Natural Draft Furnace

I built a natural draft furnace to test ideas about how hot a furnace could get without the use of bellows. Natural draft is the flow of air through a furnace due to rising hot air. The hot gasses in the fuel bed are more buoyant than the cold air outside the furnace causing them to rise. Fresh combustion air then enters the base of the furnace to replace the rising combustion gasses, keeping the fuel bed burning. This effect increases with: 1. the average temperature of the fuel bed relative to the outside air and 2. The height of the furnace. Two other important factors are the size of the tuyere (air entry pipe) and lump size of the fuel bed as these effect the resistance to airflow through the furnace. The furnace was tested with wood fuel and some ore was melted but produced no iron. High temperature were indeed produced (probably about 1200 c). These types of furnaces were once used for smelting copper and iron ores in around the world in ancient times, usually using charcoal as a fuel and in some cases wood too.

I designed the furnace using a formula from the book “The mastery and uses of fire in antiquity” by J.E. Rehder. It was designed to have a space velocity (air speed within the furnace) of 6 m per minute which is recommended for iron smelting. The furnace was 175 cm in total height but with a height of only 150 cm above the tuyere. The height between the air entry and the top of the furnace is what determines the strength of the draft, the space beneath the air entry is not included in the formula. The internal furnace diameter was 25 cm. The walls were about 12.5 cm thick at the base but got thinner with height. The tuyere (air entry pipe) was 7.5 cm internal diameter and about 20 cm long. The tuyere was placed into an opening in the base of the furnace and sealed with mud. The whole thing took about a week to make due to the slow drying time that was assisted by keeping a fire burning in side it. The furnace was designed to use charcoal (which in this case should be 2.5 cm diameter lumps) but I used wood to test it instead as it was easier to acquire. To test its melting ability, bog ore was found further down the creek and roasted. The roasted ore was then crushed and stored in a pot.

The furnace was filled with wood and lit from the top. The fire burnt down the furnace producing charcoal. On reaching the tuyere the fire then started burning the charcoal. Wood was also continually added from the top along with a few small handfuls of the roasted bog ore (not shown in the video). The temperature of hot objects can be visually estimated from their incandescence.  After about an hour, the light coming out of the tuyere was high yellow to white hot indicating a temperature of at about 1200 c. Colour temperature charts vary but white hot is usually given to be at least 1200 c, examples of these charts can be found on the internet for reference. It was uncomfortable to stare into the tuyere and doing so left an after image when looking away, indicating the strength of its brightness. After about an hour and a half the furnace was left to burn out. When opened the next day the tuyere was covered in slag with bits of slag found on the furnace floor also.

This experiment shows that high temperatures can be achieved without the use of bellows or charcoal, which might significantly reduce labour in the production of iron. The furnace was technically easy to build as it was a simple vertical cylinder. When running, the wood added to the top of the furnace converts to charcoal in the upper part of the stack and is consumed in the lower part. The ore I used was new to me, normally I use iron bacteria as an ore. This new ore produced no metallic iron so I’m inclined to use iron bacteria in future. Natural draft furnaces were once used to smelt copper and iron ores in the past, usually with charcoal fuel and less frequently with wood. The main benefit of these furnaces seems to have been the reduction in labour they provide and simplified infrastructure (fewer workers and no bellows required during operation).

Mud bricks

I made a brick mold that makes bricks 25 x 12.5 x 7.5 cm from wood. A log was split and mortise and tenon joints were carved using a stone chisel and sharp rocks. The mold was lashed together with cane to prevent it from coming apart when used.

Next, I made a mixture of mud and palm fiber to make the bricks. This was then placed into the mold to be shaped and taken to a drying area. 140 bricks were made.
When dry, the bricks were then assembled into a kiln. 32 roof tiles were then made of mud and fired in the kiln. It only took 3 hours to fire the tiles sufficiently. The mud bricks and tiles were a bit weaker than objects made from my regular clay source because of the silt, sand and gravel content of the soil. Because of this, I will look at refining mud into clay in future projects instead of just using mud.

Interestingly, the kiln got hot enough so that iron oxide containing stones began to melt out of the tiles. This is not metallic iron, but only slag (iron oxide and silica) and the temperature was probably not very high, but only enough to slowly melt or soften the stones when heated for 3 hours.

The kiln performed as well as the monolithic ones I’ve built in the past and has a good volume. It can also be taken down and transported to other areas. But the bricks are very brittle and next time I’d use better clay devoid of sand/silt, and use grog instead of temper made of plant fiber which burns out in firing. The mold works satisfactorily and I aim to make better quality bricks for use in furnaces and buildings in future.

Simplified blower and furnace experiments

Blower description

The purpose of this project was to test a simplified blower design connected to a furnace. I purposely did this to show that people in most natural environments should be able to replicate this design without difficulty. This blower differed from the previous one in several ways to simplify the construction method.

Firstly, the impellor was simply a stick as a rotor with a 40 cm wide rectangle of bark tied into a split in its end with a bark fibre cordage. A stone with a pit carved into it acted as a socket for the lower half of the rotor to spin in. If spun in the dirt the rotor can drill down and the position of the impellor can reach ground level causing the blades to bump into rocks and dirt. Later, I plastered the stone socket into the ground with mud to hold it securely in position (not shown in the video, just be aware of this solution if the socket shifts around too much).

Secondly, the housing for the blower was made in situ of ordinary mud (dirt and water on site). It was a bit more than 40 cm in internal diameter. The walls of the housing were solid mud and the roof was made of sticks covered with mud. An opening more than half the length of the impellor was left in the roof to remove the impellor for maintenance and to admit air into the blower during operation. In use, the portion of this opening near the front of the blower was covered with a tile. If left opened the blower still worked but covering it improved performance by preventing air escaping near the front. In places where water is not available, a housing shaped pit covered with sticks and dirt might work instead.

Finally, a simple length of cordage was used to drive the rotation of the impellor. This cord was placed in a notch carved into the top of the impellor rotor. The cord was wrapped around the rotor about 2.5 times. During operation the cords were pulled outwards causing the rotation. When fully unwound, the momentum of the impellor then wrapped the cord back around in the other direction. Then the cords were pulled outwards again causing the impellor to spin in the other direction. Note that this is a centrifugal blower with a symmetrical housing, therefore it doesn’t matter whether the fan spins one way or the other (clockwise or anti clockwise), the blower will always suck air in to its open top and force it into the furnace.

This design is easy to make and use. It can be made with minimal materials by unskilled people. The impellor design is simple yet effective. A stick, some bark and lashing of some sort should be available in most areas. The housing being made from mud, is easily sourced also. For the drive mechanism, I chose this method because the first blower I built had too many parts. There was a frame made of wood and vine to hold the rotor in place which kept causing issues with the rotor seizing or jumping out of the socket. Also, the bow that was used to drive the rotor added unnecessary complexity. In the new design, the simple cord in the notch of the rotor did away with the frame and the bow of the old design and the associated difficulties.

Furnace experiments

The blower was used to power a furnace attached to the front of it. Note that with minimal materials, the blower could simply force air into a hole in the base of the furnace and work satisfactorily. But I wanted to test a different configuration so I used clay grate from a previous kiln I made. Fuel in the form of wood and charcoal was used in this furnace by being placed on top of the grate instead of under it. During operation, the blower forced air up through the grate into the burning fuel bed increasing the rate of heat production relative to the use of natural draft (convection) alone.

I made 3 pots and fired them with charcoal. The first pot was painted with iron bacteria (iron oxide being the active ingredient). When fired, the oxide melted slightly showing minimal glazing. The clay became quite hard, possibly stoneware. The second pot was painted with wood ash and placed on a three sided clay plinth to hold the pot in the position of highest temperature in the fuel bed. The pot softened and sagged apart catastrophically. But the ash glaze gave a dark green smooth finish (difficult to see in the video). Finally, a pot was place upside down on the grate and a cylindrical brick made of iron bacteria, charcoal powder and wood ash was put on top of this. The brick melted over the pot, covering it in a viscous blob of slag rather than a thin glaze. On inspection, the slag had 1 mm sized spheres of metallic iron in it. Some of these were picked out and stored in a pot. The reason for these experiments like these to gain knowledge that might be of practical use in future projects that have not yet been determined.

Sandals

I made a pair of sandals from loya cane. Walking bare footed in the bush generally doesn’t cause problems for my feet. But when repetitively carrying loads of various materials the soles of the feet become cracked and split. So I made some basic footwear for the purpose of working on rough surfaces.

I cut some cane and measured out a length 6 times the length of the foot (about 1.5 m), folded it into loops and wove more cane between the loops to form the sole, adding new cane as needed. Next, I made bark fiber cordage and threaded it through the sandal to keep it on. The pair took about 1 hour to make (longer due to setting up the camera).

The sandals do protect from the ground, preventing the feet from cracking. I personally don’t like wearing footwear in the forest as bare feet give better grip, especially on inclines. But for heavy work or when my feet are injured I’ll wear these. These sandals are so quick to make that I’ve already got 2 pairs. The material used to make them (loya cane) is everywhere here but pretty much any rope like material will do. Bark fiber rope, grass, vine, flexible roots etc. will all make usable alternative materials.

Reusable Charcoal Mound

Charcoal is a valuable fuel that reaches a higher temperature than the very wood it’s made from. I’ve made some before, but with supplies running low due to furnace experiments, I decided to make another large batch of charcoal in a mound. I stacked the wood into a roughly conical shape (about 1 m wide and 75 cm high) and then built a thick wall of mud around the heap (this took 6 hours). Eight air entries were made in the base of the mound and one air exit hole was left at the top of the mound to allow the volatile components of the wood to escape while creating a natural draft to keep everything burning.

The mound was lit and the flame burned backwards down the heap in the opposite direction to the draft. This protects the coal made above the level of the fire from burning as carbon dioxide rushes past instead of oxygen, preventing combustion of charcoal. Each air entry was sealed only when fire became visible through them. This is an easy way to tell when to close them up, i.e. when the fire had burned down all of the wood in the heap. When the last air entry was closed, the air exit at the top of the mound was sealed, 5 hours after starting. The next day when cool, a large arched opening was made in the side of the mound to extract the charcoal. Despite a few unburnt brands the yield and quality was good filling almost 2 baskets.

To see if the kiln was reusable, I restacked it with timber cut from a fallen gum tree branch up the mountain. Due to the difficulty in reaching into the mound I stacked the wood in criss-crossed horizontal layers. The opening was sealed with mud and the mound lit as before. This time the mound burned quickly and I had to seal it early as the timber was burning at different rates, 3 hours after starting. Some large logs remained unburnt while charcoal that had already formed started to burn up being wasted as ash.

When I opened it the next day it had still produced an ok amount of charcoal but was disappointingly low compared to the first batch. This may partly be due to some of the wood being still green though it’s probably more likely to be due to how it was stacked. The lesson here is that when making charcoal the wood needs to be tightly stacked with few air spaces between. If not, the mound admits too much oxygen that quickly burns the timber.

Another thought I had was that wood may convert to charcoal better if laid vertically (or roughly so, like the cone in the first firing) so that the fire starts at the top of the wood and burns down. Stacking the wood in horizontal layers means that each layer has to set the one bellow alight leading to problems if the wood is green (use dry wood if stacking horizontally). By stacking wood vertically each piece is alight already and simply burns down towards the air entries. Stacking in this way also makes it easier to see fire in the air entries letting you know when to seal the mound.

For the reasons above I may make another charcoal kiln in future in the shape of a cylinder with air entries around the base and an open top. The kiln would be re-usable and easily stacked. A conical pile of wood would protrude above the walls of the kiln and be plastered in a temporary cover of mud. The kiln would be fired as with a normal mound and when finished the temporary cover of mud would be removed to extract the charcoal

Water Powered Hammer (Monjolo)

I built a water powered hammer called a “Monjolo”. I started by making a water spout from half a hollow log to direct water from the creek. This was set up in the creek and water flowed through it. The hammer was made from a fallen tree. I cut it to size by burning it at the points I wanted it cut (to save effort chopping). Next I carved a trough in one end to catch falling water. This was done first with a stone chisel that was then hafted to an L–shaped handle and used as an adze. This adze only took about an hour to make as I already had the chisel head and cordage made of bark fibre to bind it with.

To save further effort carving I used hot coals from the fire to char the wood in the trough. I put the coals in using “chopsticks” (unused arrow shafts) to transfer them from the pit. The coals were fanned or blown with a wooden blowpipe till the wood in the trough burned. Then the char was scraped out. The sides of the trough were sealed with clay to make sure the wooden sides did not burn away which would effectively decrease the volume of the trough. This was approximately 8 hours work over two days.

With the trough carved I made a hole in the middle of the log as a pivot point. Using the same char and scrape method I burnt a hole right through the log using hot coals and a blow pipe. Again clay was used to prevent wood burning where it was wanted. To burn through the approximately 25 cm diameter log it took about 4 hours and 30 minutes. Another hole was burnt in the end to fit the wooden hammer head and it took a similar amount of time.

A tripod lashed with loya cane was set up at the water spout. The axel of the hammer was tied to one leg, the hammer fitted onto the axel and the other end of the axel tied to another leg. The trough was positioned under the waterspout to collect water and the tripod adjusted so that the resting point of the hammer was horizontal (so water wouldn’t prematurely spill out of the trough).

The trough filled with water, outweighed the hammer head and tilted the hammer up into the air. The water then emptied out of the trough (now slanting downwards) and the hammer then slammed down onto an anvil stone returning to its original position. The cycle then repeated at the approximate rate of one strike every 10 seconds. The hammer crushes small soft types of stone like sandstone or ochre. I carved a bowl into the anvil stone so that it would collect the powder. I then crushed old pottery (useful as grog for new pots) and charcoal. Practically speaking, this hammer worked ok as a proof of concept but I might adjust it or make a new one with a larger trough and bigger hammer for heavy duty work.

This is the first machine I’ve built using primitive technology that produces work without human effort. Falling water replaces human calories to perform a repetitive task. A permanent set up usually has a shed protecting the hammer and materials from the weather while the trough end sits outside under the spout.  This type of hammer is used to pulverise grain into flour and I thought I might use one to mill dry cassava chips into flour when the garden matures. This device has also been used to crush clay for porcelain production. A stone head might make it useful as a stamp mill for crushing ores to powder. It might pulp fibres for paper even.

Termite clay kiln and pottery

I built this pottery kiln and some pottery from termite mound clay to test an alternative clay source to my usual one from the creek bank. I started by making a large grate from ordinary clay. It was just under 50 cm in diameter. Next, I took dry chunks of termite nest and put them into the pit in front of the tiled roof hut. The chunks were crushed and water was added to slake the clay. The clay was trodden on to mix it. Dead palm fronds were added to the clay to stop it from cracking as it dried and to add insulation to the kiln. The mixture was trodden on again and then taken from the pit. A trench was dug to form the firebox of the kiln and a wall of clay was made in the front of the trench. A hole was dug into the wall to allow air flow into the firebox.

The grate was placed on top of the firebox and the walls of the ware chamber were built around the grate. When the kiln walls were finished, grate bars made from termite clay were placed into the firebox. Grate bars are important for fireboxes as they lift the firewood off the ground allowing air to move up through the fuel bed for more efficient combustion. Burning wood as a heap on the ground allows cold air to flow up and over the coals, cooling the kiln and leaving the air unreacted with the fire wood. It still works but is much less efficient than using grate bars. The finished kiln was 50 cm tall (above grate height), 50 cm in diameter and with walls about 12.5 cm thick. The pit/firebox was about 25 cm deep and 25 cm wide with grate bars sitting half way between the ground and the circular kiln grate above.

Next, for the pottery clay, I selected a termite mound built on red clay soil. I took it to the kiln area and slaked it with water and mixed it in a small pit. I crushed up an old grate from a previous kiln and mixed it into the termite clay as grog. Grog prevents pottery from cracking as it dries and helps prevent breakage when firing. I then shaped the clay into a small urn. I also made some barrel roof tiles and a smaller pot from termite clay. I then stacked the kiln with the termite pottery (the urn, small pot and 5 barrel tiles) and some pottery made from normal clay (the housing for the forge blower and 2 barrel tiles).

To fire the pottery, I collected a large pile of dead wood and started a fire in the firebox. I heard some explosions in the kiln early on and knew something broke but continued anyway. To prevent explosions you should make sure all the pots are completely dry and slowly heat the kiln. Within an hour the kiln had heated up well and the pottery was glowing red hot. By the second hour the temperature went down illustrating an important point: if you over fill the firebox with wood the kiln will choke it and not burn efficiently. Realising this mistake I merely let the wood burn down a little so more air could get through. It’s important to watch the inside of the kiln and see how hot it’s glowing, try adding more or less wood and observe the effect on temperature. By 2 hours and 30 minutes the kiln was firing nicely again with all the pottery glowing low orange (about 845 c or 1550 f). I kept it at this low firing temperature for another 30 minutes. The whole firing process took about 3 hours from start to finish, a relatively short period of time for firing pottery.

When I took the pottery out, one tile had broken and the urn had spalled (a piece of the outer pot broke off) possibly due to still having moisture in it. The urn was still useable though and I use it to water the cassava patch. The forge blower was well fired and is now immune to water damage, no longer needing to be carefully protected from the rain. I put it in the barrel tile shed for storage. I put the broken tile and spalled piece from the urn in a special heap of broken pottery. When I make pottery in future I can crush up these broken pots and mix it into the new clay as grog to strengthen the new ceramic items. Finally, I stored the good tiles at the barrel tiled hut as replacements for broken tiles in that structure should there be any damage in future.

Termite clay is good material for making furnaces and an OK substitute for good pottery clay should it be difficult to find a better source. The termites have already processed the clay by the fact that their mouths are too small to include sticks and pebbles into their structures. As a result, the clay is very smooth and plastic. Too smooth for my liking, in fact, I’m used to working with coarser clay that has silt mixed into it naturally. I find that termite clay is either too runny when wet or cracks too easily when drier. It was difficult to form into complex shapes and it took me 2 attempts to make the urn. But for forming objects like tiles it’s OK, it can be pressed into shape and it will hold without difficulty. In future, I’d be likely to use termite clay for mass producing formed objects such as bricks, tiles, simple pots (formed over a mould) and possibly pipes, thereby conserving the dwindling clay supply from the creek bank which I’ll save for more intricate pottery. In summary, termite clay is able to be used to produce basic pottery if no other source can be found. If you have a termite nest you can make basic pottery from it.

Planting Cassava and Yams

In this video I build a garden to grow Cassava and yams, two staple food crops. Cassava is a shrub that develops large edible roots. Yams are a climbing vine that produce large, edible underground bulbs and smaller aerial bulbs on their vines.

I had 5 huts, but the wattle and daub hut (from the first video uploaded on this channel nearly 2 years ago) became dilapidated. I abandoned it in favour of the other huts I built and neglected the roof. This let water in destroying a wall. Also, the sweet potato patch behind it had a tree fall across it destroying the fence. So I demolished them both to make one large garden.

After removing the fence I set a fire under the fallen tree to burn it in half rather than spend the effort of cutting it with stone tools. After burning almost all the way through, it rained. So I came back later and cut through the rest of the log with stone tools. I eventually broke the tree in half. Using smaller logs as levers I moved the tree out of the garden clearing the space for the garden.

I then collected wood and built a simple fence that was woven loosely together with vine. The fence needs only to discourage large animals from entering to prevent them causing damage. Most times pigs and wallabies don’t know that food in growing in the garden and won’t try and enter if they see no reason to. Or at least that worked for the sweet potatoes so we’ll see if it works this time.

For the yam and cassava planting material I travelled far down stream to the site of my old stone hut that I built over 10 years ago. It had a corbelled dome roof that was damaged when a tree fell on it during a cyclone and it came down a few months later. The thick walls however have stayed standing for about a decade though.

Yams and cassava grew wild at this site which is one of the reasons I built the stone hut there. These plants are not native to Australia but grow wild here after having escaped from people’s gardens (similar to how wild pigs live here now after escaping from farms). The planting material for the yams are the bulbs that grow on the vines. The planting material for cassava are simply 25 cm long pieces of stem.

On returning to the garden, a scrub turkey was seen digging in the mounds. Protected by law, this bird has lost its fear of humans and in this case I’ve semi-domesticated it. Originally it was attracted to soil I dug up for the worms it exposed. I started leaving a pot out with small sweet potatoes in it for it to eat and now it investigates any pottery I leave for food. Now it my visits my projects and will only leave if bored or chased away. I suppose this is similar to how chickens were domesticated, in fact bush turkeys and chickens are related and will produce hybrid offspring.

Unfortunately, it has learned that the garden contains food. Originally, I was only going to plant yams but I saw the turkey digging them up and eating them. So, I planted cassava in the mounds so that the turkey would be discouraged by finding only wooden stems to peck at. I secretly planted the yams along the fence of the garden because the turkey only thinks the mounds contain yams. They can’t smell very well and only find food by sight and learned behaviour.

I planted the cassava in mounds 1 meter apart by pushing them flat into the soil. I planted the yams at intervals along the fence so they could use it as a trellis. 32 cassava stems and 12 yams were planted. Then a storm began and watered the garden. In less than a week the cassava had sprouted shoots and began to grow. The yams will take longer as I planted them deeper.

Cassava produces the most calories per time and space of any plant apart from sugar cane and sugar beet. But it requires much less fertiliser and effort. A hectare of cassava produces enough calories in 2 days to sustain a person for 1 year. It takes a year to come to harvest but will stay in the ground for a year without becoming woody. The tubers are high in starch and are what tapioca is made from.

This variety is called sweet cassava (actually not bitter cassava, it doesn’t taste sweet but starchy instead) and it needs to be boiled for 20 minutes to get rid of some cyanide it contains. The bitter variety contains such high levels that it kills if eaten raw and requires more extensive treatment to eat. There isn’t much nutrition in cassava other than the large amount calories it contains so other food would be required to provide protein and nutrients.

After I harvest the cassava I planted I’ll try fermenting it (which adds nutrition), drying it and pounding it into flour to make flat bread. Cassava flour has the same energy content as wheat flour, stores well and tastes somewhat similar. Or I could just cook it and eat it straight from the garden. I’ll use the yams like potatoes when they’re ready.

Bed Shed

I built a bed shed, a small shelter with a sleeping platform built into it. It’s quicker to build than a large hut but can be extended later on when materials and time become available. It’s not far from the dome shaped grass hut I built earlier.
The hut is 2 m long and 1 m wide. Four posts were hammered into the ground, two 1 m high posts (1.25 m long, 25 cm underground) on the low side and two 2m high posts (2.25m long, 25cm underground) on the high side. Onto this, a sloping rafters was lashed on with fish tail wait-a-while, a spiky palm with a vine like habit. To remove the needle like spikes from the plant, the leaves are pulled off so that the frond sheaths come with them. This made suitable lashings.
Battens were then tied to the rafters and bundles of long grass from the mountainside were collected. Using vine from the bush, the bundles were lashed to the battens starting at the low side and continuing to the top so that the grass would shed rain. Cross bars were lashed to the frame of the shed at each end to support the bed. These were at a height of 1m above the ground.
The bed frame itself was made from four poles (two 2m long and two 75 cm long) lashed together to form a rectangle 1.75m long and 75 cm wide (the ends of the two longer poles extending further to sit on the cross bars in the shed). Lawyer cane was then wrapped length ways over the frame to create horizontal threads. Then more lawyer cane was woven between these threads to form a sort of bed spring net. The bed frame was then put on the cross bars and tested to see if it could hold my weight. A mat I made from woven bark in a previous video was used for bedding and a bunch of grass for a pillow. In a rainstorm it was possible to make a fire in the space under the bed.
This structure is quick and easy to build. The bed is 1 m above the ground and provides plenty of area beneath to store fire wood and tools out of the rain as well as a place to sit and make things. The bed is comfortable and keeps the occupant off the ground away from ground dwelling creatures at night. The smoke coming up from the fire keeps mosquitoes away while providing heat and light reflected back from the roof. In fine weather the fire can be placed in front of the shed in the open while during rain the fire can be kept under the shelter to keep it dry. If room is needed to stand up the bed can be folded up against the roof and tied to it using cordage.
This shed is literally one half of the standard rectilinear hut I usually build (2m x2m floor plan, 2m tall ridge line and 1 m high side walls e.g. from wattle and daub hut and tiled hut videos) and was built to be upgradeable. Later, the other side of the roof could be added on and then walls of some kind built around the frame to form a full hut.

Fresh water Prawn Trap

I built a prawn trap from lawyer cane, sticks and vine. Then I caught some prawns and ate them.
Prawn (and fish) traps are simple traps designed to catch aquatic life due to their shape. It consists of a simple basket with a funnel shaped entrance. Prawns easily find their way into the trap as they are funneled in, but have difficulty finding the way out.
I wove the main body of the trap from lawyer cane then made the funnel from sticks with vines woven between them. The funnel was then inserted in the top of the basket and was complete.
I put the trap in the water under some tree roots without any bait. About 10 minutes later caught the first prawn which I stored in a pot of water. I caught another one and made a fire.
I humanely killed the prawns using the splitting method which destroys the central nervous system (boiling alive is more painful). Then I put them back in the pot with water. I collected some yams that I planted years ago from wild stock and put them in too.
I took 5 hot rocks from the fire and put them in the pot boiling the contents. The prawns turned red after cooking. They were peeled and eaten. The yams were also peeled and eaten.
This method of catching prawns is easy with the only skill needed being basketry. In practice, a long stretch of creek might have several traps collecting food each day without any effort on the part of the fisherman. Bait is not necessary to catch prawns as they will be naturally be drawn to the fish trap out of curiosity. But scraps from previous prawn may be used to bring in new ones (they are cannibalistic) or other fish like eels. The prawn trap is easy to build and can be reused many times.

Edit: This is a prawn and not a shrimp as I originally called it. Here’s the difference: https://museumvictoria.com.au/discoverycentre/infosheets/what-is-the-difference-between-prawns-and-shrimp/.