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A modern thermal cooker uses the concept of the Haybox whereby placing hay or straw around a cooking pot of heated food the meal continues to cook without fuel.
The earliest dates from medieval times and consisted of two earthenware pots one within the other. The technique involved bringing food up to temperature on the fire as usual, but rather than allowing it to cook over the flame, the hot pot was placed in a 'nest' of hay, moss, dry leaves or other insulating material, in a box or hole in the ground, and covered. The heat in the pot was conserved for a considerable length of time, and the food inside would cook slowly, without the need for supervision. Both time and fuel were saved in this way, and foods that needed long slow cooking, such as pulses or tough meat, benefited from this technique. An example of this type of cooker was found by The Monmouth Archaeological Society's year long excavation inside two shops at 69-71 Monnow Street, Monmouth. In 69 Monnow Street what maybe the first archaeological evidence of "cooking without fire". The remains of the pots that were found can been seen in The Peoples Collection Wales
Medieval Instructions for cooking without fire taken from an Anglo-Norman manuscript in the British Library 
Take a small earthenware pot, with an earthenware lid which must be as wide as the pot, then take another pot of the same earthenware, with a lid like that of the first this pot is to deeper than the first by five fingers, and wider in circumference by three; then take pork and hens and cut into fair-sized pieces, and take fine spices and add them, and salt; take the small pot with the meat in it and place it upright in the large pot, cover it with the lid and stop it with moist, clayey earth, so that nothing may escape, then take un slaked lime, and fill the large pot with water, ensuring that no water enters the smaller pot; let it stand for the time it takes to walk between five and seven leagues and then open your pots, and you will found your food indeed cooked.
In the mid 1990s the thermal cooker was developed in Asia. It consisted of two stainless steel pots, one within the other. The inner pot, was used to bring the food to the boil and the outer, twin walled with a vacuum between the walls, was used as the container to keep the cooking process continuing.
To use the thermal cooker the food is put into the inner pot and brought to the boil, simmered for about 10 minutes and then placed in the outer pot for continued cooking. There are a number of thermal cookers on the market. Some use insulation material between the outer pot walls, others, use a vacuum.
Thermal cookers with two inner pots allow two items to be cooked at the same time, such as curry and rice. All thermal cookers are capable of cooking many dishes from soups to puddings. Cakes and bread can also be cooked by partly submerging the cake/bread tin in boiling water.
- Moorhouse 1987, 26; Hieatt and Jones 1986, 874, No.6