Here's a quick video on testing fabric content. Before we get started, please be careful if trying this at home.
In the video, I've used relatively large samples so you can easily see what's happening. I'm also in a controlled environment and using fabrics I'm confident have not been soaked in anything or that may be overly flammable. If you're doing this yourself, use small samples and be very careful. Adult supervision is essential.
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Many people are unable to easily detect the presence of synthetic materials like polyester in fabrics. Milling and weaving processes have been so refined and material properties so improved that it is often next to impossible to tell the difference between all-natural fabrics and synthetics –until it's too late sometimes and you either already bought the fabric or have made something you really wanted to be natural and it turns out synthetic.
I often go to large fabric stores and markets and am faced with a dizzying array of fabrics. Here in Asia, it is often risky to believe what a vendor tells you about the fabric content. I've found the only reliable way of testing fabric content is to do this quick test myself. I've found that vendors often don't want me to do it since they realize I will probably learn the true fabric content and be unhappy with it, but I insist in any case. I've never been sorry about testing fabric content before buying an important piece of fabric. You won't be sorry either.
By doing a small burn test you can quite easily tell the difference between common natural fibers like cotton or silk and fabrics containing polyester fibers. In a nutshell, natural fibers will burn completely and leave a light ash. Polyesters, which are essentially plastic, will melt and burn with black smoke and smell like burning plastic. You'll easily smell the difference.