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Teacher Background Information

Investigating First Nations Australians’ knowledges of reversible processes such as the application of adhesives and of irreversible processes such as the use of fuels for torches

Content description 
Science, Year 6 | Science understanding – Chemical sciences

AC9S6U04

Compare reversible changes, including dissolving and changes of state, and irreversible changes, including cooking and rusting that produce new substances

Connecting the elaboration and content description

This elaboration provides students with the opportunity to learn about reversible and irreversible changes through First Nations Australians’ knowledges of resins and fuels, which have been developed and applied for many thousands of years. Students learn how First Nations Australians carefully control the application of heat to utilise reversible changes in resins in the manufacture, maintenance and repair of various implements and regalia. Students understand that the prolonged application of heat (for example, using fuel for torches to transport fire or repeated heating and cooling of resins) results in irreversible changes to the materials.

Detail 

Aboriginal and Torres Strait Islander Peoples use resin for a wide range of purposes. Resin can be used as an adhesive in the manufacture, maintenance and repair of implements, a waterproofing agent, fuel for a torch, and to add strength to a join. A wide range of plants exude resin. The type of plant used by Aboriginal and Torres Strait Islander Peoples for this purpose depends on the vegetation that is available in their specific geographical territory. Aboriginal Peoples most commonly use spinifex (Triodia spp.) and grasstrees (Xanthorrhoea spp.) to source resin. Spinifex plants are widely distributed across Australia, especially in arid and semi-arid regions, and grasstrees are found along the east and west coasts of Australia. Resin has long been a highly valued commodity and continues to be traded into areas without resin producing plants.


The use of resins by Aboriginal and Torres Strait Islander Peoples demonstrates an understanding of reversible change, and the application of this knowledge. Resins become soft and malleable with the application of direct or indirect heat. However, once the source of heat is removed, the resin cools and hardens again, demonstrating a reversible change in the state of matter. This knowledge is evidenced in the use of resins for a range of purposes by Aboriginal and Torres Strait Islander Peoples. Resins were, and continue to be, widely used as adhesives in the manufacture of implements such as attaching spear points to shafts, hafting stone hatchet heads and knife blades to handles and attaching pegs to spear-throwers. Preparation of the resin depends on its intended use. It can be enhanced, when mixed with other substances, to form a cement with stronger adhesive properties. The Ngarrindjeri Peoples of the Murray River region in South Australia mix the resin of the grasstree with sand to adhere sharp quartz tips to spears. In far North Queensland the Yidinji Peoples mix grasstree resin with beeswax, charcoal, sand or dust, to prepare a cement for fixing stone heads to wooden handles and spear shafts to tips. The reversible property of resin is important in the manufacture of such implements as it allows the opportunity to reshape and repair. In Central Australia, early European anthropologists observed Aboriginal Peoples heating the spinifex resin that bound the cutting edge of an adze to remove the blunted edge and replace the flint for a fresh cutting edge. A ready supply of resin was required so that tools and implements could be repaired or manufactured as needed. Resin was traditionally stored as a block or on sticks for accessibility when travelling, and for trading. Resins are insoluble in water, making them ideal as a sealant and to waterproof implements and vessels. The Gadigal Peoples of the Eora Nation in the Sydney basin region used grasstree resin to reinforce the joints of fish hooks and to mend damaged canoes. In the Cairns/Yarrabah region of North Queensland the Yidinji Peoples used the exudate from scrub turpentine to seal the sewn side of a bark waterbag.


In the Torres Strait, knowledge of reversible properties is applied in the use of heat to mould turtle shell, for example, in the manufacture of fishing hooks and masks. Fish hooks can be manufactured from turtle shell by cutting or scraping the shell, before applying heat to soften the shell, and bending it into shape. Traditionally, masks required the use of a hot stone to soften the shell sufficiently for it to be moulded into the desired shape. In both cases the removal of heat results in the cooling of the shell, returning it to a hardened state to set the desired form.


Irreversible changes can be indicated by a change in colour, release of odour, gas, light or sound or a change in temperature. Overheated resin undergoes an irreversible change and is no longer useful as an adhesive. Aboriginal and Torres Strait Islander Peoples treat resin carefully to ensure the thermal limits of resin are not exceeded. Early Europeans observed the Wadjari Peoples in central Western Australia heating grasstree resin too quickly so that it frothed and crumbled, and this irreversible change rendered the resin unusable. Studies have also shown that prolonged and repeated heat applied to Xanthorrhoea spp. resin causes irreversible chemical changes to the cement; it becomes more brittle and lessens the adhesive properties of the resin.


Aboriginal and Torres Strait Islander Peoples use plants, many of which are highly resinous, as fuel for torches, demonstrating an understanding of the irreversible changes that result from combustion. Spinifex is highly flammable and burns intensely for a long period of time, making it useful as a torch to transport fire. In the Torres Strait, coconut fronds are used as torches, with the peoples of Mer Island using such torches for spear fishing at night. The Ngarrindjeri Peoples of the Murray River region in South Australia traditionally sourced a resinous timber to use as a torch during spear fishing expeditions by canoe at night. This timber burns to emit a very bright light and produces little smoke. Branches or leaves from plants coated with resins are useful as torches. However, the prolonged application of heat causes irreversible changes that can be observed when the resins blacken and become brittle, and the branches are charred.


This elaboration provides students with the opportunity to learn the concept of reversible and irreversible changes using Aboriginal and Torres Strait Islander Peoples’ knowledges of adhesives and fuels. Students will also have the opportunity to learn how Aboriginal and Torres Strait Islander Peoples carefully controlled the application of heat to utilise reversible changes, and understand that the prolonged application of heat, for example, using fuel for torches to transport fire, results in irreversible changes to the materials.
 

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