Which test is performed before a chemical service to check how the hair will react, such as before perming or relaxing?

Before chemical services, cosmetologists perform preliminary tests to predict results and protect the client. A strand test applies the product to a small section of hair to see how it reacts (for example, how the hair takes color or responds to a perm/relaxer). For products that can cause allergic reactions, such as some haircolors, a patch (predisposition) test is done on the skin in advance to check for sensitivity. Recognizing the purpose of these preliminary tests is important hair-science and safety content.