Heat retention cooking has been around for a while, but is enjoying greater attention as people seek to be more economical and “green.” The concept involves bringing food to a boil for a specific amount of time and then moving the pot directly into a container filled with heat-retaining material, (like sawdust, hay, wool, or polystyrene), covering it, and letting the food sit for several hours. There are probably numerous ways to achieve this. Two that kept coming up in my Internet searches were a box design called the “wonder box,” and a (somewhat pricey) drawstring bag design called the “Wonderbag” that is being sold.

Both designs used cotton fabric and polystyrene balls, like those used in bean bag chairs. However, since I had read that shredded polystyrene packing peanuts could be used for the filler and I had an old partly cotton sheet on hand, I thought I would try to make one on my own. The instructions on how to make a wonder box were already on the Internet (http://www.iwillprepare.com/cooking_files/Wonder_Box.htm ), but I was intrigued by the Wonderbag design (it looked a little simpler!) and decided to try to make one of those.

The first step was to obtain and shred the packing peanuts. I asked family members to save them for me, and I soon had enough—about five plastic shopping bags filled and tied off. I took the precaution of checking to be sure that they really were made of polystyrene and were not the new biodegradable ones that dissolve in water! At first, I was unsure of how to go about shredding them without making a tremendous mess, but after trying a couple options, I discovered that I could hold several peanuts between my two palms, insert both hands into a pillow case, and roll the peanuts between my hands until they broke apart. Any pieces that were still too large were gathered up and rolled again until they were small enough to resemble the balls that go into bean bag chairs. This kept most of the mess contained inside the pillow case.

Now that I had my filler done, I needed to make the bag to put it in. This meant designing my own Wonderbag-style pattern. I started by measuring the pot that I would generally use: a ten-inch-diameter pot about five inches deep. You can use whatever sized pot you like, but it must have a lid and not have a long handle. I placed the end of the measuring tape at the bottom edge on one side of the pot and went up the side and over the lid until I reached the center of the handle on the top. This gave me a measurement of eleven inches. I doubled this number and went to make a paper pattern of a large semi-circle with a radius of twenty-two inches. I used paper from an end roll of newsprint which I had obtained from the local newspaper, but several sheets of newspaper ads can be taped together to form a large enough sheet of paper to work with if other options are not available. The simplest way to get an even semi-circle is to anchor a pre-measured piece of string at the center point along the edge of the paper and attach a pencil to the other end. Keeping the string taut, trace an arc from one edge of the paper to the other.

As far as I can tell, the Wonderbag is formed of a center circle with separate sections radiating from it, so I began by marking a smaller semi-circle from the same center point as the larger one. I determined the size of this semi-circle by measuring the radius of the bottom of my pot and adding an additional inch. After I had drawn what would become the center circle, I got a protractor to help me in dividing the outside edge into five equal sections. Placing the protractor at the center point on the pattern, I made marks on the pattern at 36, 72, 108, and 144 degrees (I began on the right side in my photo!). Then I used a yardstick to draw lines on my pattern from the edge of the center circle to the outside edge of the paper. Now that the pattern was complete, I turned my attention to the fabric.

I needed to have a large enough piece of fabric to make two circles that were each forty-four inches wide. I knew the fabric needed to be fairly thin, and since synthetic fibers melt in high heat, I also knew that the fabric needed to be made of a natural fiber. I had a sheet that had been given to me that was probably large enough to have cut both circles from, but the fiber content was only half cotton and the rest was synthetic so I determined to use it for the outside of the bag only. If I had had access to forty-five-inch 100% cotton fabric, I would have purchased that, but the cheapest expedient to hand turned out to be another sheet that, while not 100% cotton, was a 60/40 blend. I decided to work with that. I folded both of the sheets in half and placing the pattern with the straight edge on the folds, cut through all the layers. This gave me the two circles that I needed. Using a pencil, I began transferring the lines on my pattern to the lighter of the two pieces of fabric. By folding the pattern along the lines I had marked on it, I was able to place the pattern on top of my fabric and trace a dashed line along its edge onto the fabric.

After I had transferred all the lines, I began sewing the two circles together. I stacked the two circles of fabric wrong sides together and stitched the smaller center circle, remembering to leave an opening for filling it. After I had added the filler to that section, I stitched the remainder of the seam closed and began stitching along the lines that radiated out from it. Being sure to backstitch the beginning and ending of each seam to lock the stitching in place, I began each seam at the center circle and stitched straight out to within two inches of the outside edge. Here I turned and began sewing parallel to the outside edge until I was within an inch and a half of where the next seam would go. This formed the outside sections and left an opening in each one for them to be filled.

Now that the sections were all sewn in, I could begin the process of filling them. I used a funnel (cut from the top of a two-liter soda bottle), and a plastic container to carefully add the filler to each section. I did not pack each section so full as to be tightly stuffed, but I did fill each one moderately full and pinned the openings closed. Though the lighting in this photo is a little odd, I chose it because it does show what the sections should look like. When I was happy with the level of the stuffing, I finished sewing up all the seams.

The next step was the drawstring. I simply made a long, thin tube from some fabric cut from my sheet and turned it so that the raw edge was on the inside. If you would like to skip this step on your own bag, simply use the cording of your choice. To make the casing for the drawstring, I first had to make an opening for the string to pass through. Working with just the outside layer of my bag, I made a cut from the outside edge straight down toward the existing seam. I stopped just short of the seam and then, turning the raw edges under, I stitched them in place so that they would not unravel. This made a V-shaped opening in the outside layer of fabric. To complete the casing, I folded the edge of the inside fabric down over the outside fabric and stitched it in place. Then I threaded the drawstring through.

Now that the basic bag was completed, the last thing to do was to make a “lid” for it. This was very simple. I just had to cut out, sew together, and stuff two more circles that were a little larger than the circle in the bottom of my bag. Using the measurement for the bottom circle, I added an additional inch and a half to all sides and cut it out. Putting right sides together, I stitched them together, leaving an opening so that I could turn it right-side-out. After it was filled, I sewed the final opening together and my bag was complete. Now all I had to do was to test it.

I had a package of fifteen bean soup on the shelf, so it became my test project. It called forletting the beans simmer for two and a half to three hours, so it was an excellent one to test! The standard “recipe” for things with a long cooking time seems to be to bring the food to a boil and let it boil for twenty minutes. Then it is transferred into the bag where it stays covered for twice the amount of simmer time normally called for on the package. I let the beans soak overnight, and after pouring off that water, I added my ham, the ham stock, and all the other ingredients I wanted it to have, even the ones that the package recommended putting in toward the end of the cooking time. I made sure there was not a lot of air space left at the top of my pot, since this reduces the effectiveness of the bag. I brought the soup to a boil, and leaving the lid on, I let it boil at a good clip for twenty minutes. Then I immediately transferred it into my bag, put the bag “lid” on, and cinched the bag closed. I felt around for hot spots and adjusted the stuffing to fill in anywhere I could feel heat escaping. Then I left it on the counter (never leave it on a surface that is a good conductor of heat) and waited for five hours.

Finally, the five hours were up! With anxious fingers, I loosened the drawstring, and removed the cloth lid. I lifted the silver lid of the pot and the steamy aroma of the beans rose to meet me. I fished out some of the beans with a spoon and tasted them. They were wonderful! They were completely done and tasted great! Now I only had one problem. I had put the beans into the pot at about nine o’clock in the morning thinking that I wanted to have extra time to let them finish if need be, since the cooking times for the bag vary somewhat depending on what you want to cook. Now I had another three and a half hours until dinner time, and the beans were ready to go. I decided to close the bag back up and see what would happen.

At dinner time, I opened the bag up and without reheating it, I served the soup—over eight hours after the last heat had been applied to them. They were excellent! This was fix-it-and-forget-it at its absolute best. The bag had allowed me to put twenty minutes of electricity into something that normally took two to two and a half hours and kept it hot without scorching or burning for over eight hours! It was amazing!

This project really highlighted the issue of faith for me. I had invested hours of my time into gathering materials, designing a pattern, and putting the whole thing together—all because some individuals on the Internet said that I could save energy and money using a heat retention oven. Even after I had made the bag I had to continue trusting their witness and could not even peek to see how the food was getting along, or I would have allowed the heat to escape and the experiment would have failed! To be so close and yet still not know if the project was a success or not was hard! How wonderful to know that my faith in Christ is not like an Internet project, subject to human error and faulty equipment! It is a sure expectation that the God who promised me an inheritance in His family can and will one day fulfill His promise. I need only take Him at His word. I was reminded of the verses in Hebrews where Christians are encouraged to be “…followers of them who through faith and patience inherit the promises…” for “…without faith it is impossible to please him [God]: for he that cometh to God must believe that he is, and that he is a rewarder of them that diligently seek him” (Hebrews 6:12 and 11:6). I had gone through all the work and effort to make this oven based solely on the testimony of others, even though it may all have been a hoax, or their instructions may have been faulty. How much stronger then, should my faith be in the sure promises of a perfect God Who cannot lie! Have faith, fellow believers, for through faith and patience we shall inherit the promises!


I had been using this bag for several months to make sure that it worked well before I posted it to my blog site. However, I just discovered another advantage to the bag-style over the box-style heat retention oven. I used my bag the other day with soup that I was taking to a function at another location. My husband drove and I held the bag with it’s enclosed pot on my lap. I spent my time trying to compensate for the hills, curves, and stops to keep the liquid in the pot, which I could not see, level. I was pretty sure there was no way I would be able to arrive at my destination without having a fairly good-sized mess inside the bag, and thought that I would soon be testing to see how well it would clean up. (I was going to try hand washing it and let it air-dry.) Upon arrival however, I was surprised to see that there was no mess in the bag at all! Nothing– not even a line to show where the rim of the pot had been! The only explanation that I could come up with is that the drawstring design of the bag puts pressure on the lid and helps it stay sealed; a feature which the box-style oven does not have.


Probably the biggest detraction for heat retention ovens is the fact that they work best when used with a stainless steel stock pot that has a snug fitting lid and small handles on each side. I have tried using an enamel-coated stock pot I have that is a little bigger than the stainless steel one I originally made my thermal cooker for. Though it fits in the bag OK, it never performs like the stainless steel one does and I take a chance that the food will not be finished. Because a stainless steel stock pot is required to make the bag work well, it really is not as much use to households that have only one or two people. They simply would not need to make that much food at one time unless they had the space and resources to portion out the leftovers and freeze them. I thought it a pity that those who fit within this category of society should not be able to enjoy the benefits of heat retention cooking. So, I set out to find a solution to this problem.

Finding stainless steel pots with a snug fitting lid is not a problem. Stainless steel sauce pans of various sizes are not that expensive and can be found at most department stores. The only reason that they will not work in the thermal cooking bag is the long handle on the side. I suppose removing the handle is possible, but it would make moving the pot in and out of the bag more difficult and limit the use of the pot. But, if the bag could be constructed to allow for the handle, then technically, the pot would still be useable for other cooking needs and moved with safety. I made this goal my object.

For my trials, I made a bag to fit the pot in the above photo. Filled to the brim, the pot probably holds about eight cups of liquid, so in a normal cooking situation, probably about six cups to bring the liquid low enough in the pot to prevent overflowing during boiling. I measured the pot, made my pattern lines, and cut my fabric the same way I did for my first bag. My instructions in this post will only be for the changes I made in the construction of the bag to allow for the handle. To begin, I cut down a single line of one of the segments till I was within a 1/4″ of the center circle and zig-zagged my raw edges. I did this to both the outside and inside circles of fabric. Then I stitched strips of Velcro face down onto the right side of the fabric that would be the outside of my bag; the loop strip on one side and the hook strip on the other. I only stitched along the cut in the fabric, with a seam allowance just far enough in to catch the edge of the Velcro.

The next step is a little tricky. My inside piece of fabric (a cotton sheet remnant) did not have a distinct right and wrong side which made this step a little easier. If yours does, you want to stack the fabrics right-sides-together with the cuts in the fabric lined up. I turned the whole stack over and stitched another seam, this time attaching the cut edge of my inside fabric to the free edge of the Velcro strips. Finished, the stack looked like the photo to the left.

Once I had both circles of fabric attached to the Velcro strips, I pulled the fabric out of the way, pressed the bottom of the two Velcro strips together and stitched the bottom edge closed. This is an important step. It is required in order to keep the stuffing from coming out! 🙂

With the Velcro stitched down at the base, I turned the fabrics right-side-out and top stitched along the seams as seen in the photo to the right. I did this to reinforce the seams. I really do not want to see the stuffing coming out!



Next, I closed the Velcro strip and, making sure that my fabric circles were lined up with the wrinkles smoothed out, I pinned them together and began to stitch along the guide lines I had penciled in to form the center circle and ten segments. I did this just like I had for the larger bag except that I left the opening for stuffing the bottom circle at the base of the Velcro strip. This is illustrated in the photo to the left. You can see I left the bottom of two of the side segments open for this, but I could just as easily have left one and been fine. With the stitching in place, I proceeded to fill the bag with stuffing, starting with the center circle. This time, instead of breaking the packing peanuts up in a pillow case and having to transfer them to the bag, I inserted the whole peanuts directly into the section I wanted to fill and broke them up by working through the fabric from the outside. It took a little longer to do this way but it was a great deal cleaner!  🙂

With the stuffing completed I stitched all the remaining openings closed and prepared to work on the drawstring casing. I noticed that there was a very tiny gap where the Velcro strips met the casing, so I hand stitched this gap closed -very, very well! The rest of the casing instructions are the same as the original bag with this exception. There is no DSCF7972need to make a V-shaped opening in the outside fabric. The Velcro opening in the side of this bag becomes the natural place for the drawstring to come out. The lid instructions are the same. I just made a smaller circle. The finished bag with its opening is demonstrated in the photo to the left.


Using this bag only took one more step of preparation than using the original. Before I began cooking, I opened the bag up, put the pot inside, and closed the Velcro strip from the base of the bag to the place where the handle came out. I left the rest of the strip open so that I could easily place the pot inside when I was ready to transfer it from the stove. Then all I would have to do is close the remaining section of Velcro and tie the lid in with the drawstring.

The bag, with the pot inside and the handle sticking out is kind of  funny looking!  It looks very much like a face; in fact, it reminded me of a child with his cheeks puffed out blowing one of those party whistles that roll out and make noise! Not very flattering I suppose. Oh, well. At least it is a cheery image. 🙂


Does it work? Yes, it does! The rice in this photo cooked on the stove only long enough to begin to boil over (about two to three minutes), before I transfer it to the bag. Forty minutes later, the rice was perfect. I also tried it out on cooking a single piece of chicken (boiled) and it did that just as well. Next, I think I will try some kind of dried bean. Those typically require a long cooking time, so they are wonderful candidates for this kind of cooking. For now, I wanted to post this for those of you  who are still looking to find just the right gift to make for a loved one this Christmas.  This allows for you to give a thermal cooking bag to grandparents, parents, college students, or other households that routinely have only one or two people to cook for.

This entry was posted in Do It Yourself, Go Green, international foods, Sewing, South African. Bookmark the permalink.

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out /  Change )

Google photo

You are commenting using your Google account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s