There is an article on this at sewforum, tater baker bags, they seem to say that one should use rayon or cotton thread if possible, that also includes the bobbin thread. Some even say it is important to wrap the tater in damp paper towel to prevent scorching if you bake them a long time...

I read somewhere after I got directions and design for the tater bag that
all cotton thread, warm and natural batting, cotton material where needed. Also wrap in paper towel and put in bag. A lady said that one
caught on fire while her husband was micro waving it. She had used
poly thread. I have been sewing fifty plus years and had a lot of thread, But very little cotton. I haven't made mine yet.

I normally use an all purpose thread (6 ply cotton over poly) for its strength and cotton finish. For these bags I only used cotton. I understand that problems occur when bags overheat, poly has a higher maximum temperature than cotton and therefore holds more heat. I would wait til you get the cotton thread. Better safe than sorry.

This is new to me... cooking potatoes in cotton bags? I just stick mine on a paper towel. Where did you learn this? thanks! I am very curious and how do the potatoes taste?

I was told all has to be cotton....Others melt so when I made mine I used cotton thread.
Hugs Joann

I've made many of these & have been using mine for several yrs. I used regular Coats & Clark to construct the bag & rayon emb. thread. However, be sure & use not only all cotton fabric, BUT the new batting Warm & Natural has out for use in the microwave. My bag has the older Warm & Natural, which actually is NOT 100% cotton, but I don't recommend using it. I have not had any problems, but some have. I do wash my potato, wrap it in a paper towel without drying the potato & never had any trouble.

If the bag and the batting have to be made of 100% cotton, it stands to reason that the all the thread has to be made of cotton as well, including embroidery thread.
Hugs, Lidia

if you are worried about the thread i have seen the potato fabric that is also nice

Yes. the thread has to be 100% cotton. Everything or it wiil catch on fire.
I order everything online because to go to some stores you'd have to check all material on the bolts. I ordered a kit to try out that had everything in it. I made 2 bags so I sent sent them to my kids to try out. Also you can't make them any larger than your microwave turn table or they will scortch Read or ask questions before you make them.

here is a link & another for the free pattern from the warm company.

I have made tons of these and always with 100%cotton fabric and batting. But have used other thread with no problems since the design I use is the applique one. I remember one day I was going through my stash of fabric trying to find some more interesting patterns and couldn't determine the content so I cut 4' squares and microwaved them one at a time for about 8 minutes. Everything went fine and all came out unscathed until I came across one that wasn't 100% cotton. My husband went bizerk! It flamed up and man did it stink! HA! It caught on fire in less than 3 minutes too.

To test what a fabric is, you take a small piece, like less than and inch square, and set it on fire with a match (take precautions here like put it in a glass bowl) and let it burn up. If you get ashes, it is cotton. If you get hard lumps, it is polyester. If you get a mixture, it is both cotton and poly. Put it in chlorine bleach and leave it for 20 minutes or so. If it dissolves completely, it is wool. If it dissolves only partially, it has some wool fibers in it.

you have to use only cotton thread evening has to be Cotton if not in the microwave you get in fire flames heaped to me no more microwave bag for me .Maria

I wish I could remember the name but you might want to check around for it for safety's sake...there is a special batting made just for that. Someone somewhere had pictures of a microwave fire..pretty scary. If anyone knows the name of the batting I am referring to could you please list it here because of the number of people having a desire to make these bags.

The batting is called warm tater.

Use all cotton thread, Loralye. Make sure you check that it's actually 100% cotton, because some are cotton wrapped around a poly core for strength, that could be a nasty surprise, too. Good luck with these, love to see them in Projects when you get done. Hugs, Marji

I make them all the time and just use the embroidery Thread it does not hurt anything. my Family loves those.