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connected to a pull-push rod; when pushed shut the plate closes the base of the chimney; when pulled open the plate is withdrawn to allow free upward escape of all heat.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.