When I was writing the handwarmers tutorial, I had the basic idea and had a quick look round the internet to see what other people had done. I already had in mind to use fleece as a good material, but other tutorials differed, and I found one that categorically stated that you needed to use flannel.
Now, I’m a perverse kind of soul, and the minute I read this, my inner teenager struck a pose and asked “What happens if I don’t? Who’s going to make me?” (I had to resist the urge to slam a few doors as well.)
Once I remembered that I am not fifteen, but actually a Good Deal Older, I started to consider the question more calmly. Is flannel the absolute best choice? What if you don’t have any flannel (I don’t)? How much does the choice of fabric really matter, anyway?
My gut feeling was that the choice of fabric wouldn’t make that much difference, although perhaps it would be better to err on the side of warmer fluffier fabrics like fleece or felt or (or even flannel…) rather than quilting cottons. And then it occurred to me that by using the inner and outer pouches in my design I would add extra insulation – would this be enough to redeem myself from the crime of not using flannel?
I tried a bit of searching, and learnt far more about the insulation of specialist fabrics than I will ever need to know… And then I stopped.
Why was I researching all this generic information and trying to interpret it specifically for handwarmers, when I could just experiment, and find out once and for all, what is the best fabric to use?
So I did.
AIM: To compare the time taken for handwarmers made of different materials to cool down to the external room temperature.
- cotton jersey
- felt (the cheap acrylic kind)
- wool (a late addition to the party, hence the hacked together photo)
I know what you’re thinking – no flannel? Sadly, I don’t have any in my stash, nor can I easily lay my hands on any – which is the reason I’m doing all this in the first place…
I also wanted to know what difference using a cotton inner pouch made, so I also tested:
- cotton liner, cotton outer
- cotton liner, fleece outer
For each one, I sewed a small pouch the same size and filled it with the same amount of rice. Then I popped each one in the microwave for 45 seconds. Finally, I got my meat thermometer out and measured the temperature at 1 minute intervals for the next half hour. (Yes, I really did this. Apparently this isn’t normal…)
The Nontechnical Summary
Here’s a graph of the temperatures of the different fabrics over time (I did some fancy data modelling to smooth out the data a bit and make it easier to compare the materials. If you’re interested – and who wouldn’t be? – drop an email and I’m happy to send you all the gory technical details!):
The temperature data all follow similar curves (there’s actually some physics that says we’d expect to see this kind of shape, so it’s reassuring that we do), where initially the fabric cools down quite rapidly, and then the cooling rate slows down until eventually all of them end up at room temperature.
The actual time taken to cool down depends on the initial temperature and also on the external temperature. To make it easy to compare, the graph shows the modelled temperature of each sample over time, based on an initial temperature of 50°C (122°F). The shaded area corresponds to external temperatures of 25°C (77°F) degrees and higher- much colder than this and the handwarmer doesn’t feel warm anymore, so you can think of the edge of this area as being the point at which the handwarmers stop working.
Results – which is best?
In general, I must say I was surprised at these results. There was more of a difference than I expected – nearly ten minutes between best and the worst – and the order was not what I was expecting either.
Both the felt and the wool in particular were much poorer than I expected. Even though it’s just cheap acrylic felt I still assumed that being thicker it would be better than just plain cotton, and I thought that the wool would outperform the rest. Not so.
The two layers definitely makes an improvement, but the improvement depends how good the fabric was on its own. So a double layer makes the fleece last an extra 5 minutes, but only adds a couple of minutes to the cotton.
Overall, the best combination was the fleece outer with a cotton inner pouch – coincidentally exactly the combination I chose for my handwarmers (okay, so I cheated, and did the experiment first…).
So what? What does this mean?
So what is all this good for, apart from the opportunity to use a bit of fancy maths?
It means we can conclude the following:
- how long handwarmers keep warm for depends on the fabric you use, and the external temperature; they tend to keep warm for around 15-30 minutes.
- fluffier fabrics with more fibres keep warm for longer, but I saw no obvious benefit from either thicker fabrics or natural fibres.
- using two layers rather than one keeps them warm longer, especially if you use at least one layer of warmer fabric.
- Quilting cottons are not the best choice here, even if you use a double layer. If you have a favourite cotton you’re desperate to use, a better option would be to applique it onto fleece.
Of course, there are a lot of criticisms you could make of my experiment here; while I tried to control as many factors as possible I didn’t take it as seriously as a proper scientific experiment. So it’s possible that some of my more surprising results are actually a result of error on my part. Even so, it was fun to do, and I found it interesting to see how much difference the choice of fabric made. I’ll be less likely to take things for granted and make assumptions in the future.
Of course, we’ll never know where flannel fits into the hierarchy, but at least I can rest assured that using fleece instead didn’t cause the world to explode…