When customers for the large ovens are finding out about technical details before they place an order, it is clear that they are often confused about the position of the chimney, and the basic simplicity of an oven that is fired on its floor.
The basic oven that is fired on its floor is the ancient and classic design that we have had for millennia. The domed chamber is built with only one opening – a doorway that is used for loading the fuel and subsequently releasing all exhaust from the firing phase; and later, it is used as the aperture through which the food to be cooked is pushed in and pulled out. This simplicity requires that the chimney base has to be placed directly outside the doorway.
An internal chimney would require a mechanism called a damper, being the method of closing the base of the chimney so that all the heat built up in the internal masonry of the oven does not escape freely up the chimney during the long cooking cycle. Dampers are usually simple metal plates which are connected to a pull-push rod; when pushed shut the plate closes the base of the chimney; when pulled open the plate is withdrawn to allow free upward escape of all heat.
It is worth pointing out that an internal chimney in a small oven is not recommended, because as all heat rises to travel across the oven dome, a considerable amount of heat will simply rush up the chimney in a wasteful manner.
In large ovens, however, internal chimney openings are more common, and are essential if the oven is to be fired from a separate furnace. Compared with firing on the floor, firing through the separate furnace box is relatively passive, but does bring the contingent benefit of having no mess of ash and coals strewn across the oven floor. Time and effort do not have to be expended cleaning this mess off the floor before the bread is set in the oven. At the bottom of the furnace box is the grate (fire bars) and underneath the bars is the ash box. While firing, the flames roar out into the oven chamber leaving ash and coals falling neatly into the ash receptacle below. My oven in Australia was a “side-firing Scotch” type, with the furnace placed on one side of the facade wall, and the amount of inside cleaning was simple: to rake back gently into the furnace a small amount of ash that had spewed forward carried by the velocity of the draught feeding the flames. This is a far cry from the labour involved in raking back through the doorway all the coals and ash left from a hearth firing.
With the furnace to one side, the doorway in the middle, the other side of the facade wall features the damper mechanism at the base of its chimney. All these facade wall features will entail expensive cast iron. Handsome to look at, the cast iron is essential because it will endure great temperatures without rusting or warping.
Should the oven purchaser decide that he will prefer the clean floor, and fit a furnace in the wall or underneath the doorway, he must brace himself for two major expenses – the first expense is the cost of purchasing and fitting into the building project all that complex and costly cast iron; the second is a much bigger on-going fuel bill. I estimate that a furnace style masonry oven uses about 30% more wood than one fired on its floor. This is because the energy required to hurl the flames out of the furnace box and deep into the oven chamber is already using up some of the fuel, and further loss of fuel occurs when you consider that much heat from red coals is being lost as it falls downwards through the fire bars.
To the prospective purchaser I present it as a simple and challenging question: is the convenience of a clean floor after firing worth the huge expense that it entails?
In France I have been surprised to see how many professional bakers with large masonry ovens are content to clean routinely the ash and coals from the floor after each firing, content to eschew the cast iron fittings that make life easier but more expensive. In many cases there is no cast iron at all on the oven, with the doorway being stopped up at baking time with a simple prop door of steel that is pushed firmly into the opening creating a tight enough seal.
Paul Merry
Firing a cold oven
FIRING INFREQUENTLY – A COLD OVEN
Life is easy with a wood-fired masonry oven when your routine involves much baking and daily firing of the oven. The masonry and the whole structure stays permanently warm and hence the daily firing is a topping up affair, with less time taken, less fuel used.
This article concerns the other extreme, when your baking in the wood-fired oven is sporadic, and it is allowed to get completely cold. Maybe you are an amateur baker that only gets to it at weekends, or perhaps you are a semi-pro who only fires and bakes on the days when you prepare bread for your local market. At PANARY when I am in the phases of the year when there are no courses being held, I fire and bake in it only once a week when I have my commercial baking day to sell the bread in local shops. If it is only fired once a week, while it will not have become damp, it has definitely gone back to stone cold by the next time you fire.
For these cold starts I recommend a long firing with larger fuel – more like logs than split skinnies. Before setting the fire I begin by placing, or fanning, logs around each side perimeter, none at the rear. In my little Panyol 120 (that means diameter of the oven is 120 cm) I would place about four logs around the perimeter, two on each side, each log as thick as your leg (or arm if you are a huge person). Leave the blank at the rear because it is into that space that you will later gradually push your fire when it is really blazing well.
Then, in the normal way, build the fire inside the doorway, between the doorway and the centre. As it catches properly load the kindling starter fire with several medium sized pieces that will consolidate your blaze. When you have a healthy blaze that will not be set back by moving it, gradually push the fire to the rear of the oven. Two large flanking sticks on left and right are a good way to do this – see the picture of the French baker firing his Panyol 180 on the Gallery section of the Ovens part of my website. Keep the medium sized – perhaps split – pieces up to the fire so that the blaze becomes intense enough to light the perimeter logs from the rear and start fanning around the sides.
Remember that a well managed fire is achieved by the maxim “load a little, often”. A sure sign that you have become carried away is to see naked flame gushing out the doorway and going uselessly up the chimney. If the hottest point of a flame is its tip, that is a lot of wasted heat going up the chimney.
For the next two or three hours I settle into the steady rhythm of loading a log whenever one has burned away at the rear or down the sides. If a piece comes along that is fairly large, therefore you get the feeling that the fire will be dampened for a while until it catches, I then accompany the too big piece with a split skinny to put a quick blaze right beside it and allow it to catch fairly quickly.
Because so much heat will be lost to warming the cold structure of the oven’s surrounding masonry, you must go on firing for a very long time. Long ago the inside crown will have burned itself clean, but ignore that signal on an occasion like this. While it is a well known indicator that you have bread baking temperature when the crown clears its soot away (to appear whitish), it really only stands as a good indicator when the oven is fired regularly and has warmth retained in its structure. In many instances, if you proceeded to bake immediately after the soot has cleared, the heat will be so shallow you would only get one round of baking at proper bread temperatures.
Now you are soaking the oven in this high level of heat, singly replacing the logs that burn away, making sure the heat is sustained rather than reducing. Perhaps two hours would do it for this stage. By now the heat will have ceased disappearing into the structure, and can be doing its proper job of heating the refractory masonry in the oven chamber.
When you estimate that there is about two more hours in the bread proving time, that would be the marker to allow the fire to die away. No more logs are loaded, and all around the perimeter the dancing flames can be allowed to gradually reduce to red coals. Throttle the draught by part-closing the doorway opening so that the rush of incoming air is not blowing much of the heat away and up the chimney. Every twenty minutes or so, stir the coals with the poker to keep introducing oxygen to them. Allow a trickle of draught so that coals continue to reduce but massive heat is retained.
After about an hour, rake the coals into the doorway opening where they are mounded in a great heap. Close the door(s) so that only the merest trickle of oxygen can feed this mound, enabling them to glow a little. Now the coals can soak the masonry of the doorway with a forceful heat. Previously the doorway will have been a coolish zone because the cool air feeding the fire will have been steadily rushing through the doorway for hours. It is important to heat the doorway like this because it means the oven will bake evenly when all the bread is set and the door(s) closed. Every so often I stir the mound of coals here.
At this point you have to concentrate on the bread proof, to ascertain that it is not actually faster or slower than you had first estimated. If it is showing faster, you now have to hurry to rake out the coals, clean the floor, and fully open the doorway to blow some excess heat away so that the oven is not too hot for the bread. If proof is slow, you can retain as much heat as possible by entirely shutting the door(s).
If things seem to be going as you had planned, choose a lull in your work to take the time to rake out the coals and sweep the ash off the floor. I finish this task by swinging in a rotating action a damp cloth on the end of a chain around and around, across the floor of the oven. It was called “the scuffle”, and hessian is a good material for it.
Now you proceed to give the oven its final “settling” period, when it can further even itself out, and heat that would be too fierce can fade away. A typical settling time with closed doorway may be thirty or forty minutes. You are now ready to bake two rounds of bread, and such a long and careful firing should have given you the sustained heat to be able to do two rounds – two full oven loads.