This post is long past due. I promised I write about this. I roughly mentioned it on my ETSY hand-made site about silks and polyester.
I had to take a class (part of the curriculum) at FIT my freshman year..(gosh, it's been more than 10 years ago). Very helpful I must say. The only thing I learned from the class that I use constantly is the fabric test. I use this when I go shopping for fabric in the fabric district. The fabric sellers know by now to do the burn test to show customers the true nature of the fabric..that is if they think you know your stuff. If you don't, they can claim, but how would you really know what you're buying is exactly what you are getting???..you don't.
I have once purchased some silk brocades from an online vender from China...thinking that it was 100% silk since the site emphasize on 100% silk products. It wasn't super cheap, but cheaper than buying the silks here. When it arrived, I did the burn and only to find out that it was polyester...which for the price, I could have bought it here in the States. Why would I need to buy from China and pay the shipping. duh! Lesson learned.
If in person, most of the time I can tell what the fiber is once I touch it or by looking at the threads. This takes a lot of practice though of course. Designers who have worked for a design company that mass-produces as I did will get tons of swatches in everyday, look at it and approves the quality and color. Not much fun at all. Very tedious, but essential part of the designing process.
The burn test will identify most fabrics. Can't determine the percentage but will let you know if it's natural or synthetic.
Natural Fibers-
100% WOOL: Smells like burnt hair. Very hard to ignite a flame. You will most of the time see the edges burn, but with no flame. Doesn't bubble, just burns clean.
100% COTTON: It burns pretty clean and you can blow out the flame quickly. The burnt edge ashes pretty easily at a touch. Smells like burnt paper.
100% SILK. You can see the edges bubbles a little when burning but ashes when you touch it. Smells like burnt hair. Also a little harder to blow out. DO remember that qualities such as chiffon, satin, taffeta, shantung, organza..etc. do come in silk or polyester fibers. Pricing is a big difference. DO not assume it's silk.
Synthetic Fibers:
NYLON: is made from Petroleum. It hold a steady flame. The end will bubble and curl as you burn and will be hard after cool down. Does not ash.Will smell like plastic.
POLYESTER: have a very strong plastic smell as it burns and melts. The burnt area will bubble and very hard after cooling.ACRYLIC & ACETATE: both are usually used only for linings. Acrylic is a little better quality than acetate. Acetate will shred if you use bleach at all. Both will be hard after burning.
Blend fibers are hard to identitfy. If it's nature with synthetic, I have notice that it will burn ans ash, but the edge of some threads will be hard and scratchy if you rub against your skin.
Please make sure that the burnt area is cool before examining with hands.
To sum it up all natural fibers will ash when burnt. And only one man-made fiber(RAYON) will ash.
All man-made/synthetic fibers EXCEPT for rayon, edges will be hard after burning.
Why is it important?? Well for one, if you paid a lot of money, you don't want to ruin it under the iron. Synthetic fibers usually burn very easily, meaning you can never have the iron on high when ironing.