Aboriginal and Torres Strait Islander Peoples have a long-held scientific understanding of the techniques that can produce heat and the knowledge that heat can be transferred from one object to another. For millennia Aboriginal and Torres Strait Islander Peoples have used a variety of techniques to make fire by friction, including methods of generating heat through the friction between two pieces of wood (fire drill, fire saw, fire plough) or between two rocks (percussion), resulting in the production of heat. Generation of heat through friction is used to initiate combustion (the rapid oxidation of such materials in air); however, sufficient heat must be attained to reach an ignition point that results in flames. The process of initiating combustion has long been understood and employed by Aboriginal and Torres Strait Islander Peoples to produce fire for many purposes, including the generation of heat for cooking processes.
Heat transfer is the process whereby heat (thermal energy) moves from a warmer object to a cooler object until thermodynamic equilibrium is reached, that is, the objects are at the same temperature. Aboriginal and Torres Strait Islander Peoples have long used this fundamental scientific concept to transfer heat from one object to another for specific purposes. Many Aboriginal and Torres Strait Islander Peoples’ methods of cooking provide clear examples of the knowledge, understanding and application of heat transfer between objects.
For example, for millennia the Barindji Peoples of south central New South Wales built a substantial fire to heat stones for cooking large animals in a ground oven. Immense heat is applied to the stones until they glow red hot. Such stones have considerable thermal inertia and long radiate (transfer) heat to their surroundings. The hot stones are placed into a pit oven and the animal is then placed into the oven on the hot stones. The oven is covered with grass, more hot stones placed on top and the oven completely enclosed with earth. The process of using hot stones both underneath and above the meat ensures that heat is transferred from the stones in both directions so that the meat is cooked through without needing to be dug up and turned.
The success and efficiency of ground ovens is due to the heat transfer principles of conduction and convection. The method of transferring heat from one object such as hot stones, to another object, such as food, is called conduction. Convection is the process of heat transfer due to the bulk movement of particles within fluids such as liquids and gases. Cooking food in a pit oven acts in a very similar manner as a pressure cooker on a stove. The stove conducts heat into the pressure cooker and the contents are cooked evenly and efficiently through the convection of gases that form within the cooker. Similarly, the earth that encloses a ground oven acts as a lid that retains the heat and pressure generated by the hot stones.
The Bindal and Wulgurukaba Peoples of far North Queensland also cook meats in earth ovens, digging a hole in the ground and setting a large fire in the pit. Stones are placed in the fire, heated until they are red-hot, then removed from the pit and the fire is extinguished. Once the fire is cleared away a number of the hot stones are returned to the pit, the uncooked meat is placed on leaves covering the hot stones, and the remaining hot stones are placed on the top of the meat. Again, the practice of surrounding the meat with hot stones ensures that heat is transferred to the meat to thoroughly cook from all directions. This cooking process rests on a thorough understanding of the process of heat transfer, as using only a single layer of hot stones may result in only one side of the food being cooked due to an uneven transfer of heat.
Ground ovens have long been used, and continue to be used, widely on the Torres Strait Islands. Amai in the Meriam Mir language of the eastern Torres Strait Islands and ame or netebu in the Kala Lagaw Ya language of the western Torres Strait Islands, that translate to “earth oven” in English, consist of a large shallow hole built in sandy soil lined with stones. A large fire is set on top of the stones and is kept burning until the stones are red hot. The fire is then extinguished and some of the stones are removed from the oven. Food is layered alternately with hot stones, and the number of layers depends on the amount of food that is to be cooked. Stones are not the only material that can be heated to facilitate cooking of food through the process of heat transfer. The Wik-Mungkan Peoples of the western Cape York Peninsula in northern Queensland use heated pieces of termite mound (ant-bed) to cook food. A fire is set in a hole in the ground containing the termite pieces, and once they are sufficiently heated, food is placed on top and the hole is enclosed with earth and bark to create a hot oven for cooking. Industrial steel offcuts are frequently used, in combination with rocks or as substitutes for rocks, in many contemporary ground ovens to take advantage of their excellent thermal properties.
Aboriginal and Torres Strait Islander Peoples also employ heat transfer by convection when heating liquids over a fire. When air is heated by fire, the warmer air becomes less dense than the cool air and it rises, and is then replaced by the cooler air underneath. Placing a vessel of water over a fire allows the heat energy from rising air to contact the vessel and transfer heat to the object. The water closest to the heat source in the vessel warms and rises, being replaced by cooler, more dense water molecules to create a convection current. The continual exchange of water molecules in this way results in the entire vessel of water being heated to the desired temperature. Prior to colonisation, the Kuku-Yalanji Peoples of the rainforest region of far north Queensland used large bailer (melon) shells or bark troughs for boiling water over a fire. The Meriam Peoples of Mer Island in the Torres Strait boil water or coconut milk in a bailer shell over a fire, with the large shell supported over the fire on stones. Alternatively, water is heated by adding hot stones into the vessel of water. Tubers, roots, fish and meats are then cooked or boiled in the heated liquid. These processes require an understanding of the transfer of heat through both convection and conduction processes.
This elaboration provides students with the opportunity to investigate Aboriginal and Torres Strait Island Peoples’ long-held scientific understanding and utilisation of methods of heat production and heat transfer. For many thousands of years Aboriginal and Torres Strait Islander Peoples have produced and controlled heat in a variety of ways, including the use of different fire-starting techniques. Fire has long been used as a source of heat and the understanding of methods of heat transfer, including convection and conduction, is evidenced in cooking methods. Through exploring this context students can learn that heat can be produced in many ways and can move from one object to another.