After reading about some problems that have arisen with the use of 'bowl cosies' I decided to carry out a few experiments myself regarding the safety of materials used and just what are the problems involved. Now, I do not have a lab, or have access to one to do this test, just my kitchen, microwave and a box of matches .
Some writers state that
" wadding or fabric that has polyester included in it's composition can sometimes catch fire in the microwave oven and you should only use cotton thread, wadding and fabrics."
The problem that I see with this is that although goods are marked cotton that doesn't necessary mean that there is NO synthetic fibre included in the fabric. Because of modern weaving methods a synthetic component is often added to the warp fibres to give strength to them , otherwise they would break under the pressures that are used by modern machinery, and labeling laws do not require this component be stated on the label. In other words it could still be labeled cotton.
For this experiment I chose to look at my left over fabrics from my last quilt. (I thought that they would make good cosies as gifts but I couldn't give them to anyone if there was a danger involved. I had visions of some teenager putting the item into microwave and setting the timer for 15 mins on high and the next thing you know there is an explosion.)
There were no problems with the outer cotton fabrics. They got a bit warm but that was all.
As I cut the samples from the wadding I noticed that there was a colour difference between the two I had selected.
I put them into the microwave oven for 10 minutes each and and they became warm to touch but not hot and they didn't catch fire.
I then did a burning test and if they had been 100% cotton the should have burnt like paper which didn't happen. They burnt very slowly and there was a clear oily residue left on the clay plate. A clear sign that there was some synthetic content.
I also tested some wool wadding. The microwave test returned the same results and in the burning test the fibres smoldered but did not catch fire and there was that smell of burnt feathers.
I repeated both the tests again and got the same results.
So for this component of the tests and if the time in the microwave was 10 minutes or less there didn't seem to be a problem but if the fabrics were exposed to an open flame there was a problem. Those fabrics may react differently if they were in the oven for a longer period of time.
I then tested some wadding which had two pieces sewn together, again for 10 minutes in the microwave and this returned clear burn marks where the sewing thread had burnt. The thread was labelled cotton but it must have been cotton around a polyester core.
I repeated this and got the same result.
These results show that in my experiments the most dangerous component is the sewing thread.
For myself, I would heat my food in a container or plate and then put that plate onto the cosie but not put the cosie into the microwave. I believe there is a real element of risk involved and there is the potential for a fire.
I will use cotton thread in the cosies and I will tell my recipients they are not to go into the microwave oven.