In regions where much of the daily cooking involves a long simmering period (required for many beans, grains, stews and soups) the amount of fuel needed to complete the cooking process can be greatly reduced by cooking with retained heat. This is a practice of ancient origin which is still used in some parts of the world today.
In some areas a pit is dug and lined with rocks previously heated in a fire. The food to be cooked is placed in the lined pit, often covered with leaves, and the whole is covered by a mound of earth. The heat from the rocks is retained by the earth insulation, and the food cooks slowly over time.
Another version of this method consists of digging a pit and lining it with hay or another good insulating material. A pot of food which has previously been heated up to a boil is placed in the pit, covered with more hay and then earth, and allowed to cook slowly with the retained heat.

This latter method is the direct ancestor of the Haybox Cooker, which is simply a well insulated box lined with a reflective material into which a pot of food previously brought to a boil is placed. The food is cooked in 3 to 6 hours by the heat retained in the insulated box. The insulation greatly slows the loss of conductive heat, convective heat in the surrounding air is trapped inside the box, and the shiny lining reflects the radiant heat back into the pot.
Simple haybox style cookers could be introduced along with fuel-saving cook stoves in areas where slow cooking is practised. How these boxes should be made, and from what materials, is perhaps best left to people working in each region. Ideally, of course, they should be made of inexpensive, locally available materials and should fit standard pot sizes used in the area.

There are several principles which should be kept in mind in regard to the construction of a haybox cooker:
Insulation should cover an six sides of the box (especially the bottom and lid). If one or more sides are not insulated, heat will be lost by conduction through the uninsulated sides and much efficiency will be lost.
The box should be airtight. If it is not airtight, heat will be lost through warm air escaping by convection out of the box.
The inner surfaces of the box should be of a heat reflective material (such as aluminium foil) to reflect radiant heat from the pot back to it.

A simple, lightweight haybox can be made from a 60 by 120 cm sheet of rigid foil-faced insulation and aluminium tape. Haybox cookers can also be constructed as a box-in-a-box with the intervening space filled with any good insulating material. The required thickness of the insulation will vary with how efficient it is (see below).

Good Insulating Materials Suggested Wall Thickness
Cork 5 cm
Polystyrene sheets/pellets/drinking cups  5 cm
Hay/straw/rushes  10 cm
Sawdust/wood shavings 10 cm
Wool/fur  10 cm
Fiberglas/glass wool 10 cm
Shredded newspaper/cardboard 10 cm
Rice hulls/nut shells 15 cm

The inner box should have a reflective interior: aluminium foil, shiny aluminium sheeting, old printing plates, other polished sheet metal’ or silver paint will all work. The box can be wooden, or a can-in-a-can, or cardboard, or any combination; a pair of cloth bags might also work. Be inventive. Always be sure the lid is air tight.

There are some adjustments involved in cooking with haybox cookers:
Less water should be used since it is not boiled away.
Less spicing is needed since the aroma is not boiled away.
Cooking must be started earlier to give the food enough time to cook at a lower temperature than over a stove.
Haybox cookers work best for large quantities (over 4 lifers) as small amounts of food have less thermal mass and cool faster than a larger quantity. Two or more smaller amounts of food may be placed in the box to cook simultaneously.
The food should boil for several minutes before being placed in the box. This ensures that all the food is at boiling temperature, not just the water.

The boxes perform best at low altitudes where boiling temperature is highest. They should not be expected to perform as well at high altitudes. One great advantage of haybox cookers is that the cook no longer has to keep up a fire or watch or stir the pot once it’s in the box. In fact, the box should not be opened during cooking as valuable heat is lost. And finally, food will never burn in a haybox.


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