Vacuum is space that is devoid of matter. The word stems from the Latin adjective vacuum for “vacant” or “void”. An approximation to such vacuum is a region with a gaseous pressure much less than atmospheric pressure.
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.