There are many tests for fiber identification. They can be broadly classified as nontechnical tests and technical tests. The nontechnical tests include feeling test and the burn test. The feeling test is the most simple one which requires identifying fiber just by touching the fabric. For example, wool will feel warm whereas cotton or linen will feel cold. However, this is not a very reliable method as many fabrics are made of different fiber contents. In the other test called the burn test, a sample fabric is burnt and observed for the various characteristics shown by it after burning so that its fiber contents can be determined.
There are fiber testing techniques that are carried out in laboratories. These technical tests for fiber identification are the microscope test and the chemical tests.
Microscope Test: In this technique, powerful microscopes are used for testing and identifying the fiber contents of different fabrics. Different fibers show specific characteristics regarding shapes, length, marks etc. under the microscope. It is very effective for testing the natural fabrics. Since many synthetic fabrics have similar appearance, sometimes, it becomes difficult to test them under microscope. Dark color fabrics also can't be tested as light doesn't pass through dark substances. Additionally, dyes of the fabrics also have to be removed by stripping, bleaching etc. so that they can be seen clearly and without any wrong effects that can misguide and result in incorrect test reports.
Chemical Tests: They can only be conducted in well equipped laboratories. There are two key techniques for chemical testing- stain and solvent. The stain method uses acid and alkali on different fabrics to identify the fiber contents. A fiber stained with acetic acid turns to a specific color but when the same fiber is stained with mild alkali, it turns to a different color. For example, acetate changes to light green color when acetic acid is used and turns orange when dilute carbonate of soda is used. In the solvent method, different solvents are used to distinguish one kind of fiber from another. There is no single solvent or chemical that can be used on all fibers. The fabric has to be cleaned thoroughly and the finishing chemicals have to be removed completely before immersing sample fabric in solvents. Animal fibers are destroyed in strong alkalies while plant fibers destroy in acid.