Thermal cooking

After heating over a fire, a hot, earthenware pot containing food was placed in another, larger pot, box or hole in the ground, insulated by hay, moss, dry leaves or other material, and covered.

The heat conserved within would slowly cook the food inside, saving fuel and work.

The hollow evacuated wall of the cooker thermally insulates its contents from the environment, so they remain hot for several hours.

[citation needed] The slow cooker is used for a similar purpose; but instead of minimising heat loss, sufficient heat is applied to the non-insulated slow cooker to maintain a steady temperature somewhat below the boiling point of water.

The pot is filled with food and water and heated to cooking temperature outside of the flask on a stove, usually to boiling.

The flask minimises heat loss, keeping the food hot enough to continue cooking and avoid bacterial growth for many hours.

[4] If a large part of the cooking time is spent at temperatures lower than 60 °C (as when the contents of the cooker are slowly cooling over a long period), a danger of food poisoning due to toxins produced by multiplying bacteria arises.