Natural Fibers:

Cotton – Burns completely. Smells like burning paper. Produces gray ash. 

Types of rayon (Rayon, Viscose, Bemberg, Modal, Tencel, Lyocell, etc.) will burn the same as cotton. They are regenerated cellulose fibers, which are man-made from natural material. They are all essentially the same fabric, but there are some major differences in how sustainable they are. One way to tell between cotton and these fibers is based on the drape of the fabric. Regenerated cellulose fibers have a much more fluid drape.

Linen – Burns completely. Smells like burning grass. Produces gray ash.

Jute and Hemp also burn like linen, but they are not as commonly found in textiles. Hemp has a very distinct smell if it is not pre-washed, so if you come across any secondhand/vintage hemp fabric, it may make itself known.

Wool – Burns briefly then goes out on its own. Smells like burning hair. Produces dark ash.

Other animal fibers will burn similarly. Wool is commonly blended with synthetics, so pay close attention to if any other smells are present or if there is visible melting. If it is mixed with polyester you may smell a slight sweet smell mixed with the burning hair smell. 

Silk - Burns briefly then goes out on its own. Smells like burning hair. Produces black ash that sometimes beads up.

 

Synthetics:

Nylon – Burns briefly and melts. Smells like celery. Produces a hard, grey bead.

Acetate – Burns for an extended time and melts. Smells like vinegar. Produces hard black ash.

Polyester – Burns briefly and melts. Smells sweet/chemically. Produces a hard, black, round bead.

Olefin – Burns briefly and melts. Smells like burning asphalt. Produces a hard, tan bead.

 

Other Tips:

1. You are probably never going to know exactly what comprises a blended fabric (some fabrics have 5 or 6 different fiber types blended in them) but you can get close. One common blend is to mix polyester with cotton or wool. To identify this, just look for melting and a sweet chemically smell in addition to the normal smells you would expect for cotton or wool. It is fairly easy to identify polyester since it has such a distinct smell.
2. Silk and wool can be tricky since they smell the same when burning and the difference in appearance of ash is minor. I recommend not getting too caught up in the burn test itself and rather using your other senses to determine what type of fiber it is. Once you know it is one of the two you can check these things:
   • Is it itchy?
   • We all know wool can be itchy, even if it is very slight. This indicates it is wool.
   • Is it sticky?
   • Sometimes raw silks, or silks that have been washed, have a very slightly sticky feeling.
   • Is there a visible nap?
   • Wool will likely have some shorter fibers sticking up from the fabric. Again, this may be slight, but wool fibers tend to be very short, so they will produce this nap that is not present on silk.