Sometimes we need to know the exact fabric content of something we are planning to use. I’ve always done a rudimentary burn test but sometimes it is hard to know from that anything other than it is a natural fiber. When dyeing fibers, you need to know if it is cotton, silk, wool, rayon or something else because of mordants used.

I found a wonderful chart at http://nvg.org.au/. The New Varangian Guard Inc. (NVG Inc) is a historical re-enactment organization with branches throughout Australia, and sister organizations in several other countries. I am copying the chart below and giving them credit since the original source of the information is a dead link.

How to tell what the fiber is:
Burn Test
Snip a piece of fabric equivalent to 1″ square. Using butane lighter and holding the fabric with a pair of tweezers ignite the fabric over a non-flammable surface in a well ventilated area.

Examine the quality and color of the flame, the odor produced, and the quality of the resulting ash or cinder.

Use this table to help determine your fabric’s content.

FABRIC FLAME QUALITY ODOR ASH QUALITY COMMMENTS
WOOL orange color, sputtery burning hair or feathers blackish, turns to powder when crushed flame will self-extinguish if flame source is removed, no smoke
SILK burns slowly burning hair or feathers grayish, turns to powder when crushed burns more easily than wool but will self extinguish is flame source removed
COTTON yellow to orange color, steady flame burning paper or leaves grayish, fluffy slow burning ember
LINEN yellow to orange color, steady flame burning paper or leaves grayish, fluffy takes longer to ignite than cotton, but otherwise very similar
RAYON fast orange flame burning paper or leaves almost no ash will continue to burn after flame source removed
POLYESTER orange flame, sputtery sweet or fruity smell hard shiny black bead black smoke
ACETATE burns and melts, sizzly acidic or vinegary hard black bead will continue to burn after flame source removed
NYLON burns slowly and melts, bluse base and orange tip, no smoke burning celery hard grayish or brownish bead will self extinguish if flame source removed
ACRYLIC burns and melts, white-orange tip, no smoke acrid black hard crust will continue to burn after flame source removed
Bleach Test:
To determine content of fabrics I recommend conducting this first with fabrics that you are sure about the content of – so you will know the outcome of the test yourself. In a non reactive pan (I use a pyrex pie plate) take snips (small pieces) of the fabric you will be testing. Use straight bleach and put about ½” of bleach in the pan. Please do this in a ventilated area. Add the snippets of fabric and let them sit for about 24 hours. The next day, look at what you have left.

100% Linen or Cotton any color should be removed from the fabric, but there will be no damage to the fibers themselves
100% Silk or Wool the fabric will have fully dissolved (unless its worsted/gabardine, there is a fabric treatment that protects the fabric from the bleach and keeps the fabric shiny even after washes).
100% Polyester, Rayon, Acetate, or Nylon the fabric will have become a cloudy mess within the bleach. Fully dissolved into an opaque cloud within the bleach.
Blends – The individual fibers will act as described above. The result will be different for each combination
Rayon / Linen linen fibers with a opaque cloud;
Silk / Polyester and Wool / nylon fully dissolved with an opaque cloud;
Linen / Cotton no damage to the fibers;
Silk / Wool fully dissolved.