If you generally shop for fabric in small chains, they often have all of it labeled with its appropriate content, generally on the cardboard bolt somewhere. But for many of us who are bargain basement fabric hunters (cough cough), it can be a challenge to find a price let alone fabric content.
I liken determining fabric content to color theory. If you know your primaries (red, yellow, blue) and secondaries (green, orange, purple), then you can generally determine with decent accuracy the content of any mystery fabric.
So you are there at that store and you want to know what kind of fabric it is. Any reputable place will do a burn test for you. Just take it to a counter and ask for one. If they can tell you by feel (a lot of people can) you can take them for their word, or request the test anyway. If they won't do it, ask for a swatch and take it home or outside and do it on your own. I've done that. I often carry wood matches with me when I fabric shop.
Taking the idea that fibers are like colors, I'll give a basic rundown of some of the most common fibers, how they burn, and what they look like when they burn. You will notice that fiber blends will take on characteristics of what they are blended with.
Cotton is pretty simple, from flannel to denim, its burn is pretty consistent.
(the first is a cotton twill, the next two fabrics are cotton flannel)
-Burns like paper.
-Slow crawling flame across the fabric. Now, this can vary depending on the weave. The looser, the faster the flame because the more air between the fuel of the fabric.
-It smells like burning paper. Big surprise with it being a plant fiber and all, right?
-The ash is often singed on the outer edges with a ring of brown, then black at the center. Its a soft crumbling ash that smears on the fingers.
Another plant based fiber, it as a lot of characteristics to cotton, but some of its own.
-The burn of linen is slow, crawling, but has some flicker, because the fibers of linen are hollow.
-Also smells like paper.
-The burn is like cotton as well with a ring of discoloration, and a center of ash.
-The ash is also soft like cotton when pressed on the hand. It smears as well.
Rayon.It's a standard when making vintage garments, and not too easy to find with a lot of variety. I honestly didn't know how it would burn. Its a synthetic fabric based on plant fibers. Still, now I know why it's not plentiful to find. Lights up like kindling!
-SUPER fast burn thats steady and ravenous.
-Slight discoloration farthest from the flame.
-Leaves little to no ash.
You can see some of the burn characteristics from both fibers in this one.
Most would be surprised at how wool burns.
But then, maybe not if you have ever used a hot iron on your hair.
-Has a popping quick burn. The burn bubbles and pools as it ashes. Maintaining a flame on wool is difficult.
-Smells like cooking bacon.
(top is wool gabardine, bottom is wool felt)
-The ash melts to a sheen hard and crunchy, like charcoal.
Another animal protein.
(top is douponi, bottom is silk satin)
-Has a quick popping flame that is hard to keep lit.
-Like with wool there is a sheen on the hard crumbling ash.
-It smells 'burnt', like ash.
Polyester and polly blends.These types of fabrics you are most likely to come across depending on your price point. Most blends that are not labeled are most likely polly blends. Because they are a petroleum product, they tend to have some variations of the burn properties they are blended with.
(silk polly blend)
-Melts when on fire. No ash to speak of.
-Material contracts as it melts curving itself into a ball.
(perhaps a polly/rayon blend)
-Very hard to get it to flame. No ash at all.
-Smells like burned plastic. the burn is black but a thin line around where the flame used to be.
So there you go, the basics of burn testing. There are many, MANY other types of fabrics out there, but unless you are specifically looking for them, I doubt you will run across them in the bargain fabric bin. And if you do, try a burn test and look at how it behaves.
Good luck and burn safely!!
A burn test chart from DitzyPrints.
A great list of burn test fibers from fabric.com.
Her is a great list of fabric definitions.
If you all have anymore links you wish to contribute on the different types of fabrics and weaves, please share!