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Franciscomv 09-13-2010 05:03 AM

Two ways to use the Sun (FTF Bushcraft School Lesson 3)
Hey, guys. Here's this week lesson. It's time to mix some modern science with a bit of primitive knowledge. I thought you would appreciate something a bit easier than last week's bow drill, so instead of adding three more friction fire methods to that lesson I decided to go with something different. Give this a try, you might like it.

Free effortless heat: using the Sun.

If you gave friction fire a try, I’m sure you’ll agree that getting a flame going can be quite challenging even in ideal conditions. In some circumstances, even when equipped with such technological marvels as butane lighters, it could be almost impossible.

We’ll look at two simple ways to use the sun for important tasks: making water safe to drink and cooking food. While these methods have their limitations, they take very little work and are suitable for use in extended emergencies. I thought it would be nice to balance the hard assignment from last week with something fun and quick to make.

Solar Oven

I started looking into different alternatives at my family’s mountain home. During the summer I didn’t want to fire up the wood burning stove because it would make the house really hot, and during the winter I wanted to save fuel. The sun was the obvious choice for a heat source that demanded no fuel. I remembered having lamb stew in Morocco that was cooked by just leaving the clay pot out in the sun, but the temperature around here never gets that high (thank God!).

The solution came from a crazy hippy neighbour who does sustainable farming while he waits for some super advanced aliens to come pick him up (he really is waiting for them). His kids would routinely come over to my house, usually drawn by the smell of freshly hunted boar roasting for dinner. One afternoon I proudly served a rather cold and half cooked dessert that had been cooked in my new invention, a piece of technology so ground breaking that it would save people millions of dollars in fuel and make me a rich man. The kids were quick to remark that my new invention looked like a poorly made solar oven, an appliance that apparently has been around forever and which their father used quite often.

So off I went to talk to him about solar ovens, I’ve got to say I was quite impressed with what he showed me. Bear in mind that I had no internet access at the time, so I couldn’t just Google the term and find all the available options in five minutes. I made a few different ones and slowly learnt to use them properly. They won’t replace my regular stove but they are a good option that is worth being familiar with. If anything, they can be a fun DYI project (I know you can just buy one, but that’s not the self reliant caveman way!).

For those of you wondering what a “solar oven” is, it’s a cooker that works by focusing the sun’s heat onto a pot. Reflective material is used to reflect the sun’s energy onto the pot and heat up its contents. Although several aboriginal peoples used (and still do) the sun to cook, the first true solar ovens began to be developed in Europe during the 17th century. E. W. Von Txchirnhausen, in Dresden (Germany) used a glass lens 1,6m in diameter to focus the sun’s heat for use in pottery. In 1774 Joseph Priestly (the bloke who discovered oxygen) managed to reach the staggering temperature of 1700 C (his solar powered contraption actually melted platinum!).

However, the true father of the solar oven pretty much as we know it today is the Swiss naturalist Horace de Saussure. In 1767 he built a “hot box”, an insulated box with a clear lid through which the sun’s rays could pass to heat the interior. He run a series of tests at different altitudes (to see how the change in atmosphere composition affected his box) and different outside temperatures. He managed to get excellent results even in cold weather, while the outside temperature was 1C his box managed to produce 85C. These were just simple wooden boxes, painted black and using wool as insulation.
Saussure's first "hot box" design.

In the 19th century several scientists played around with this concept. In 1830 the English astronomer John F. Herschel made hard boiled eggs with his solar cooker on a trip to South Africa. French scientist Auguste Mouchot came up with a solar heater that could be used to run a steam engine (it was used to power the presses of a magazine called “Le Journal du Soleil”). He also designed a portable cooker for the French army. William Adams experimented with solar cooking applied to large quantities of food in India.
Finally, American astrophysicist Samuel Pierpont Langley conducted a thorough investigation on trapping solar radiation (for use in greenhouses and such), and during an expedition to Mount Whitney (California) routinely managed to boil water even though he was surrounded by snow.
Auguste Mouchot's design for the French army (1857).

There are three basic types of solar cookers: box cookers, parabolic cookers and screen cookers. I’ve had the most success with box cookers, they are easy to insulate and more panels can be added around the top to focus more heat into the food. They are easy to make with a minimum of fuzz, and can handle large quantities of food.

Screen or panel cookers are really simple. They consist of flat screens that reflect the sun’s rays onto a container inside a glass bowl or plastic bag. They can be made with cheap, easy to find materials.

Parabolic cookers use a concave reflective disc to focus the sun’s rays onto the bottom of a pot. They aren’t very easy to improvise and I’ve only had good results using them for kettles and stuff like that. In fact, I made one that keeps my “mate” (a local infusion) water at a perfect 82C temperature. Mind you, I’ve only tried homemade ones.

Cooking times and water quantities need to be adjusted for solar ovens. They are slower than a regular gas stove, and tend to use a lot less water. They work pretty much like a crock-pot or slow cooker. In an extended emergency situation (like the aftermath of a flood or hurricane) where clean water might be hard to come by, this cooking method is great because it will help you save some precious drinking water. Everything takes about twice as long as it would in a conventional oven. Some recipes work better than others.

They are also a good way to make water safe to drink. Even solar cookers that can’t make water boil will reach a high enough temperature to pasteurize it and kill all the nasty pathogens in it. Since they don’t consume any fuel and are easy to make, they can be used to treat large quantities of water when fuel is scarce. They are also very safe, there’s no fire to worry about.

Try out a few different pots. My beaten up and blackened camping pots work great, since they don’t reflect any of the sun’s rays back as a shiny one would. Depending on where you’re located you’ll be able to get away with any sort of pot (for instance, during the hot summer months I bake potatoes by just covering them in tin foil and putting them in the oven).

I've also used solar ovens to heat rocks than I then used to warm up my shelter.


The second use of the sun’s energy we’ll discuss in this lesson is the SODIS water disinfection method. I’m sure at least a few of you have heard about it, but it’s worth sharing for those who haven’t.

Starting in 1991 the Department of Water and Sanitation (SANDEC) at the Swiss Federal Institute for Aquatic Science and Research (EAWAG) began conducting a series of tests to develop a field worthy method of disinfecting water through solar ultraviolet radiation. These experiments were inspired by anecdotal evidence of the sun’s ability to render water safe to drink that date as far back as 2000BC. The results showed that ultraviolet radiation from intense sunlight that shines through a nonopaque container filled with water will radiate and kill nasty bugs.

And so the Solar Water Disinfection method (SODIS) was born. In the past decade a lot of effort was put into making this method available to the countless people who don’t have access to safe drinking water and over 2,000,000 people use it to treat their water every day. Surveys have shown an enormous decrease in illnesses that are tied to unsafe water (like dysentery, which went down as much as 50% in some South African townships).

SODIS does not make water absolutely sterile, which is just fine since humans can actually handle quite a few critters. It has been shown through years of research that it will get rid of some big meanies like Escherichia coli, Salmonella typhii, rotavirus, Giardia spp, etc.

Choosing the right type of container is crucial for this method to work. Some research has shown that plastic freezer bags are the best, since they expose the most surface area to the sun. They lay flat when compared to a bottle of equal capacity. And, lucky for us, are very easy to pack in a survival kit (thus making SODIS a very viable outdoors emergency tool). Plastic bottles are the most common container used, pick clear ones with no more than a quart or two capacity (too much “depth” of water can keep UV radiation from achieving the best results). If at all possible, choose PET over PVC. PET is better at transmitting UV radiation.

Although it’s not necessary, I like to lay my water containers on either tin foil or a mylar blanket. That way more UV radiation is reflected onto the water and the heat also increases. Remember to place the bottles laying down and in direct sun light. It takes around 6 hours for SODIS to work.
If the water you’re trying to disinfect has stuff floating around, filter it first (use a bandana, a sock, loo paper, whatever). These particles will otherwise get in the way of the UV radiation.

SODIS will not get rid of chemical pollutants in the water. The only way to deal with those is distilling, which can be done with your solar oven. It also has some limitations, treatment time has to be increased of the sky is too cloudy (the standard six hours work for up to 50% cloud coverage). Complete cloud coverage can reduce UV radiation to as little as 30%, so the bottles need to be exposed for two consecutive days. The Swiss, being Swiss, felt it was important to point out that SODIS isn’t reliable during days of constant rain... on those days just gather the rain water!

For more information on SODIS you can go to SODIS: Willkommen bei SODIS

Field tested

I’ve had the chance to put both solar ovens and SODIS to the test during a real emergency. In April 2003 the Santa Fe province (Argentina) suffered some of the worst floods in my country’s history. The combination of torrential rain fall, poor Government planning and careless digging of crop watering canals that messed with the natural drainage of the land, caused a huge disaster that was mostly man-made.

My mother’s side of the family comes from a rural town in Northern Santa Fe, so when the country wide calls for assistance started I signed up to help. I knew my SAR expertise would probably be worthless, since I’m a mountain tracker. The area where my team works is dry, fire is often the problem not water! But I wanted to lend a hand and was told that anybody with some degree of emergency training was welcome, they were thrilled to know I could bring my own horses.

A lot of people from all over Argentina came to help. We each tried to bring as much of our own stuff as possible to avoid straining what few resources were available to help the victims. However, water quickly became an issue. Everybody, emergency workers and flood victims alike needed clean water. The army brought a couple of mobile units that purified water and sealed it in plastic bags, but they just couldn’t work fast enough. Boiling was complicated, because of the lack of containers and fuel rationing. It also required people to watch the fires.

One of the army doctors asked if anybody had heard about SODIS, those of us who knew how the method worked were asked to explain it to as many people as possible. The army had an almost endless supply of clear plastic bags from their mobile water purification plant and they worked like a charm. Zinc roofing had been donated to build shelters, and it proved to be a great reflecting surface for UV rays. For a week, most of the water my team drank was disinfected using SODIS. This allowed to free up more bottled water and water purified by the army for the flood victims.

Cooking was also a problem, it required fuel and water. Working with a group of local agricultural engineers we made a couple of big box solar ovens that helped save fuel, water and provide emergency workers with warm food. Since there was no fire, nobody needed to stay and watch over the ovens, freeing up more hands to do important work. A lot of the food we had were legumes and stuff like that, the slow cooking of a solar stove works great for them.

This week’s assignment

We’re going to build a solar box oven! We’ll keep it simple, but it can be improved if you feel like it.


• Mylar space blanket or heavy duty aluminium foil.

• Glue or tape (or both).

• Clear plastic oven bag.

• Piece of wire.
A box!


• Insulation material (wool, crumpled newspaper, old pillow, etc.)

• A box that’s big enough to fit the first box surrounded with insulation material.

• Additional cardboard and reflective material to make more panels.
The process is quite simple and straight forward. Start by lining the inside of your box with the space blanket (the cheap thin ones work well). Glue it to the sides and bottom, try to keep it as flat as possible (with no “wrinkles”) for maximum reflection.

Take the lid and cut a big window in it. Save the piece of cardboard you’re cutting off, it will be your main reflective panel. Cut the oven bag open (so that only one layer covers the window) and use it to cover the window, glue it to the edges of the lid. Glass can also be used, but oven bags are lighter and they’ll easily withstand high temperatures (plus they won’t shatter).
Cover your reflective panel (the piece of cardboard you cut off from the lid) with aluminium foil or a space blanket. Use the wire to prop it up in a position that will direct sun light into the oven’s window.

That’s it, you’re done.

If you want to improve your cooker’s performance, just place it inside another box with a layer of insulation between them. Wool, Styrofoam, even paper work. You can also make more reflective panels to focus more sun light onto the window.

Anybody who actually cooks something in their oven gets bonus points.

"Someday some usefulness might be drawn from this device . . . [for it] is actually quite small, inexpensive, [and] easy to make."
Horace de Saussure

Angry_bald_guy 09-13-2010 05:16 AM

This one I should be able to do. By the way, you left a box off the list of required materials.

c3shooter 09-13-2010 10:45 AM

These can be a lot of fun. Over the years, have built a traditional solar oven that worked well enough to make a batch of beef stew, a LARGE (5 ft) round parabolic from carboard and foil- that would boil water or fry meat on a bright day- and did a small parabolic that the kids loved- it was a hot dog roaster.

Franciscomv 09-13-2010 11:23 AM


Originally Posted by Angry_bald_guy (Post 349758)
This one I should be able to do. By the way, you left a box off the list of required materials.

Cheers for the heads up. I posted around 2am, my brain goes to sleep around 12. :)

Shihan 09-14-2010 09:32 PM

Just moving it back to the top. We will be working on this this weekend. Should be fun.

Dillinger 09-14-2010 11:34 PM

Hey Man..... I live in the PNW. Our sun left about two weeks ago. I don't think I will be able to cook a piece of PH litmus paper to change color. :o

But, I'll give it a try anyways. :p


Franciscomv 09-15-2010 09:28 PM

Look at the bright side, mate. You've got loads of clean water literally raining down on you quite often. And plenty of firewood.

Franciscomv 09-15-2010 09:44 PM

Oh, and just to make it more interesting everybody who cooks something or makes an interesting solar oven (beyond the basic one I described) will take part in a drawing for a little prize.

So get to work! :)

Dillinger 09-19-2010 08:25 PM

3 Attachment(s)
So, we have very few competitors for the Bushcraft Title. Guess I am going to be a front runner and get instantly promoted to Master Chief's Right Hand Man. :D

So, I started this challenge looking to add a little flair and make use of a cool stryofoam cooler that had showed up with a shipment from last week.

Now, the melting point of stryofoam is 167 or so degrees F, so while this is not the best choice, with the added cardboard, the aluminum foil and the cool seal, I decided to give it a try and wait for some sun to see what I can do with it.

Here is what I started with: A cardboard box and a stryofoam cooler.

Dillinger 09-19-2010 08:29 PM

3 Attachment(s)
I cut the box down to panels that would slide in the stryofoam cooler and lined them with aluminum foil. (Note to anyone else doing this: make sure there are no plans for aluminum foil involving dinner or else you too will be going to the store to secure more foil before finishing your project. :rolleyes:)

I lined the bottom and 3 sides with alumimum, slid them in place and had a nice little aluminum foil oven going. Slapped the lid on and it was self contained. Cool.

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