Testing fabric for natural or synthetic content

How to test fabric for natural fiber content:
(Mostly I'm looking at testing for cotton in fabric. You would have to look at other things as well as how the fabric burns to tell cotton from linen from wool.)

Note about handling fabric:
All fabric should be prewashed before you use it for anything. It removes sizing, allows the material to do any shrinking (it's nicer to have it shrink BEFORE you spend hours sewing an item rather than afterwards! If it shrinks before it's washed, you will be cutting the pattern to the size you need. If it shrinks afterwards, it may not fit or the fabric may pull or pucker the finished article.) Wash the fabric as you will the item you're making it into. So, if I'm making napkins, I put the fabric into hot water and a hot dryer. If I'm making a dress, the fabric would be cold washed and dried on medium heat.

You will need a couple of things:
a small swatch of the fabric you want to test.
a fire source, a lit candle is great, but matches or a lighter will work too.
tweezers or tongs to hold the swatch (so you don't burn yourself)
Non-flammable surface to work on

Set yourself up in a safe place so that you can burn a small swatch of the material you want to test. By this I mean, we're going to actually burn a small amount of the fabric to see how it burns, so set up where you have water close by in case you can't blow the fire out. Make sure you are standing where you can drop the burning piece without setting anything else on fire!
Standing over your kitchen sink is a really good place to do this kind of testing.

First, I use a candle. I had a bunch of fabric that I wanted to test and the candle is the best option - it leaves both hands free to work and you don't have to worry about burning yourself. Set the candle down into your sink (in a holder, if it needs one) and light it. Next, using tweezers, take the swatch of fabric and slowly start moving it towards the fire, the let it burn for a couple of seconds, then put the fire that's burning on the edge of the swatch out. (Leave your candle lit.)

You're looking for three things.
How did the material react to being burned
How the burnt part smells
How does the burned and cooled fabric react at the burned area.

How the material reacted to being burned?
The reason for slowly moving the fabric towards the fire is so that you can see if the fabric starts to shrink away from the fire, how close to the fire you can get before it catches the swatch on fire and can you just blow it out easily or does it want to keep burning. If the fiber starts to shrink way from the fire before it gets to the flame you have a high synthetic count in the fabric. Cotton/wool/linen have to almost touch the flame before it will ignite. Man-made fibers shrink - melts actually and wants to stay lit, even after you've blown the fire out. More about that in the last section.

How does it smell?
Man-made fibers, of which polyester is one of the most common, are made of chemicals and when any of these fibers burns, they have a chemical smell. Natural fibers have a milder, more natural odor.

How does it look and feel after being burned?
Fabric containing synthetic fibers will have a melted "ball" or "beads" at the ends of the burnt area. Make sure the fabric has cooled down, then if you run your fingers along the burned edge and it feels rough, or the fibers separate in "clumps", you have synthetic fibers in the fabric. Natural fibers will break off in an ash. The ends will feel soft and the fibers will separate individually, instead of looking like they have been glued together in small groups.

I read on one tip site that if the fabric sample being burned was natural, as soon as you blew on it to put out the fire, it would all go out. But that if it "glowed" it was synthetic. Usually, but not always so. If the natural fibers are unevenly woven - as when they ran out of thread and added another piece of thread to continue the weaving - it will cause the fibers to be thicker there than in the surrounding area, and thus not go out evenly. Also, if the fabric has been treated with sizing, it may also not go out as quickly. But the edges will be soft and the smell as it is burning is not "chemical" smell.

So, I took my many fabric samples and held them level with the side of the flame, but about 2-3 inches from it. I slowly moved the fabric closer to the flame. I didn't have any of the fabric swatches recoil from the flame, they had high enough cotton content that it didn't melt away from the heat. I would touch the edge of the flame and the material would ignite. I would blow it out. Some of my fabric, after being blown out, glowed for a few minutes. I knew that that fabric had a good amount of synthetic fibers in it (though it was still mostly cotton). I had a few pieces that glowed, for a moment or two. But then I noticed that the individual fibers were bunched closely together, as it does when I run out of yarn and have to add another piece to continue knitting. The edges were soft, brittle, grey-ashed, and there was no chemical smell. I know these are 100% cotton. (I know don't have any linen or wool fabric, just cotton and cotton/polyester.) Some of my fabric didn't smell, perhaps I just didn't let it burn long enough. But the edges did clump together or bead. I knew those have polyester in them.

Most of the 100% cotton did extinguish as soon as I blew on it. They all had a softer smell to them and the edges were soft with the fibers separating as I ran my finger across the edge. The fibers were also brittle and would discinerate to ash when touched.

I hope this helps someone distinguish between their materials, natural, combination and synthetic.