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Made up of hydrogen peroxide and a few other things to make it smell pretty, hair color developer is the most important part of the dye mixture. It does all the work. Color -- merely a bunch of pigments -- just has to lie there and allow the developer to push it into the hair. As you can imagine, expired developer doesn't have what it takes to make the magic happen. How long does it last? It depends.
Sealed and In Storage
Unopened developer lasts the longest, with a shelf life of about five years from the manufacture date. Keep in mind, though, that any bottle of developer could sit on someone's shelf before it got to yours. Warehousing, shipping and retailing all cut into the life of developer.
Once opened, the shelf life of developer becomes shorter. Unused portions of developer generally last one year. Other things may affect the life of the product, though, including storage temperature, sun exposure and contamination.
Mixed with Color
Mix developer with color and strange things start to happen. Over time, the mixture becomes thicker, darker. This process, known as oxidation, kills the effectiveness of developer pretty rapidly. Once mixed, developer lasts only about an hour. Color processes that take longer require rinsing and the application of a new mixture.
Not all manufacturers use the same secondary ingredients or packaging, all of which can affect shelf life. If you're wondering how long you've got with a particular bottle, it's always a good idea to read the packaging for special instructions unique to that product.