Potpourri

Testing the Fabric Content of Your Textile
Interpreting the Results of the Standard "Burn Test"
By Kristin White

The burn test is a great test to start with when testing the fiber content of your textile. However, there are many ways to interpret the results. Incorrect interpretation, of course, will give you false information. Whether a fiber burns or self extinguishes is not dependent upon whether it is a "natural" fiber or a "synthetic" fiber.
Because of their chemical make-up, both silk and wool will self extinguish when the flame is removed. Rayon is regnerated cellulose (cellulose is the polymer of which cotton, linen, and wood are comprised) so even though it is mad-made it burns extremely well and will continue to do so after the flame is removed. It will burn until the sample is exhausted if unhindered. The smell given off when rayon burns will be similar to that of paper (also cellulose based), but silk will smell like burning hair.
The best way to view this " shrinking" of silk I mentioned is to slowly bring it close to the flame but not into the flame. They will shrink away due to the heat given off even before they burn from contact with the flame. While a synthetic like Polyester will also self extinguish it does not behave in the same manner as silk when exposed to the flame. Silk will "shrink" away from the flame and form small beads on the ends of the fibers after the flame is removed. Polyester will also form beads but it does not shrink away from the flame. (It does melt, but it doesn't "jump back" like silk.) The color, shape and size of the beads at the end of comparable sized threads of polyeter and silk will differ, also.
Different solvents may be used to determine fiber content, also. For example, silk and wool are degraded by alkaline solutions, but can withstand acidic solutions. It is exactly the opposite for cotton, rayon and linen. (This is why laundry detergents are harmful to silk and wool.)
I have two suggestions when testing fabrics. The first is to separate the weft from the warp before testing especially burn testing). It is not uncommon for fabrics to be comprised of weft yarns of one fiber and warp yarns of another. If cotton and silk fabric were burn tested whole, the presence of silk may not be discernible. Since the cotton will continue to burn, the silk will likely be consumed also.
The other is to keep in mind that certain fibers are often blended at the point of yarn formation, cotton and polyester, for example. Separating the weft and warp or even the piles in the yarn many not separate the different fibers.