Cotton or polyester? 7 ways to tell without a label
Cotton and polyester can look nearly identical, especially in T-shirts, quilting prints and poplins. The difference matters: polyester won’t take dye made for cotton or a hot iron, and cotton won’t hold a sublimation print. Here’s how to tell them apart.
1. The water drop
Drop a little water on the fabric. Cotton usually soaks it in within a few seconds and darkens. Polyester tends to let the drop bead up and sit on top. A water-repellent finish can make cotton bead too, so try an inside seam allowance if you can.
2. The crush test
Squeeze a corner tight in your fist for ten seconds, then let go. Cotton keeps the creases. Polyester springs back with few wrinkles. A cotton-poly blend lands in between.
3. Sheen and feel
Cotton is usually matte and feels dry and soft. Polyester often has a faint shine, feels smoother or slicker, and can feel warmer and less breathable against your skin.
4. Static and pilling
Polyester builds static: it clings and crackles in a dry room. It also tends to form tough pills that stay attached. Cotton pills less, and its pills usually fall away.
5. Heat, carefully
A hot iron on the cotton setting can leave a shiny, glazed mark on polyester or even melt it. Cotton takes high heat without melting. Only try this on a scrap, never on something you care about.
6. The burn test
The most reliable home test. Cotton burns like paper and leaves soft gray ash. Polyester shrinks from the flame, melts, smells chemical and leaves a hard bead. See the burn test chart for the safety steps.
7. A photo
Fabric Identifier estimates the likely fiber content from a photo (for example, “likely cotton, possible polyester”) and names the fabric and the weave. It’s quickest at a thrift-store bin or a remnant table; confirm with the tests above when it matters.
Why the difference matters
- Sublimation needs polyester; on cotton the print washes out. See what fabric works for sublimation.
- Dyeing: fiber-reactive dyes made for cotton won’t color polyester, which needs a disperse dye.
- Ironing: cotton takes a hot iron; polyester needs a much cooler one.
- Quilting: cotton shrinks a little in the first wash and polyester barely does, so mixing the two in one quilt can make seams pucker.