-up,
so it is flowing up through the chimney and generating an under-inflation, which sucks new air from the bottom of the chimney through the fireplace and keeps the fire alive by feeding it with oxygen in that way.
How big should a chimney be?
5-6 metres tall is minimum size,
also the diameter should be not too big, minimum size is 18cm = 220cm²
You can calculate the cm²:
For round chimneys with following formula: r²*π (square radius, multiplicated by pi)
For cornered chimneys multiply the length and width of the chimney-sides.
In older buildings, the chimneys where built very big so that the chimney-cleaners were able to climb into them.
For the most ovens its too big, and sucks too bad, so it much more efficient when you build in a smaller tube, you will have much more sucking power. But be careful, ask your chimney-cleaners, there are everywhere different strict rules about ovens and chimneys.
Also when the chimney is too big the smoke will circulate inside and go much slower up. When the chimney is small it goes straight-up and fast.
When you build the chimney with bricks you will have the effect that the bricks will be heated by the hot smoke of the oven(s) and heat the areas where the chimney passes by creating a heater at the same time.
When you use a tube inside the chimney which is isolated from the walls, you will not have this effect.
You also always have to look when you build a chimney,
which main wind-direction you have, this is important for knowing how you should build it.
Also when your chimney is smaller than the top of the roof it can happen that the wind hits the top of the roof, starts to squirl and press air into the chimney, so the smoke cannot go out and gets pressed back into the house.
Also very important is the location of your house and your surrounding.
Is the house built on the top of a hill, where the air runs up the side and helps to press the smoke out of the chimney. This is better than a house which is built in a forest, where if many tall trees are growing beside the house
the air can fall down between the trees and could press the smoke back into the chimney.
For starting the fire it is also bad if the air pressure outside is too low, or the chimney is too cold, for that you can use a trick.
Go down to the cleaning-clap of the chimney in the basement and start a short and very hot fire with newspapers there.. so you will heat up the chimney very fast and you can more easily start a fire in the oven.
In german it is called “Lockfeuer”.
Chapter two: The Oven¶
There are different ways of building up an oven depending on what you want.
If you just want a simple oven for heating the room you just need a fire-room and a channel to the chimney,
but you can use the hot air going out of the fireroom much more efficiently when you lead the hot smoke of the fire around in some channel-systems before you let it flow into the chimney.
This way you have much more stone-area which gets heated up and radiates the heat into the room.
You also can build a sitting-bench on this smoke-channel-system so you have heated bench – there are no borders for your creativity.
I just want to show you one example of ovens and its air circulation, that you get an first impression about the functionallity of ovens, and some very important things you should attend.
First and most important:
Fire produces different gases.. some of them stick on the top of the ovens fireroom. They are very explosive and when they get in touch with some fresh air they will explode and destroy the whole oven, some people have even died because of this.
So it´s extremly important that in every kind of oven you build, you need to build a so called “shortcut”, this is the shortest connection between the fireroom and the chimney, so the gases can always leave the top of the fireroom and flow suddenly into the chimney.
For that you build a small tube and also a clap (like a gate) between the fireroom and the tube where you can open and close the tube. (though it is necessary to make a small hole in that clap, so the chimney is never completly blocked.)
The clap helps when you start the fire, first you open the clap, so the chimney can suck the air of the fire the shortest way and you will have a stronger sucking and pressure, which helps to start the fire easily, and the hot air and smoke dont get lead through the channel-systems.
When the fire is burning you close the clap and the smoke gets lead through the channel-system and heats up the hole oven.
The hole in the clap has the function, of constantly sucking all the gases out of the fireroom and lead them suddenly into the chimney.
Next you have the fireroom, which is the room of the oven where you burn the fire in.
This will be the most hot place of the oven and it is important to build this with good stable materials. Don´t use pure clay-bricks and normal clay-sand-glue for it, as it will break.
Better is to use schamott or firebricks (the red ones).
Schamott is made of a mixture of clay and little stones and pressed under much higher pressure.
The Schamott-bricks has to be clue also with much more fine clay-sand-mixture.
The pure clay-bricks just get clue by more gritty mixture of clay and sand.
this is because the schamott-bricks can handle much more heat than the clay ones. they dont get brittle so easily like the more gritty gluey-mixture and the clay-bricks.
The fireroom should be between 90cm and 120cm tall.
At the exit of the fireroom the smoke has a temperature of about 1000 degree.
From the fireroom the smoke will flow through a channel called “Ãœberbrandt* (german) there it is 900 degree and runs down the (Sturzzug) to a channel which is under the fireroom,
there you lead the smoke through different channels up to the chimney.
The more ways you have the more stones will get heated up, but also the slower the smoke will flow and when it is too long the chemney will not suck it anymore. There is no standard data how long it can be, it always depends on the power of the chimney and how the oven is built.
The diameter of the channel leaving the fireroom should be the biggest one,
the smoke gets more and more cold on the way to the chimney, so we can reduce the channel-diameter.
resulting in maintaining the air pressure in the chimney so that it can suck the smoke through better and more consistantly.
On the bottom channel of the oven we have to add 20% more because there ash will collect over the years and we need more space to suck the smoke through.
