6 April – 12 April 2020

During the COVID 19 Shutdown period, we will be sharing a few fun “renewable” based activities that are fun for you and the family to try at home.

Enter our weekly competition, by sharing evidence of your constructed activity (relating to that specific weeks activity) on Facebook, and make sure you tag the Out the Green Box Facebook page in your post

Weekly activity competitions will run from Monday – Sunday. The winner for each weeks activity will be announced the following Monday, before the next activity is released.


You will need:

  • Mason Jars or large glass jars;
  • Black paint (OR dark paint)
  • Masking tape
  • Shiny reflective material (Tin Foil)
  • Oven bag or larger glass enclosure
  • Optional – Soda Can

Step 1: Basic Premise & Science

The basic idea is to use heat from sun light captured in a container containing food or water – to cook, to heat water, to pasteurize it, to make it safe to drink. When light hits any material many things can happen. It can be transmitted (passing through), reflected (bouncing off), refracted (change direction as I passes through media), absorbed (taken in), diffracted (bending and spreading), or scattered (reflected in all directions at random). We are hoping to use three of these to gain the heat we need to cook and pasteurize water: transmission, reflection, and absorption.

We want transmission of the light though a protective cover, reflection from the background material to concentrate more light on than would normally strike the target, and absorption of the light into the target. We want to minimize reflection off the target, and minimize absorption and reflection from the protective cover.

Dark flat colors on the target help to absorb as much light as possible. Clear protective cover protects the target from losing heat radiating from the target and cooling it. This heat loss can be increased when it is windy or if there are intermittent clouds or haze that limits the amount of light striking the target. The protective cover helps hold what heat gain you do get.

Step 2: Can Method

Find a Soda can that fits in the mouth of your jar. Clean it out and wash it well. Paint the outside black and let it dry.

The top may be painted, but you need to keep the paint out of the inside of the can. {While this is not an ideal situation as soda cans are lined with a thin layer of plastic, in an emergency the need for pasteurized water will be greater than the short term concerns of what may be given off from heating the plastic. Upgrade to a better option when you can}

Place a larger glass jar inverted over the can with both of them on the reflectors in the sun.

Step 3: The Mason Jar

>> The Jar:

Get a Mason jar that is free from any chips or cracks.

There are two basic types of mason jars; regular and wide mouth. The regular sized jars have an opening or mouth slightly smaller than the size of a pop can.

The wide mouth jars have an opening almost as large as the jar itself. Wide mouths are good for cooking or as protective covers for the soda can method. Weigh your options as to what you need and what you have. If you only have limited wide mouths save some for the soda can method.

Tip: If you only have regular sized jars, you might have to try and find a smaller soda can to fit inside the mouth, like an energy drink can or similar slim can styles.

>> The Lid:

Most canning lids come in two pieces the lid and the ring. A solid lid may come with it if you are using a commercial jar like pickles, mayo, and others.

The ring and lid are traditional canning lids consisting of a round flat metal lid, and a ring that has the threads that hold onto the jars threads and a lip that hold the lid to the jar. Ring and lids work by putting a lid on the jar and a ring over the lid and threading it down to hand tight. When canning the heating of the jar makes steam and air leave the jar, and the contents to expand. When taken from the heat a vacuum is made by the contents cooling and constricting thus pulling the lid down tight and sealing it.

>> Clean It:

Prepare your mason jar by making sure it and the lid is clean. Most importantly it needs to be clean of oils and residue that can make the paint not stick. You can also rough up the surface of the jar by using sandpaper. Put the masking tape on the jar before to remind yourself what not to rough up.

Make sure it is dry and place a strip of masking tape up the side of jar from bottom to top. The tape will mask off the area to be used as a viewing window to see how what inside the jar is doing while in the sun. You can see if the water is boiling or your food is cooking.

>> Paint It:

Put the lid on so you can paint the lid and jar at the same time. You also don’t want to have paint get on the threads of the jar. If you do it can stick the lid down tight when placed in sun and paint under the lid gets hot.

Once dry peel the tape off.

Step 4: The Reflectors

The reflective material can be anything you can find with a shiny surface. You can use tin foil shiny side facing the jar put on cardboard

{Before We Start Playing with the Reflectors}

**CAUTION – Whenever you use reflected or focused sun light you need to use caution and prevent damage to your eyes. Just the light from the sun can damage your eyes, but multiplying the sun light into one spot increases the danger to your eyes. 

>>Connecting the Reflectors

It is a good idea to work on putting them together inside or in the shade so you don’t blind yourself shining the sun in your eyes.

Once you have your reflectors we are going to make half a cube with them. A bottom and two adjoining sides

When connecting your reflectors just remember depending on how good your reflectors are at reflecting the sun light and how hot they get can limit the effectiveness of what you use to connect them. Tape and glue may get hot and fail, clamps can get hot to touch but will hold. Modeling clay may help hold things together to get your shape right but in the sun it can melt and things fall apart. Tape and glue seems to work just fine for foil and mylar reflectors

Place the reflectors in a sunny spot early in the day, on a stable area. You don’t want it being knocked over, or falling over in wind or your base collapsing. Using these little cooker methods take time to build up heat and cook so pick an area that gets sun from early morning until late in the day.

It will so need to be an area you can regularly turn the reflector setup to realign with the sun. Start with the open corner of the base reflector pointed toward the sun and the closed corner lined up with the sun and front corner. Since the sun is a very busy guy always on the move, you will need to come out and turn the entire setup every so often to keep it aligned with the direct sunlight.

If you align it just ahead of the sun’s position, you will still be catching enough sun to start heating, the sun will be moving into the best position for direct light, then start to move out of it. You can set it so you don’t have to move it as often. That is unless you want it to be constantly in the best position for maximum light collection and maximum heat, then you will have to move it constantly to keep up. If you turn it too far ahead you can lose too much of the reflected light from the angle of incidence of the incoming light bouncing out of the target area.

Put your water or food in the jar but don’t fill it much above where the jar narrows or about 1-1.5″ from the top. You don’t want what you are cooking to swell and block your pressure relief hole. Trust me it can be just as dangerous as tightening the lid on too tight.

Step 6: Place the Jar

Once the reflectors are set up place the jar on the reflectors near the rear corner. As the sun shines on the jar and reflectors it will begin to heat up. The dark color helps absorb more of the incoming light.

To increase the amount of heat retention in the jar and shield it from heat loss to wind put the jar inside an oven roasting bag before placing it on the reflectors. This helps shield the jar from wind and increases the ambient heat around the jar inside the bag, the clear bag allows the light to come through and be converted to absorbed energy. Any radiated heat coming from the jar is held inside the bag and near the jar to help increase the heat around and inside the jar.

Since the roasting bags are already made for food use and to be used in a hot environment they are perfect for the job.

Monitor the jar and reflector. Adjust the reflectors angle it to keep up with the changing sun position. Check the jar that is is heating properly, is venting so it won’t explode, and remove when the food is done.

Step 7: Mason Solar Cooking Jar

There you have it – You have a solar cooking jar! You don’t have to use your fuel storage while there is sun. You can use the sun as often as it is available.

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