Since I was in a hurry to get in time for the Slow Cooker Challange, this instructable is a bit of a mess. I was writing it in a parallel to proceeding with the project, so it contains some experimental and fail bits. If you want more compact "in a nut shell" version of all this - go to the shortened verison of the tutorial.
Some time ago, while researching on different solar powered... stuff, I've came across David Delaney's web page. In particular I want to mention the article on Roger Bernard's Solar Panel Cooker, since it was the thing that inspired this project.
Basically The Solar Panel Cooker's design involves a dark colored metal pot enclusured within bigger transparent glass pot with reflective panels enhancing the amount of sunlight provided to them. While inner pot absorbs heat and transfers it to food within and surrounding air, the outer one incapsuletes the air inside, thus creating greenhouse effect which allows to achieve reasonably great temperatures.
In general, I'm suggesting pretty much the same thing, but instead of inner pot there'll be magnetite rocks. Why? Because I'm very original.
Step 1: Questions
What is magnetite anyway?
Magnetite is a rock mineral and Iron oxide... kind of at a same time. Here's Wikipedia article on it. It was used as a black pigment for paints for centuries. Nowadays, due to it's high density, magnetite is widely used for producing so called heavy concrete and in particular weights for building machinery, like cranes and similar.
Why using magnetite for a solar cooker thing?
1. Because you dont have a small black pot;
2. Because it naturally black, so it absobs solar energy good. In fact you can use any black or dark colored stones, but, let's all agree, "Cooking With Any Dark Colored Stones" doesn't make for a cool title and sound rather like a really lame superpower.
3. Because magnetite cooller than other stones. And by "cooler" I mean: "It can get hotter". Magnetite has great heat capasity as well as high density, which means, it can accumulate a lot of heat. This way, stones can stay hot for some time after the Sun is no more, and you can prolong cooking process a bit by wrapping the pot in some warm blanket. So, use magnetite instead of random dark rocks if you can.
4. It's readily available.
Is it safe?
Yes, it safe for cooking with (as far as my research and underastanding of English language tells me). And, although I do not recommend consuming magnetite itself, it is widely used food aditive (coloring), registered in EU as E172 (which is also the first letter of the street and the number of a house I lived in previously).
Where to find it?
Read next step.
Step 2: More Questions
Where to get magnetite?
Although you can buy magnetite online, I see no fun in that. If you want to find your own magnetite - go to the nearest beach or river. Sometimes you can find it right in the sand. Othervice, look for patches of rubble washed out by water, or you can try your luck with pile of regular gravel.
I've gathered mine at local river (Dnipro is forth longest and third largest by area coverage river in Europe). I've spent more than hour magnet fishing fine magnetite gravel at one spot first, and then I discovered that I can just collect large chunks right from the beach sand at a bit different locatin. So I filled a bag like in 15 minutes... Do some exploration first.
How to identify magnetite?
It's pretty easy: it's black (may include differently colored strips of other minerals), glossy, reasonably heavy and it's one of few minerals reacting to magnet (if it's red/brown and reacts to magnet - it is hematyte; you can make red pigment out of it).
Large or small?
Preferably take reasonably large pieces of magnetite. Since finer gravel will fill more gaps in the pot it's less convenient than chunkier rocks. Here's my thinking behind it: even if you're going to wrap your food products in aluminium foil and protecting them from direct contact with rocks, some juices may still sip through. It is much easier to wash individual pieces of magnetite (since you're going to reuse them) if neaded, rather than dealing with tiny rocks that can clump together with greace and stuff... Also with bigger rocks it's much less likely to get one into the food and then break your teeth.
Step 3: Test Run Stage One
When you have your magnetite and suitable glass pot in your command, one last thing you have to make is a reflecting panel/s. With all that gathered together you'll be able to make a test run of your cooker.
Since I already have this parabolic dish all covered in aluminium tape and shiny and pretty, I decided to use it for a test run before commiting to making new reflective panels. For you, I'll cover them in further steps.
So, the main idea here is to get the temperature inside the pot consistantly higher than 60*C. At this point all the bacteria start to die and food start to cook. This whole thing is basically a solar slow cooker and all the cooking routine is identical to it.
How do you know if it gets hot enough?
This is why you need a test run. Conveniently, I got this handy thermometer with remote sensor with two dials, which takes measurement up to 70*C. If you don't have a thermoter with high enough temperature gauge, use this trick...
And here's where I goofed out a little bit. The initial idea was to place a piece of parafin wax inside the pot (what you can see on the photos). I believed that its melting temperature is above 60*C. This way it could be used as a indicator. But when wrighting this instructable I checked the data and it turned out to be 45-65*C, so it can't give sure indication. Buy a propper thermoter.
Anyway, what I did first Is tested the pot without any reflectors. I was checking the data from thermometer every half an hour (the thermometer was placed in a shadow ("IN" dile)). The results you can see on the photos. At first the temperature inside the po ("OUT" dile) was rising for 8*C for every thirty minutes. But at some point the rate started to slow down.
Step 4: Test Run Stage Two
At this point I've added a dish (antenna). Although it was a parabolic dish I didn't mean to place the pot at exact focal point. All I wanted is to collect maximum amount of reflected light. It is not essential to use a parabolic dish, so don't copy me here mindlessly.
The temperature within a pot started to rise and soon enough it exceeded 70*C, which is more than perfect. Interestingly, the dish I'm using is pretty small and at previous attempts to use it I wasn't even able to boil a cup of water.
At this point I still had enough of a light day left to try and bake some potatoes...
Step 5: Test Run Stage Three
...So I baked some potatoes. When I had no Sun anymore I decided to test one more thing, so I wrapped the pot in few blankets and let it to cool down gradually.
For one hour the temperature stayed higher than 60*C, which means it still was cooking. And for two more hours it was kept warm enough to be eaten.
Step 6: Real Deal & Washing Rocks
It's a next day, and now, when I know that the whole thing works I'm ready to cook some stuff in a propper way.
So, one thing you want to do before solar cooking with magnetite is to clean your rocks. Firstly, I rinced them with fresh water, and then I brushed each one of them with a brush with addition of some dish cleaner. Then I rinced them once again and left to dry.
Since I decided to experiment with fine magnetite gravel as well I washed it too. I just poured dish cleaner liquid ontop of it and rinced it good.
Step 7: Chicken
I decided to cook two things in a bit different way. First one was a pair of chicken legs. I placed them into a glass pot and filled the rest of the pot wit magnetite rocks (a layer of rocks on the bottom was layed first). The clear glass plate went ontop.
Step 8: Potato
The second thing was a potato. For this porpouse I used 3L glass jar. And this is the main advantage of using rocks: you can cook in any transparent glass vessel without it being specially designed pot.
I filled the jar with magnetite gravel and potatoes making sure the last ones are surrounded with rocks all the way around. I also decided to try one more thing here and poured a glass of water into jar as well. I planned to achieve a steam cooking effect in a way, but now, when I know the results, I'm not so sure abot that though.
Step 9: Making Stand
I also decided to keep using my pot along with my parabolic dish antenna, so I've made a simple stand for it. All it takes is three metal pipes (aluminium in my case) and a metal ring (it doesn't have to be a tie ring - just a metal ring will do). It's ajustable and foldable as well as primitivelly simple to make.
Step 10: Making Reflector
On the other hand for the patato jar I've made this simple reflector. It's just a piece of corrugated cardboard with aluminium foil glued to it (I used latex based glue (seems like it workes)). Some reflective tape was used to reinforce the structure.
Step 11: Water Heating Experiment
One particular problem with this method of cooking, as you, probably, have already figured out, is unability to cook liquid stuff. Concidering this issue I've commited another experiment. I was wondering if you can heat water in enclosed glass container with some magnetite rocks added. The idea here is the same as with greenhose effect double pot, but insted of trapped air within you have water.
I filled one jar with magnetite rocks. Then I've added water. Then I poured all the water from first jar into another, and then I filled the first jar again. This way I was sure that each jar contains equal ammount of water. I've left them then on the direct sun standing on a wood board to insulate them from colder groung.
I concidere this experiment a fail. I've measured the temperature in both jars after few hours of exposure to the Sun... and the difference seemed to be not that great. At this point my thermometer went amok and I can't certify it's readings for all 100%, but to the touch the water in a jar with magnetite seamed to be not much hotter that that one with only a water in it. So... I guess you can't make soups with magnetite.
Step 12: Cooking
I've started cooking the chicken legs and the potato at roughly half past twelve, and finnished at 5 P.M.
Step 13: Post Cooking
At this point had no sufficient Sunlight to continue cooking anymore (shadows from the house and the trees), I've wrapped both the pot and and the jar in few blankets and left it for about half an hour to continue cooking.
Step 14: Results
And finally st the evening I had a nice supper. Chicken leggs cooked completelly, but I had to put potatoes into a microwawe for 20 minutes since they weren't cooked enough. I'm not quiet sure if it was because of me adding water or not sufficient enough work of reflector. I'm going to work on this issue though and I'll let you know If I'll get some progress.
Step 15: Washing Stones
While cooking (chicken legs) there was some gravy gathered on the bottom of the pot that you can see on previous photos. I used collected it later and used as a seasoning for potatoes. But since you want to reuse your stones in future, you want to clean them after cooking. Here I just "sprinkled" some dish washing liquid ontop of rocks and rinced them thoroughly with water. You also want to dry them afterwards.
Step 16: Conclusion
So, cooking with magnetite... what it's all about? For me it was an experiment that have proved that my idea works. In general though, it's an alternative way of cooking that has its con's and pro's. You have to deal with all those stones, on one hand: gathering them and keeping them clean after each cooking; but you also can use any glass jar to cook food with nothing more than solar energy, on the other. I have an idea for a survival-ish variation on solar-magnetite cooker. I'm going to work on that, and I feel like I'll be able to came up with some other ideas as well. I'll keep exploring magnetite as a material.
In any case thst's it for now. Thanks for your attention and if you want to vote for this project in a Slow Cooker Challenge, please, vote for the "in a nutshell" instructable. I decidet to make two separate tutorials on this project to make it more convenient for you, but I'll have a little chances to win anything if votes will be spread between two articles.
I have a Patreon page, in case you want to support my efforts, but also I have a Facebook page where I share some inspiring stuff as well as additional glimpses on my projects in process of working on them, so don't be shy to follow me.
Runner Up in the
Slow Cooker Challenge